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 The most common insect pests in Oklahoma alfalfa include:
Foliage-Feeding Caterpillars - always present during summer months. As a perennial legume, alfalfa provides a favorable habitat and food source for a large number of insect species. Many of these species are considered pests because of injury resulting from their feeding on leaves, stems, and root systems of alfalfa plants. These insect pests of alfalfa are so varied in their life cycles and habits that feeding injury caused by one or more species may occur anytime throughout the entire year. Often, the damage resulting from feeding by insects threatens not only the yield or quality of a particular harvest of alfalfa, but may also result in thinning of stands and reduction in productive stand-life.
Figure 2-1. Graph of Alfalfa weevil egg and larval occurrence Fortunately many beneficial insect species prey on plant feeders and help to reduce their damage potential. Among these natural enemies are lady beetles, aphid lions, and other types of predators, as well as parasitic wasps and flies that attack foliage-feeding caterpillars and alfalfa weevils. Pollinators are another important group of beneficial insects essential to seed production in alfalfa. This section of the manual provides descriptions of the various types of insects (both pests and beneficials) found in alfalfa, and outlines procedures for sampling to assess population densities. Additional information on insect life cycles, sampling procedures, photographs, and advisories related to current insect infestations can be obtained through Oklahoma Alfalfa on the Internet. Insect Pests in Alfalfa Forage Production Life Cycle and Damage. The alfalfa weevil is a snout beetle about 1/4 inch long, light brown with a dark brown mid-dorsal band (Photo 1). This insect completes one generation per year. The life cycle begins in late fall when adult weevils enter alfalfa fields from their summer resting sites (dense vegetation such as in fence rows and along creek bottoms). Adults normally begin laying eggs inside stems of alfalfa plants during November and continue until April or May of the following year. Egg lay (oviposition) continues throughout winter, and large numbers of eggs may accumulate before hatching begins during January or February. As they hatch, larvae leave the stems and crawl to growing alfalfa terminals to begin feeding. Newly hatched larvae are a yellow color and feed on developing leaf tissue within plant terminals (Photo 2). As they grow, larvae feed in more exposed locations on leaves and can be recognized by their green color and shiny black head capsules (Photo 3 and Photo 4). Larval numbers usually increase rapidly as plant growth begins during February and March, and the potential for defoliation in early vegetative growth is great. The peak in larval numbers and greatest amount of damage occurs during March and April (Figure 2-1). Yield reduction of alfalfa due to defoliation by weevil larvae is about 170 pounds per acre in the first crop for each increase of one larva per stem in the population. An additional reduction of about 140 pounds per acre occurs in the second crop due to loss of vigor in damaged alfalfa stands. With combined losses due to actual feeding damage in the first crop and residual effects later totaling more than 300 pounds per acre for each increase of one larva per stem, timely, effective insecticide applications are essential. Insecticide applications should be made when results of field scouting indicate that weevils are present in high numbers and potential losses due to feeding will exceed the cost of control (at the economic threshold). The time period when larval numbers exceed the economic threshold may vary greatly by year and location in Oklahoma, ranging from late February to mid-April.
Figure 2-2. Graph of infestation vs. yield reduction Sampling and Control. The 'shake-bucket' scouting procedure is designed for use in decision-making relative to application of chemical insecticides takes into consideration 1) accumulated heat energy (degree-days) for weevil development, 2) alfalfa plant height, and 3) number of weevil larvae collected in a 30 stem sample (OSU Current Report No. 7177). Degree-day values for each county can be obtained from the Oklahoma Mesonet System. Specific values obtained for these three variables at the time of sampling are combined in a recommendation chart to determine if spraying is warranted. If spraying is not recommended on a particular sampling date, a time interval for resampling is given. This is the most accurate sampling procedure for decision-making, particularly when sampling after application of insecticide or following a period of freezing weather that may have killed some larvae. When a recommendation to spray is indicated by the scouting process, it is important to follow label instructions for rates of insecticides to apply. Applications should be made with favorable weather conditions; temperature above 50o F and wind velocity less than 10 mph. For both ground sprayers and aerial application, swath width should not exceed the length of the spray boom to avoid skips in coverage. For effective control of weevil larvae, it is important that adequate spray volume is used for thorough coverage of alfalfa foliage; a minimum of 10 gallons per acre with conventional ground equipment and 2-4 gallons per acre for aerial application is recommended . The value of increasing spray volume for effective control of the alfalfa weevil is illustrated in Table 2-2.
