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Extension Entomology

Category: Alfalfa

Fall armyworms, Armyworms, and Army Cutworms in Wheat

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Where wheat has emerged, fields need to be checked for fall armyworms, armyworms, and army cutworms. If you see worms on your wheat this fall, the first thing to do is to determine which worm is present. Proper identification is important because they have different feeding and overwintering patterns.

We have been hearing about and seeing a mixture of both armyworms and fall armyworms on wheat and other host plants this fall. These small worms start by causing small “windowpanes” in wheat or alfalfa. No army cutworm infestations have been verified yet on wheat.

Flocks of birds in wheat or alfalfa fields in fall or early spring are often indicative of a “worm” infestation as the birds are feeding on the larvae. Fields with 25-30% of the plants showing “windowpane” feeding need to be monitored frequently as these larvae consume more as they get larger. Treatment should be applied before stands become threatened.

Fall armyworms

When scouting fields for fall armyworm damage, look for “windowpane” injury caused by tiny larvae chewing on seedling leaves. Each individual field should be scouted in several locations, including the field margins and the interior. The larvae themselves are usually too small to be easily observed after they first hatch, and hide in or around the base of seedlings. Within a few days of hatching, the larvae become large enough to destroy entire leaves.

The suggested treatment threshold is 2-3 actively feeding larvae per linear foot of row in wheat. Fields with 25 to 30 percent of plants with windowpane injury should be re-examined daily and treated immediately if stand establishment appears threatened. Larvae increase in size at an exponential rate, and so do their food requirements. Later instars do the most damage, sometimes destroying entire stands, and are the least susceptible to insecticides. Without treatment, problems can continue until larvae reach maturity or until a killing frost. Thin strands of wheat are especially at risk.

 

Fall armyworms will feed until the temperatures cool into the mid-20’s or they pupate, whichever comes first. If a killing frost does not occur soon after the treatment threshold is reached, fields may require chemical treatment.

 

Armyworms

Armyworm larvae are green to black with stripes of various colors. The head capsule is medium brown with dark markings. Most damage to wheat in Kansas occurs in southern and eastern areas of the state during warm, moist periods from late April to early June rather than in the fall. Like fall armyworms, armyworms will feed until the temperatures cool in the mid-20’s or they pupate, whichever comes first.

Most armyworm damage occurs during the last three to five days of larval feeding. When leaf feeding is observed, look for larvae curled up on the ground under litter, especially in patches of lodged plants. Treatment is usually not necessary below levels of four or five larvae per foot, but is probably justified at infestations of five to eight per foot depending upon larval maturity in relation to crop maturity.

 

Army cutworm

The army cutworm is a late fall /early spring pest in Kansas. Leaf damage by early stage army cutworm larvae  looks very similar to that of fall armyworms. However, army cutworm larvae are typically very small in the early fall – smaller than fall armyworms or armyworms. If the worms causing defoliation in wheat in the fall are relatively large, ½ inch or more, they are probably armyworms and/or fall armyworms.

Adult moths lay eggs in soil in the fall. The brown, faintly striped larvae hatch during the fall and early winter. They will feed throughout the winter (unlike armyworm and fall armyworm larvae), burrowing in the soil to escape frost and emerging again to feed during sp

Unlike other cutworms, only above ground plant parts are consumed, giving plants the ap­pearance of being grazed by cattle.

Infestations in well­-established stands will probably not require insecticide appli­cations while wheat is dormant, but some fields never green up in the spring because of cutworm feeding. Along with fall scouting, frequent inspections during warm periods in February, March, and early April are strongly encouraged, particularly when preceded by a dry fall.

Moisture availability, crop condition, and regrowth potential are all factors influencing potential losses to this pest. Late­-planted fields under dry conditions with poor tiller­ing may suffer economic damage with as few as one or two larvae per square foot.

In most fields, treatment will not be necessary until populations average four to five worms per square foot. Vigorous, well-­tillered fields under optimal growing conditions can tolerate even higher popu­lations, as many as nine or 10 larvae per square foot, without measurable yield loss. Infestations in later stages of crop develop­ment are less damaging than early ones because established plants can compensate for considerable defoliation and larvae normally finish feeding before wheat enters reproductive stages.

Mixed populations

Mostly the same insecticides are registered for control of these species of worms, but higher rates are recommended for fall armyworm. Any fields with mixed populations should be treated with the fall armyworm rate.

For treatment options, please refer to the latest K-State Wheat Insect Management Guide 2017 at: http://www.bookstore.ksre.ksu.edu/pubs/mf745.pdf

Soybean Update — Green Cloverworms

— by Dr. Jeff Whitworth and Dr. Holly Schwarting

 

Green cloverworm larvae have been very numerous throughout north central and south central Kansas for the past 30-45 days.  However, the larval stage and the leaf defoliation that they’re known for, has pretty much ceased as most larvae have entered, or are entering, pupation.

 

 

A few green cloverworm larvae can still be found in late planted beans and alfalfa, but for the most part, their feeding will soon cease.  The most common question is, with all these green cloverworm adults around, will they be in the same fields next year?  They do not overwinter in Kansas and most soybean fields will be rotated, so the answer is no.  If the same fields are infested next year, it will not be because of the green cloverworms that survived the winter in that field.

 

Soybean Update – Green cloverworms and Stink bugs

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Green cloverworm adults are quite numerous and are laying eggs in alfalfa and soybeans.  So, there are, or will soon be, small larvae present.  Feeding by green cloverworms will probably not impact alfalfa or most soybean fields unless there are significant larval populations in really late planted fields.

 

 

Stink bug populations seem to be increasing in north central Kansas but most beans should be far enough along in their development that stink bugs should be of little concern.

