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Extension Entomology

Category: Corn

Corn Update

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Have received several inquiries relative to corn earworms in field corn and there does seem to be a good infestation of them throughout north central Kansas.  100% of the ears we have randomly checked currently are, or have recently been, infested with corn earworm larvae.

Many of these larvae are still relatively small and thus will be feeding for another week or two.  The two most common questions received this week relative to these pests are; 1) Will a rescue treatment work?  The answer – no.  Once the larvae have hatched from eggs deposited on the silks and moved into the husk, they will be protected from contact insecticides.

 

 

2) Will they re-infest these corn ears?  The answer – no.  Field corn will be too tough by the time these larvae finish feeding, pupate in the soil, emerge as adults, mate, oviposit, hatch, and larvae initiate feeding.  But, the adults of this generation will move to soybeans (soybean podworms) and/or sorghum (sorghum headworms) to oviposit and larvae can do considerable damage by feeding on soybeans within the pods and/or directly on the kernels of the heads of sorghum plants.  So, the larvae currently in corn are the “spring board” for the next generations moving into soybeans and sorghum.

 

Japanese Beetles

— by Dr. Jeff Whitworth and Dr. Holly Schwarting

Adult Japanese beetles have been detected around north central Kansas in the last 7-10 days.  These adults may feed on corn, sorghum, and soybean leaves, as far as field crops are concerned, and may cause some “window paneing” much like the leaf feeding of adult corn rootworms.  However, this leaf feeding usually is of little consequence.  In corn, these beetles will be attracted to the silks and, as they can be very veracious feeders, may clip these silks at a pretty good rate.  Fortunately, they are usually localized to small “hot spots” in some fields and thus do not really justify any insecticide application.  These adult Japanese beetles may be active for another couple of weeks, after which only eggs and larvae will be present, and these life stages are not a threat to these crops.

 

Chinch Bugs

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

 

Chinch bugs are numerous and very active throughout north central Kansas, and have been for at least the last month.  The overwintering adults deposited eggs in wheat and oats, as far as our agricultural crops are concerned, and apparently the overwintering survival was relatively high because there have been huge numbers of chinch bugs migrating from these two crops.  Fortunately, most of the corn and sorghum have developed enough to be able to withstand relatively large numbers of chinch bugs as they suck plant nutrients.  Chinch bug populations sampled this past week consisted of 90% nymphs (both the very small reddish orange and larger gray nymphs, both of which have a transverse white stripe).

 

 

These nymphs, for the most part, are around the base of the plants feeding behind the leaf sheaths.  These bugs will feed and develop for approximately another couple of weeks, then mature into adults.  Mating and oviposition then will start another generation of chinch bugs that will continue to feed in corn and/or sorghum fields.  With good growing conditions, most of this feeding will go unnoticed and have little effect on yield.  However, if growing conditions deteriorate but bugs continue feeding, they can cause stalk lodging, which makes harvesting much more difficult.  Spraying for chinch bugs at this stage of crop development is usually not effective as most bugs are relatively inaccessible to insecticides at ground level behind leaf sheaths.

 

Corn Rootworms

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Western corn rootworm development seems to be progressing rapidly.  In north central Kansas, corn plants have been struggling to germinate and grow up until last week.  But, the rootworm larvae in the soil seem to be doing well.  So, these smaller root systems will be more impacted by this rootworm feeding, thus stressing these small plants, and in some cases, even killing them.  Corn rootworm larvae were first detected on 17 May this year.

 

Corn Earworms/Sorghum Headworms/ Soybean Podworms – Helicoverpa zea (Boddie)

— by Dr. Jeff Whitworth, Dr. Holly Schwarting and J.R. Ewing

There has been much activity this year by this particular insect, starting in whorl stage corn and moving to sorghum (both whorl and heading stages) and now in soybeans.

soybean podworm pod

Likewise, there has been some concern that all this activity and resultant insecticide applications have cause an increase in insecticide resistant Helicoverpa zea populations.  So, thanks to great effort on the part of Ethan Kepley, consultant in south east Kansas, who collected all the Helicoverpa zea larvae in one morning from an untreated soybean field, and Steve Freach, FMC, who was kind enough to transport all the larvae directly from the field to our lab, it was possible to conduct a bioassay.  The results of this insecticide bioassay are shown below.