Application of insecticides has been an essential component of control programs for the alfalfa weevil in Oklahoma since 1970. Nevertheless, populations of this pest and the potential for losses can be reduced by other means such as planting varieties with tolerance to insects and by grazing of alfalfa stands for 2-3 weeks during December and January. Grazing can reduce number of alfalfa weevil eggs in March by 50-70 percent (compared to nongrazed). Recent studies have also shown that grazing during March can remove nearly all eggs and larvae present and eliminate the need for spraying. Grazing in March, however, is usually not recommended in hay fields due to bloat and increased weed problems (see "Integrate Grazing into Harvest of Thinning Stands" in the Weeds Management section of this chapter). Two natural enemies of the alfalfa weevil are wasps that parasitize weevil larvae and a fungal disease that kills both larvae and pupae. These beneficials are most important for killing weevils remaining in stands near the time of first cutting. The fact that weed growth is increased in areas that are defoliated gives an added incentive for maintaining an effective control program for the alfalfa weevil. Additional information relating to control of the alfalfa weevil can be found in OSU Extension Facts No. 2097 and Oklahoma Alfalfa. Species Identification. Although aphid infestations can occur at nearly any time of the year in Oklahoma, damaging populations are most common from February to May during growth of the first alfalfa crop. Two species usually found during this time each year are the pea aphid and blue alfalfa aphid. The pea aphid is the largest of the species found in alfalfa, and is typically a light green color (Photo 5). This species prefers to feed on stems and developing leaves near plant terminals. Blue alfalfa aphids are somewhat more blue-green in color and are typically smaller than pea aphids. Preferred feeding sites are similar to those of the pea aphid, except that the most common biotype of the blue aphid in Oklahoma (designated BAOK90) may be found in greatest numbers on lower portions of stems. The most reliable way to distinguish pea aphids and blue alfalfa aphids is by careful examination of the antennae. Using a hand lens or magnifying glass (10X), dark-colored rings can be seen at regular intervals on antennae of pea aphids, while those of blue aphids have no dark rings, but rather become gradually darker in color near the tip (Photo 6). Pea aphids and blue alfalfa aphids are adapted to relatively cool temperatures, and populations seldom persist beyond early July in Oklahoma. A third species, the spotted alfalfa aphid, may infest alfalfa fields during February and March, but the occurrence of damaging populations in winter is relatively infrequent (5-10 year intervals). Infestations of spotted aphids are found commonly during warm, dry weather, typical of late summer and fall in Oklahoma. Size of this species is relatively small in comparison to the pea aphid or blue aphid. Its color ranges from light green to tan with several rows of dark spots (Photo 7). Spotted alfalfa aphids may be found in greatest numbers on leaves and stems in the lower portion of the plant canopy, near the soil surface. An additional species that typically occurs in low numbers during late winter and spring is the cowpea aphid. This species ranges from gray to shiny black in color. The cowpea aphid prefers to feed on leaves near plant terminals and on stems. Interest has increased regarding this species as it has been found in relatively large numbers throughout the summer months since 1999. Damage Caused by Aphids. All aphids feed by removing fluid from plants with piercing-sucking mouthparts. When large numbers of aphids are present, their feeding typically results in wilting of foliage and stunting of plant growth. Often, feeding causes leaves to become chlorotic, having a yellow discoloration. Of the four species found in alfalfa, the symptoms observed with infestation by the cowpea aphid have been the least severe. There has been little evidence of necrosis (death of leaf tissues) and no mortality of plants observed with feeding by this species. It is important to note, nonetheless, that seasonal occurrence of the cowpea aphid has changed radically in recent years, with high population densities occurring during summer months. It is unclear at this time if damage potential from this insect will increase and warrant spraying in the future. In the presence of high population densities, feeding by the pea aphid typically results in stunting of plant growth and chlorosis of leaves. Only with large numbers and an extended period of infestation does wilting of plants with necrosis of leaf tissues occur. There is little threat of stand loss due to damage by pea aphids. From the time data were first collected in Oklahoma in 1977 until 1990, the extent of damage by the blue alfalfa aphid