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Oklahoma State University Stillwater, OK |
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Oklahoma Alfalfa |
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Oklahoma State University Stillwater, OK |
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Oklahoma Alfalfa |
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Alfalfa Aphids |
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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 light-green in color. 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. 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. 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 it was 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 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 called "veinbanding". 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 either by sweepnet sampling or by pulling samples of stems (whole plants in seedling stands). Examining whole seedling plants is the only effective option for newly planted stands and in cases where foliar growth is less than 6-8 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 twenty 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 and aphid lions should also be counted. If aphid counts are below the economic threshold and numbers of predators exceed 5-7/sweep, aphids may be kept under control by the predators. For stem/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/stem. Estimate plant height, and refer to table 2-3 for treatment guidelines.
The most important factor in 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 due to aphids 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, the 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 Oklahoma Alfalfa. |
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| Richard Berberet,
Alfalfa Entomologist Department of Entomology and Plant Pathology Oklahoma State University
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