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Extension Entomology

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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

Sugarcane Aphid Update

–By Dr. Sarah Zukoff, Dr. J.P. Michaud, Dr. Brian McCornack and Dr. Wendy Johnson

  1. First report of sugarcane aphid on grain sorghum in Kansas this year

The sugarcane aphid (SCA) has now been reported in Sumner County. Sorghum producers in Kansas should begin scouting their fields on a routine basis. More information on scouting and thresholds for treatment can be found in the Agronomy eUpdate article “Start scouting soon for sugarcane aphid” from July 7, 2017, and at the myFields web site at: https://myfields.info/pests/sugarcane-aphid

 

 

Figure 1. Current status of the SCA. The map indicates only the counties in which the SCA has been found, and does not indicate how many or how few aphids were found in that county. Source: https://www.myfields.info/pests/sugarcane-aphid

 

 

Chemical control

Two insecticides are labeled for use on sugarcane aphid on sorghum in Kansas this season:

Sivanto (flupyradifurone)
4.0 – 7.0 fl oz/acre (0.052 – 0.091 lb ai/acre)

Transform (sulfoxaflor)
0.75 – 1.5 oz/acre (0.375 – 0.75 oz ai/acre)

Field trials show good efficacy of the above materials against the sugarcane aphid. Both have the ability to penetrate leaves through translaminar movement and kill aphids feeding on the undersides.

Maximum efficacy will be achieved by application in a large volume of water, preferably 20 gallons per acre or GPA (minimum 10 GPA) by ground or 5 GPA from the air. Laboratory trials indicate that Transform (sulfoxaflor) is relatively safe for important aphid predators such as lady beetles and lacewings and thus can be considered IPM-compatible. This is true to a lesser extent for Sivanto (flupyradifurone), but various trials have indicated a much longer period of residual activity for this material.

Both insecticides have annual application limits and growers are advised to rotate them if follow-up applications are required. Note also that preharvest intervals will be a factor to consider when treating late-season infestations, so applicators should read labels carefully and keep a log of all treatments for each field. Because Transform and Sivanto are absorbed by leaves and eventually metabolized by the plant, reinfestation can occur if large numbers of winged aphids continue to settle in the field.

When inspecting fields for treatment efficacy, note whether any live aphids are winged or wingless, as the former may indicate continued immigration rather than control failure. DO NOT attempt to control sugarcane aphid with contact insecticides that have broad-spectrum activity; these include all pyrethroid and organophosphate materials and combinations thereof. Replicated field trials indicate these materials are not effective, harm beneficial species, and often result in higher aphid numbers than unsprayed control plots.

Sorghum headworm infestations are often present when SCA is observed in a field, since this pest migrates using the same weather events. When choosing an insecticide to control headworms, use products that are less harmful to natural enemies such as Prevathon or Blackhawk, as these have proven compatible with Transform and Sivanto and less selective materials risk flaring the aphids.

The myFields web site: Keeping updated on SCA in Kansas and reporting findings

For ongoing current information on SCA in Kansas, check out the myFields web site often in the coming weeks and months: https://www.myfields.info/pests/sugarcane-aphid

It would be helpful if producers would report findings of SCA in their fields on the myFields web site as soon as the insects are found. Reports of findings are used in developing the map seen in Figures 1.

The reports used to develop each map are, in part, those submitted through the myFields web site from account holders that also have special permissions as “Verified Samplers.” Only reports submitted by these verified samplers get mapped so that we can account for data quality. However, we do encourage any account holder to report their observations on the SCA. Web site administrators can see these reports and can contact the submitter for a confirmation, a great way to get an early detection in new areas. Web site visitors will need to: 1) sign up for an account, 2) log in, 3) to get access to the ‘Scout a Field‘ feature to make reports. The Scout a Field tool is easy, you just map the observation location and select yes or no for SCA presence.

Here is the sign up page: https://www.myfields.info/user/register

Also, if sorghum producers are interested in receiving alerts, which are triggered by new reports submitted by verified samplers, they just need to sign up for a myFields account. Signing up for an account automatically signs them up for SCA alerts, but they can also opt out of them in their user preferences. The alerts include a statewide email notice when SCA is first detected in the state, and then are localized by county as SCA moves into the state. The notices will also contain latest recommendations and contact info for local Extension experts.

 

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.

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