was similar to that of the pea aphid. High population densities resulted in deformation of leaves near plant terminals and moderate to severe stunting of plant growth. There were few reports of stand reductions due to blue aphid infestation. Since 1990, however, a virulent biotype (BAOK90) has been identified with the potential to cause death of plants and severe stand reduction. This biotype induces a severe toxic reaction in alfalfa plants, with symptoms that typically including stunting and death of plants during relatively short periods of infestation (2-3 weeks). Until 1990 and the identification of blue aphid biotype BAOK90, the spotted alfalfa aphid clearly had the greatest damage potential of any insect pest in alfalfa. This species induces a severe toxic reaction in susceptible alfalfa plants that results in chlorosis and necrosis of leaves, beginning in the lower portions of the plant canopy. An unusual damage symptom often caused by the spotted aphid is clearing of veins of newly-formed leaves near plant terminals, which is called "veinbanding" (Photo 8). Heavily infested plants are killed within 1-2 weeks, and stand loss may be severe, particularly in those infrequent years when infestations occur in late winter and spring at a time when there is relatively little foliar growth. The spotted aphid is a serious threat to newly planted stands because its seasonal abundance is greatest from August to October when alfalfa is planted. The threat of stand loss due to this species is great in newly planted alfalfa because seedlings, even those of resistant cultivars, may have limited ability to withstand feeding. Sampling and Control. Aphid numbers may be estimated by sweepnet sampling, by pulling samples of stems, or by pulling whole plants in seedling stands. Examining whole seedling plants is the only effective option for newly planted stands and for stands where foliar growth is less than eight inches tall. Sampling should be done at least once per week following emergence of seedlings in new stands of alfalfa and during growth of the first crop in established stands as numbers of aphids may increase quite rapidly during optimal weather conditions. Sampling may be conducted less frequently during summer months, unless infestations of spotted alfalfa aphids have been detected, in which case weekly sampling should be continued. For sweepnet sampling of aphids, use a 15-inch diameter net with a heavy muslin bag. Take 20 sweeps at each of a minimum of five locations per field. For each location, place contents of the net in a pan and estimate the numbers of aphids of each species present. Determine average population densities for each species in the field and refer to Table 2-3 for treatment guidelines. Numbers of predators such as lady beetles (Photo 9) and (Photo 10) and aphid lions (Photo 11) should also be counted. If aphid counts are below the economic threshold and numbers of predators exceed 5-7 per sweep, aphids may be controlled by the predators. For stem or seedling plant sampling, a sample of 30 stems should be collected at random in each 10-20 acre area and aphids shaken into a container for counting. Divide aphid numbers of each species by 30 to calculate the average number per stem. Estimate plant height, and refer to Table 2-3 for treatment guidelines.
The most important factor for effective aphid management in alfalfa is variety choice. All improved varieties recommended for production in Oklahoma have resistance to one or more of the aphid pests discussed. Most have high levels of resistance to pea aphid and spotted alfalfa aphid. Although the virulent biotype of blue aphid is adapted to most resistant cultivars, new variety releases are being made with resistance to biotype BAOK90. For a modest increase in seed cost for improved varieties compared to Oklahoma common alfalfa, a high degree of protection against losses can be obtained for the life of a stand. It is an investment that is well worth the cost. Several types of predators including lady beetles, aphid lions, and damsel bugs assist in limiting aphid populations in alfalfa. As indicated earlier, when relatively large numbers of predators are present and aphid populations have not reached the economic threshold, natural control agents usually provide effective regulation of aphids. Parasitic wasps may be important in limiting population growth of spotted alfalfa aphids during summer months. In order to preserve these beneficial species, proper sampling and comparison of aphid population densities with economic threshold levels should be done before insecticide applications are made. Additional information relating to control of aphids can be found in OSU Extension Facts No. 7184 and at Oklahoma Alfalfa. Life Cycle and Damage. Species that commonly infest alfalfa include the army cutworm and the variegated cutworm. Although damage by these species may be quite severe, high population densities are