 

For management decisions for all soybean pests, please refer to the 2017 Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

Alfalfa Update – Alfalfa caterpillars and Potato leafhoppers

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Alfalfa caterpillars are currently active throughout north central Kansas.  These caterpillars are feeding on foliage, and based upon the size of the larvae in fields, will soon be pupating.  In many fields, swathing is currently, or has just been done and this should help lessen alfalfa caterpillar populations.  However, if the timing is such that these larvae are pupating in the soil and thus swathing does not remove/destroy the pupae, the emerging butterflies may lay eggs on recently cut fields and the developing larvae may feed on regrowth.  This may retard regrowth for a couple of weeks until larvae pupate.

Potato leafhoppers are also very numerous throughout north central Kansas in uncut alfalfa fields.  Thus, their characteristic feeding damage, called ‘hopper burn’, is common.  Swathing should reduce potato leafhopper populations significantly and, hopefully, they will not rebound.  Continued monitoring is prudent as alfalfa caterpillar feeding and potato leafhopper damage may lessen the plant’s ability to store reserves in their roots for overwintering.

 

For management decisions for all alfalfa pests, please refer to the 2017 Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF809.pdf

 

Alfalfa Update

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

 

Potato leafhoppers are still common in all alfalfa fields sampled in the last 7-10 days, unless they have been swathed during that period of time.  Fields swathed within the last week did not have enough potato leafhoppers to reach a treatment threshold.  However, fields swathed just 10-14 days earlier are once again loaded with these little lime green, wedge-shaped leafhoppers.

There do seem to be good populations of green lacewings in uncut alfalfa fields.  However, they do not appear to be impacting the potato leafhopper populations.  We did pick up one alfalfa weevil larva and one adult in an uncut alfalfa field but the alfalfa weevils should not be of concern as major defoliators until next spring.

 

 

Alfalfa Update

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Potato leafhoppers have been infesting alfalfa fields throughout north central Kansas for at least the last month.  They continue to be very active, which is causing quite a bit of leaf yellowing, often called ‘hopper burn’, and even stem and whole-plant stress.  Typically, swathing is sufficient to manage leafhopper populations.  However, they have been actively reproducing and there are many nymphs, so it will be especially important to continue to scout these fields after swathing.  If a stubble spray is deemed necessary after swathing, one application is often highly effective and re-infestation is unusual.

 

Alfalfa caterpillars are also quite common in alfalfa fields, where they feed on foliage, although they rarely do enough damage to warrant an insecticide application.  They will eventually pupate and then turn into a yellow or white sulphur butterfly.

Bean Leaf Beetles in Soybeans

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Soybeans seem to be growing really well around north central Kansas.  As these plants start germinating and growing, however, they provide very attractive hosts for bean leaf beetles.

 

These soybean pests have been inhabiting alfalfa fields and grassy areas since last fall, waiting for these first soybeans.  They are very efficient at finding young, succulent soybeans when the plants start emerging.  They are also relatively unique as foliage feeders because they usually chew round and/or oblong holes in the leaves.

These beetles can eat an alarming number of holes in these small plants.  However, the young plants are very resilient at overcoming this leaf feeding and so there is normally very little impact on yield.  For more information on bean leaf beetle biology, treatment thresholds, and management options please see the following:

Bean Leaf Beetles: https://www.bookstore.ksre.ksu.edu/pubs/MF2824.pdf

Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

 

 

Alfalfa Update

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Alfalfa weevil feeding activity has slowed significantly in north central Kansas, at least south of I-70.  North of I-70, larvae are still developing and thus feeding, but even in the northern counties this feeding and resultant damage should be significantly reduced by the end of the next week.  There are still some small larvae but the majority of populations are pupating or have pupated.  Adult weevils are still hanging out in alfalfa fields and probably will until that 1st cutting, or temperatures get into the mid-80’s or warmer.

Alfalfa Update

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

 

Alfalfa weevil populations in north central Kansas seem to be developing as expected.  Fields sampled this week that had not yet been treated had greatly reduced foliage and were showing obvious signs of significant larval feeding, especially compared to fields that had been treated.

 

 

The good news is that the treated fields seem to be doing well and the untreated fields will probably only continue to be seriously impacted for about another week.  Weevil populations are predominately pupae, pre-pupae, or mature larvae which should cease feeding in the next few days.

 

 

This should give the alfalfa a chance to recover.  However, in untreated fields, and even fields that were treated later, after a majority of the larvae pupated, may have some adult feeding, as they may remain in fields until the 1st cutting or until temperatures start getting into the higher 80’s °F.  Adults feed a little on foliage and/or may cause ‘barking’ on stems, but this usually doesn’t stress plants too much unless there are significant numbers of adults.

 

Very few aphids were detected, but there were a few so periodical sampling should continue to ensure that these aphid populations remain at insignificant levels.

 

Alfalfa Update

— by Dr. Jeff Whitworth and Dr. Holly Schwarting

Alfalfa weevils are cool weather insects, thriving when temperatures are between 50-75°F and thus mostly affect alfalfa prior to the 1st cutting. That has been the case for about the last two weeks in north central KS. Temperatures have been on the cool side and the alfalfa has not been growing as well as most producers would like.  However, alfalfa weevils have been very active to the point where there are many pupae and even pupal webbing with holes indicating the adult weevils have already emerged and dispersed.  Infestations as of 6 April were composed of very small 1st instar larvae to larger, more mature 3rd instar larvae, as well as pupae, and newly developed adults.  Also, lady beetle larvae were quite common in most alfalfa fields, but no aphids were detected.

Alfalfa weevil feeding damage

 

Alfalfa weevil feeding damage – close

 

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