QUESTION #30 

My fall-planted alfalfa has yellow areas where plants appear to be dying. In these yellow areas, there are tiny aphids that resemble greenbugs. What are they? Should I spray them? 

EXPLANATION: Careful identification and scouting should be conducted to determine the levels of infestation of the various aphids. Newly established stands are highly susceptible to even low numbers of spotted and blue alfalfa aphids (1 or 2 per seedling). 

When present in alfalfa during fall and early winter, aphids are likely spotted alfalfa aphids, which are generally yellow in color with dark spots. When aphids are present in late winter or spring and are a green or blue-green color, they are probably pea aphids or blue alfalfa aphids. 

The spotted alfalfa aphid is the most devastating aphid pest of seedling alfalfa. It causes a severe toxic reaction in susceptible varieties that often results in discoloration (yellowing) along veins of leaves near plant terminals. This characteristic symptom is called "veinbanding." Heavily infested plants turn yellow and often some leaves have reddish discoloration. Eventually, these plants may take on a bleached appearance and affected spots across a field may die. New stands of susceptible varieties are particularly vulnerable to stand loss because of the size of plants and lack of genetic resistance. 

During winter and spring, following establishment of an alfalfa stand in fall, other alfalfa aphids may be present that can also account for thinning of stands or death of plants. The blue alfalfa aphid can be devastating because, like spotted alfalfa aphids, it causes a toxic reaction that can kill plants. Generally, blue alfalfa aphids do not cause yellowing. Plants remain green until just before death. High numbers of pea aphids cause wilting similar to drought. Spotted alfalfa aphids cause yellowing. Differences in reaction of alfalfa to feeding by aphids stresses the importance of identification and timely control applications. 

SCOUTING: Aphid populations in seedling alfalfa are best sampled by pulling stems and estimating numbers per stem. Sampling should be conducted at least once per week after emergence of new seedlings or during the growth of the first crop in established stands. Sampling should not be conducted within 24 hours following hard rains or high wind, as most aphids will not be on plants and accurate estimates of populations cannot be obtained. 

Select 30 stems at random over the field and place them in a container (preferably white in color). If the field is larger than 20 acres, additional sets of 30 stems should be taken. Shake aphids from the stems into the container and carefully estimate the number collected. Divide the total number of aphids by the number of stems to calculate the average per stem. Measure ten stems to determine the average plant height. 

ECONOMIC ANALYSIS: When aphid populations reach thresholds indicated in Table 5, insecticide applications should be considered. Assuming a $12.00/A cost for insecticide and loss of stand if left untreated, then treatment is warranted. When infestations already resulted in several dead or dying spots across the field, then appreciable losses in stand (25% or more) and subsequent yields ($80/A or more) may be experienced. 

In Oklahoma, where chronic aphid problems occur (particularly spotted alfalfa aphids), growers should plant resistant varieties. While none of the well-adapted varieties show a high level of resistance to blue alfalfa aphids, several varieties possess a high level of resistance to pea and spotted aphids. The additional cost ($1.50/lb, at most) of an improved, aphid-resistant variety is easily offset by increased establishment success, increased stand longevity (1-3 years), and greater yields (500-1,000 lb./A/year). A 10% increase in yield alone is worth at least $200/A, over a 5-year stand life. 

Insecticides should be considered when threshold numbers of aphids are observed (Table 5). Those listed as most effective for aphid control in Oklahoma include: 

Apply insecticides according to the instructions on the label for the best action and for safety. Other insecticides may also sometimes be effective.

Back to Table of Contents   

Back to Top 

DI'JA'NO  (Pronounced in some areas "did you know") 

QUESTION #31 

In addition to aphids, what other insects might infest my alfalfa stand? When should I look for them? 

EXPLANATION: Insect species prevalent in fall include grasshoppers and foliage feeding caterpillars such as: 

-- Yellowstriped Armyworm 

-- Corn Earworm 

-- Fall Armyworm 

-- Alfalfa Webworm 

Anytime after plant emergence, and usually before the first significant frost, these insects can be a problem. Cold temperatures either kill many of these pests or drive them into overwintering. 

Caterpillars that attack newly planted alfalfa in fall can quickly destroy all plant tissue above the soil surface. This includes the fall armyworm, alfalfa webworm, yellow striped armyworm and corn earworm which generally cause defoliation. 

Cutworms can be a serious threat in the fall under dry conditions, including the black cutworm which feeds at or below the soil surface, cutting or defoliating plants. 

In late winter and early spring army cutworms can cause damage to alfalfa. This cutworm species is often associated with dry weather conditions and poor fall growth. 

As weather warms in February and March, army cutworms may appear. Larval populations of this pest feed on alfalfa forage causing delays in spring growth or destruction of new stands when heavy infestations are present. 

During spring growth alfalfa weevil can be a problem on early fall-planted alfalfa that makes abundant fall growth. Levels of infestation are generally not as severe on seedling as on established alfalfa and chemical control is seldom justified. 

Variegated cutworms are generally a problem after first cutting and can delay regrowth of the second crop up to three weeks. This pest is not limited to seedling stands but tends to be less troublesome on older stands. 

To determine the species of insects present in alfalfa, consult OSU Extension Fact Sheet No. 7159, "Field Key to Larvae in Alfalfa." 

THRESHOLDS: Infestations of 2-3 caterpillars or cutworms per ft². can be used as a general threshold. At that level of infestation, producers should monitor fields closely, identify the pests, and obtain exact counts for each species present. 

Scouting for grasshoppers in new alfalfa stands should begin as soon as alfalfa emerges and continue until the first hard freeze (20⁰ - 25⁰ F). Grasshopper populations are normally highest around edges of fields near areas with tall weeds and grass. Grasshoppers are always a problem when alfalfa is sod-seeded into grass pastures. 

In seedling stands, 1 or 2 grasshoppers/sq. yard should alert producers to apply chemical control measures. Frequently it is only necessary to spray border areas before they move into the interior of fields. 

ECONOMIC ANALYSIS: Even when assuming a $15/A cost of insecticide application, chemical treatments may be justified for these insects. Insecticide costs can be reduced by spraying only those portions of a field that have severe infestations. Less expensive applications ($6/A) of insecticides may also prevent damage from these insects.

Back to Table of Contents  

Back to Top