relatively infrequent, typically once every 6-8 years. Damage from cutworm larvae typically occurs at consistent times during the year and usually results from a single generation per year for each species. Army cutworm adults (moths) lay eggs during October in newly planted or cleanly harvested fields. Eggs hatch during the fall, and the small larvae overwinter. Most feeding by these larvae occurs as weather warms in March. Army cutworm larvae are gray to tan in color and finely mottled with white and brown, but without prominent markings (Photo 12). Fully grown larvae may attain a length of 2 inches. When present in large numbers, army cutworms consume all crown growth of alfalfa and may delay spring "green-up" by 4-6 weeks (Photo 13). The variegated cutworm overwinters in the pupal stage and adults emerge to mate and lay eggs during growth of the first crop of alfalfa in April. Eggs hatch and larvae begin to feed on leaves in the lower portion of the foliar canopy before the first harvest. Larvae are gray to black in color with distinctive white to yellow diamond-shaped markings along the center of the back (Photo 14). The most serious threat of damage occurs on regrowth following the first harvest of the year (usually early May). Variegated cutworms feed primarily at night and tend to seek protected habitats within plant foliage or debris during daylight. They aggregate beneath windrows when hay is cut, and the greatest damage to regrowth is often where windrows have lain in fields. There is frequently a ribbon-like appearance of alternating heavy damage (under windrows) and light damage (between windrows) in infested fields after baling (Photo 15). Sampling and Control. Detection of cutworms and assessment of population densities is often difficult because of their nocturnal feeding habits. Whenever alfalfa seems slow to green up during March or after first harvest, fields should be checked for the presence of cutworms. Sampling requires sifting through plant debris and the upper inch or two of soil around plants. A minimum of ten one-square-foot areas should be sampled in each 10-20 acre field area. When numbers of small larvae (up to ½-inch long) exceed an average of three per square foot, an insecticide should be applied. If large larvae (>½-inch long) are present, spraying is recommended when numbers exceed two per square foot. Control of cutworms with insecticides may be difficult. It is important that applications be made in late afternoon or evening, near the time when larvae begin to feed, using high spray volumes (10 gallons per acre for ground application and 2-4 gallons per acre for aerial application) to ensure thorough coverage. Additional information on control of cutworms can be found in OSU Extension Facts No. 7150 and Oklahoma Alfalfa. Life Cycle and Damage. The larval stages or "caterpillars" of several species of butterflies and moths are common foliage feeders in alfalfa from May through October. The most important of these are the corn earworm (Photo 16), yellow-striped armyworm (Photo 17), fall armyworm (Photo 18), alfalfa caterpillar (Photo 19), green cloverworm (Photo 20), and forage looper (Photo 21). Each of these species completes several generations during summer and fall, with each one lasting 4-5 weeks. When present in large numbers, these caterpillars may completely defoliate alfalfa, consuming the leaves for their high nutrient content and leaving only the stems, which have limited value as livestock feed. Sampling and Control. The most effective sampling approach for these caterpillars is sweeping with a standard 15-inch diameter net. A set of 20 sweeps should be made in each 5-10 acre field area. If numbers exceed six larvae per sweep, insecticide should be applied or, if the time for harvest is near, alfalfa may be cut to remove the caterpillars' habitat. Additional information on control of foliage-feeding caterpillars can be found in OSU Extension Facts No. 7150 and at Oklahoma Alfalfa. Life Cycle and Damage. This small, wedge-shaped insect is light-green and about 1/8- inch long (Photo 22). This species does not overwinter in Oklahoma, but migrates northward from coastal areas of the Gulf of Mexico on storm fronts each spring and is usually first collected in May. The most characteristic symptom of damage by the leafhopper is chlorosis and eventual necrosis of tissues at leaf tips called, "hopperburn" (Photo 22). In addition, growth of alfalfa plants may be severely stunted, resulting in both quality and yield reductions. Sampling and Control. The primary means for sampling to assess the need for control of leafhoppers is sweepnet sampling. At least five sets of 20 sweeps should be taken in each field up to 40 acres. More samples should be taken in larger fields. The greatest potential for losses due to potato leafhopper occurs with infestations on new growth after a harvest has been taken. Although sweeping is difficult in alfalfa stubble, the effort should be made because the economic threshold in six inch or shorter alfalfa is an average of 0.5 leafhoppers per sweep. When alfalfa is 12 inches or taller, the threshold is two leafhoppers per sweep. When growing conditions are optimal for alfalfa, applying insecticide when these thresholds are reached can be profitable. However, when exceedingly dry conditions prevail as often occurs in mid-summer in Oklahoma, the limited potential for alfalfa growth may not warrant an added expenditure for insecticide. Leafhoppers survive from one flush of growth to the next by living on stems that were missed by the harvester. Making sure that all standing alfalfa in a field is cut can help control this pest. Additional information relating to control of leafhoppers can be found in OSU Extension Facts No. 7150 and at Oklahoma Alfalfa. Life Cycle and Damage. Several species of blister beetles are common throughout eastern and southern areas of the U.S., and in no way is the problem with blister beetles in alfalfa restricted to Oklahoma. The species found in alfalfa range in color from a uniform black or gray to a striped pattern of alternating brown and orange (Photo 23). It is the striped blister beetle that most commonly forms large aggregates or "swarms" in alfalfa fields. The blister beetles found in alfalfa complete one generation per year, which begins during late summer as females lay eggs in cracks and cavities at the soil surface. Eggs hatch and tiny long-legged larvae called "triungulins" crawl over the soil surface in search of clusters (or pods) of grasshopper eggs, which are also laid within 1-2 inches of the soil surface. Upon finding a site where grasshopper eggs have been laid, each triungulin tunnels into an egg pod and begins to feed. The larva continues to grow and develop while consuming the grasshopper eggs, then overwinters in the soil and emerges as an adult beetle the following May or June. These beetles feed on foliage and blooms of many plants including alfalfa. However, it is not their feeding activity that gives them pest status, but the fact that the beetles contain a chemical called "cantharidin", a blistering agent that is highly toxic and may cause illness or death in livestock, particularly horses, when consumed in forage. Cantharidin is a highly stable chemical that remains active even within the dried remains of beetles. If the beetles are killed during alfalfa harvesting, as would occur when a crimper/conditioner is used for swathing, they may be baled into hay and inadvertently fed to livestock. In the case of the striped blister beetle that frequently forms "swarms" in alfalfa fields, remains of many beetles may be found in baled hay if they are killed at the time of cutting either by crimping hay or by wheel traffic over windrows. If they are not killed when alfalfa is cut, the beetles leave the windrows as hay dries before baling and the threat of cantharidin toxicity is eliminated. All cases of mortality in horses resulting from cantharidin toxicosis that have been confirmed by Oklahoma State University College of Veterinary Medicine have involved the striped blister beetle. Cantharidin toxicosis causes ulceration of sensitive skin areas (lip, nose, and mouth) and mucous membranes (esophagus and stomach). Severity of toxicosis symptoms depends primarily on the number of beetles consumed. Cantharidin is highly damaging to tissues of the digestive and urinary tracts. If large amounts of cantharidin are ingested, death may occur within six hours. When smaller amounts are consumed, horses may display symptoms of colic (pawing and stretching). Due to reduced levels of electrolytes (calcium and magnesium) in the blood stream, animals may exhibit stiffness or an exaggerated "goose-stepping" gait. Small amounts of darkened urine (blood in urine) may be voided as a result of damage to the kidneys and bladder. Horses displaying these symptoms should be promptly examined by a veterinarian, and hay should be examined for the presence of blister beetle remains. Sampling and Control. There is no way to guarantee that alfalfa hay harvested in the southern United States is completely free of blister beetle contamination. However, several precautions can be taken to greatly reduce the chances that hay will be contaminated:
As there is no way to completely eliminate the threat of blister beetles in alfalfa, the prudent approach for management is to take all possible precautions to reduce the likelihood that they are present, particularly when marketing hay to horse owners. Additional information on blister beetles in alfalfa can be obtained from OSU Extension Facts No. F-2072 and at Oklahoma Alfalfa. The three-cornered alfalfa hopper completes several generations per year while feeding
in legume crops such as alfalfa and soybeans. However, populations are rarely high enough in
alfalfa in the southern plains to cause noticeable damage until September or October.
Nymphs and adults of this species make repeated punctures to feed on plant fluids while
moving around stems, which results in girdling of stems. Stems are typically girdled within
2-3" of the soil surface. Leaves on girdled stems become chlorotic and often turn red to
purple
in color. Stems that have been weakened by girdling often break and lodge. Insect Pests in Alfalfa Grown for Seed Some species described as pests in forage production may also interfere with profitable alfalfa seed production. When growing alfalfa for seed, it is important to scout fields before bloom to prevent stunting or defoliation of plants due to aphids, leafhoppers, and foliage-feeding caterpillars. Profitable seed production depends on healthy, vigorous plants that have not been subjected to stress during vegetative growth. To the greatest extent possible, use of insecticides must be avoided once pollinators (bees) have begun to work. Insect pests included in this section on seed production are those having the greatest potential to cause injury or destruction of buds, blossoms, or seeds. Life Cycle and Damage. Grasshopper species that damage field crops such as alfalfa typically complete one generation per year. In Oklahoma, three of the most common species are the differential (Photo 24), two-striped (Photo 25), and red-legged grasshoppers. These grasshoppers overwinter as eggs laid in soil, usually along fences, roadsides, and in pastures rather than in field crops. Nymphs hatch from these eggs in late spring (May and June) and feed on grass and broadleaf weeds outside of fields until mid-summer when. When they have become large nymphs or adults and plants in non-cropland areas typically mature and become less palatable, grasshoppers then begin migrating into adjacent fields. Depending upon population densities, they may feed extensively on alfalfa or other available crops. Although grasshoppers may defoliate alfalfa in areas near field borders, they pose a much more serious problem in seed production because they often feed primarily on the fruiting structures when alfalfa is in bloom, causing nearly 100 percent loss of the seed crop near field margins. Sampling and Control. Grasshopper population densities are typically estimated while walking in areas near field margins. As grasshoppers fly out of foliage, numbers per square yard are estimated. After alfalfa begins flowering, the economic threshold of 15-20 insects per square yard for alfalfa forage production does not apply to seed production because this infestation level may result in seed losses greatly exceeding the cost of control, and because insecticides and application rates sufficient to reduce grasshopper populations may be quite damaging to pollinators. It is critical that grasshoppers be controlled in nearby fence rows, pastures, and other adjacent areas before migration into alfalfa begins. Controlling grasshoppers in areas adjacent to fields in June is effective since hoppers are sprayed while they are small nymphs. Applications made at this time may prevent serious losses in seed production later and also preserve pollinators. Additional information on control of grasshoppers can be found in OSU Extension Facts No. 7150 and at Oklahoma Alfalfa. Life Cycle and Damage. Although there are numerous species of plant bugs found in alfalfa, those that pose the greatest threat to seed production such as the tarnished plant bug, are classified in the genus Lygus and are usually referred to as Lygus bugs (Photo 26). Nymphs of the Lygus bugs are a light green and are often mistakenly identified as aphids. However, they are much more active than aphids, often crawling rapidly over the plant foliage. Adults are yellow to brown in color and typically about 1/4 inch long. These insects overwinter as adults in alfalfa fields or adjacent areas with abundant grasses and weeds, which provide a protective habitat. Lygus bugs complete several generations per year and may be present in large numbers in summer months. Nymphs and adult Lygus bugs feed with piercing-sucking mouthparts to remove plant fluids from buds, blossoms, and green seed pods, causing these structures to die and drop from plants. In the presence of high population densities of Lygus bugs, seed set and maturation may be reduced by nearly 100 percent. Much of the flower drop in alfalfa seed fields that is often attributed to dry weather may in fact be due to feeding by Lygus bugs. Sampling and Control. Fields in which seed production is planned should be sampled by sweeping before bloom so that, if necessary, controls may be applied before pollinators are present. As Lygus bugs are flying insects and may migrate into fields in large numbers within short time spans, it is important that sampling be conducted as alfalfa enters the bud stage. At least five samples of five sweeps each should be taken in each 30-acre area of alfalfa. Spraying is recommended when there are an average of two Lygus bugs per sweep. Spraying after the onset of blooming is not recommended as it may kill pollinators. However, if large numbers of these insects have migrated into seed production fields after bloom has begun, it may be necessary to apply insecticide. If bee colonies have been placed in the field for pollination, they must be removed before spraying. Spraying should be done in late evening or early morning when pollinators are not active. Additional information on control of Lygus bugs can be found in OSU Extension Facts No. 7150 and at Oklahoma Alfalfa. Life Cycle and Damage. Webworms are larvae of small moths that complete several generations per year and often become quite abundant in alfalfa during summer months. When fully grown, these worms are about 3/4-inch long and have light green coloration with numerous black spots. As they develop, webworms spin silken webs over plant terminals, then feed on leaves, buds, and blossoms within the webs (Photo 27). Although extensive defoliation may not result from their feeding, webworms often consume fruiting structures, and their webs interfere with pollination and seed set. Infestations by these worms can result in serious losses in seed production. Sampling and Control. As with other pests that have been discussed, it is important to assess population densities of webworms before alfalfa begins to bloom. Webworms can be counted in sweep samples taken for foliage-feeding caterpillars (20 sweeps in each of five areas per field). If the numbers of webworms exceed five per sweep (even in the absence of other foliage feeders) and silken webs cover 10 percent or more of the terminals, application of insecticide should be considered to prevent serious losses in seed production. It is important to use relatively high spray volumes (10 gallons per acre for ground application and 2-4 gallons per acre for aerial) to penetrate webs for effective control. Additional information on control of webworms can be found in OSU Extension Facts No. 7150. Life Cycle and Damage. The adult of this species is a tiny black wasp (Photo 28; 1/10-inch long) that lays eggs in developing alfalfa seeds within green pods. A single larva develops within each seed, destroying all contents, then enters a pupal stage (Photo 29 and Photo 30). Upon emergence from the pupal stage, adults chew holes in seed coats and seed pods to escape and lay eggs for a new generation. Several generations are completed each year, with increasing population densities and greater potential for damage through mid- and late summer. Larval and pupal stages of the fall generation overwinter in seed to emerge as adults the following spring. Surveys of damage in seed produced in Oklahoma have shown up to 80 percent loss due to the seed chalcid, with an average loss of 10-15 percent. Sampling and Control. Seed chalcids cannot be controlled effectively with chemical insecticides, and spray applications made during bloom and seed set may cause high levels of mortality to pollinators. Thus, the primary means for limiting infestations is reduction of populations entering seed production fields through sanitation. Methods of sanitation include:
Additional information on alfalfa seed chalcids can be found at Oklahoma Alfalfa. It is fortunate for alfalfa growers that many of the insects found in this crop are natural enemies that attack the plant-feeding insect species. Although natural enemies do not typically maintain populations of some insect pests, such as the alfalfa weevil, below economic threshold levels, others such as aphids and foliage-feeding caterpillars are effectively controlled in most years. The need for insecticide applications would be increased greatly were it not for the presence of these natural enemies. A second group of beneficial insects that is of critical importance in alfalfa seed production is the pollinators. Without pollination by several species of bees, seed production in alfalfa is not profitable. (More on pollinators later in this chapter.)
Predators of Insects The most well known among the predators are the lady beetles, a group in which both larvae and adults prey on a variety of insects, most commonly on aphids and small stages of foliage-feeding caterpillars. They also eat larvae of the alfalfa weevil. Species that are most common in alfalfa have an oval body shape and are usually bright orange and black (Photo 9). Among them is an imported species called "C-7" or the seven-spotted lady beetle, which has become abundant throughout Oklahoma. The larvae of lady beetles typically have elongated bodies with long legs and orange and black mottled coloration (Photo 10). Laboratory studies have shown that individual lady beetle adults and larvae consume 20-30 aphids per day.
Also common in alfalfa are the aphid lions, or larvae of the lacewings. Aphid lions resemble tiny alligators with long, curved, hollow jaws that they use to clamp onto insect hosts and suck out body fluids (Photo 11). These predators readily attack not only aphids, but foliage-feeding caterpillars as well. Adult lacewings are typically brown or green, with large, fragile wings (Photo 31).
The damsel bugs are slender-bodied insects with gray to tan coloration (Photo 32). Their front legs are adapted for grasping prey, and they possess piercing-sucking mouthparts that they use to suck body fluids from their prey. Damsel bugs attack any small insects they encounter including aphids, Lygus bugs, weevil larvae, and small caterpillars.
There is also a variety of spiders in alfalfa that prey on insects. The most effective of these are the crab spiders, which search for prey in the foliar canopy without use of webs. Recent studies have indicated that these spiders may be quite important as natural enemies of insects in field crops. Additional information about beneficial insects can be found at Oklahoma Alfalfa.
Parasites of Insects The most common parasites of plant-feeding insects in alfalfa are tiny wasps that range in color from black to brown or orange. Several species parasitize aphids by inserting eggs into their hosts using an ovipositor shaped like a needle. The larval and pupal stages of the parasites grow inside the hosts, transforming the hosts into "mummies" before the adult wasps emerge. In a similar manner, two species of imported parasitic wasps attack larvae of the alfalfa weevil by inserting eggs into hosts (Photo 33). The parasitic larvae consume the internal organs of their hosts, and after the weevil larvae have spun their silken cocoons, the parasites spin cocoons within those of the weevils (Photo 34). Numerous species of parasitic wasps attack foliage-feeding caterpillars. There are also many parasitic flies that attack foliage-feeders, typically sticking eggs on the body surface. The larvae hatch and bore into the bodies of their hosts to consume the internal organs. Studies conducted in Oklahoma have shown that as many as 50-60 percent of foliage-feeding caterpillars may be killed by parasites. The most effective pollinators of alfalfa in Oklahoma are bumble bees (Photo 35) and leafcutter bees (Photo 36). While both of these bees occur naturally, their population levels are typically not high enough to provide all of the pollinators needed for a highly productive seed crop. There is no process available for rearing bumble bees for use as pollinators. Although leafcutter bees are reared as pollinators in some areas of the western U.S., they are not used extensively in Oklahoma. The most common species used to supplement naturally occurring pollinators for alfalfa seed production in Oklahoma is the honey bee (Photo 37). With exception of those who place honey bees in fields, most alfalfa seed producers in Oklahoma rely on native bee populations for pollination. Over the years, there have been adequate numbers of bees of various species to sustain profitable seed production. However, wild populations of honey bees, in particular, have been reduced greatly by diseases and tiny parasitic mites. The need to supplement native bee populations by moving colonies of bees into seed production fields is greater than in the past. Also, the need to restrict the use of pesticides in seed fields once plants start blooming is critical. Follow recommendations related to sampling fields for potential pest problems before the onset of bloom. If applications are required after alfalfa plants start blooming, take care to minimize mortality of pollinators (spray in late evening and remove bee colonies from fields). Additional information on pollinators can be found at Oklahoma Alfalfa. Insects are infected by a variety of disease agents including bacteria, viruses, and fungi. The most common diseases observed in pest species of alfalfa in Oklahoma are caused by fungal pathogens. The fungal diseases are quite inconsistent in their occurrence because they require frequent rain showers and high humidity to become infective. These conditions often do not exist in Oklahoma, even during spring. One of these fungal diseases has infected larvae and pupae of the alfalfa weevil since 1983. This disease kills alfalfa weevil larvae within 2-3 days of infection, with the remains of dead insects being either tan in color (Photo 38), from which the fungus forms spores to infect additional weevils, or black (Photo 39). Host remains that are black contain resting spores that survive until the next season. When pupae are infected by the fungus, they invariably turn black and produce resting spores. Under optimal environmental conditions of frequent rainfall and moderate temperatures, this disease has been found to kill over 90 percent of weevil larvae and pupae. Unfortunately, these conditions do not occur often enough for the fungal disease to provide consistent control of the alfalfa weevil in Oklahoma. There is also a fungal disease of pea aphids that may occur during periods when high population densities are present with frequent rainfall. Infected aphids die within 2-3 days, leaving remains that are brown to tan in color. As with the disease of the alfalfa weevil, this pathogen may kill a high percentage of aphid populations when optimal environmental conditions prevail.
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Alfalfa Production Guide for the Southern Great Plains, 2001
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