 

Approximately 300 corn earworm/soybean podworm/sorghum headworm, Helicoverpa zea (Boddie), were collected from an untreated soybean field in south east Kansas.  Larvae (mixed sizes but predominantly relatively large instars, which are known to be more difficult to kill quickly) were equally divided into 6 treatments (Table 1).  Larvae were placed individually in small petri dishes that had been coated with the selected insecticide at the rate listed and set aside to dry for 4 hours prior to adding the larvae.  All treatments were individually evaluated 24 hours after the larvae were placed in the petri dishes.  Larvae were evaluated as live: no apparent effect; moribund: larvae very sluggish, little or no movement unless prodded and then only very slow, unnatural movement, and; dead: no movement even when prodded.  From this bioassay there does not appear to be any insecticide resistance to those insecticides and rates utilized (Table 1).

 

Treatment % of Larvae Live % of Larvae Moribund % of Larvae Dead
Hero @ 6 oz/a 0 3.4 96.6
Lorsban @ 2 pts/a 0 8.6 91.4
Mustang Maxx @ 4 oz/a 0 10.9 89.1
Baythroid @ 2.8 oz/a 0 15.5 84.5
Warrior II @ 1.6 oz/a 0 3.4 96.6
Untreated 87.5 9.4 3.1 (parasitized)

 

 

Corn Pests

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Most corn pests have come and gone throughout north central and south central Kansas, with a few exceptions.  Japanese beetle adults have been causing concern in north eastern KS, from about Topeka to the Nebraska border and east to the Missouri border.

Japanese Beetle

Green june beetle

 

These beetles are attracted to green silks and can feed so voraciously that they eat into the husks and damage some of the kernels on the tip.  These populations usually do not occur in such numbers to affect pollination over a large area of many fields.  However, in small areas they can cause concern but are relatively well controlled if a foliar insecticide is justified.  Some adult green June beetles are also feeding on corn silks and/or ‘naked ears’ and have been mistaken for Japanese beetles.  Japanese beetles usually migrate to soybean fields to feed on pollen when fields start pollinating but probably not to the extent that they affect yield.

 

Corn Update

–Dr. Jeff Whitworth and Dr. Holly Schwarting

The first western corn rootworm adults were collected on 17 June from a corn field in north central Kansas.  Development is being completed very quickly, which should be expected based upon the temperatures we have been experiencing.  However, there are still some larvae feeding on roots.

CRW adult 2016

CRW larva mass in root

None of the plants sampled have started tasseling and therefore adults are feeding on leaves.  This typical leaf feeding by adults will not impact yield.

WCRW adult leaf feeding

Adults will probably start feeding on emerging tassels and then shift to silk feeding when silks start emerging.  Remember, corn plants are very efficient pollinators, so as long as a little silk is showing above the husk, the pollen will be successful.  There are still many adult tarnished plant bugs in the corn fields and these are quite commonly confused with adult western corn rootworms.  However, they will NOT clip silks.

tarnished plant bug adult

 

Some leaf feeding is also evident by corn earworms and fall armyworms.  Again, these leaves can look ragged, thus the name ragworm, but will not impact yield.

CEW1

CEW2

Corn and Sorghum Update

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

As wheat senesces, or matures, the chinch bugs are migrating to adjacent corn and/or sorghum fields.  V6+ stage corn can withstand considerable chinch bug feeding but younger plants may be stressed, especially dryland corn.

Chinch bug nymph

chinch bug adults

Seedling sorghum adjacent to wheat will be somewhat protected by insecticide seed treatments if chinch bug populations aren’t overwhelming as they migrate from the wheat, but only for about 28 days, maximum.  Waiting about 2 weeks after wheat harvest to plant sorghum will mitigate chinch bug nymph infestation problems. For more information on chinch bug biology and management, please visit: https://www.bookstore.ksre.ksu.edu/pubs/MF3107.pdf

Corn Update

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

We sampled several corn fields in north central and south central Kansas this past week.  The youngest field was at V2 and the oldest was mostly at V8.  The lady beetles and green lacewings are still relatively plentiful.  I did not find any aphids, or many pests of any kind, really, for these predators to be feeding upon.  All of the lady beetles or lacewings were adults, no larvae.  Hopefully, all these adults of both species will successfully reproduce and therefore there will be numerous individuals present to help control any future pests.  There were however, numerous adult tarnished plant bugs in all fields sampled.  These will not cause any problems to the plants, but later are often confused with western corn rootworm adults when they are found on the corn silks.  Western corn rootworm larval feeding is still proceeding, probably for about another two weeks.  The only real plant damage noted was leaf-feeding by fall armyworm larvae but not even very much (>1%) of that.

May 2016 133

Corn Update

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Lady beetles are unusually common in corn fields right now.  Most corn plants in north central Kanas are from V2-V6.  Checking corn rootworm plots revealed western corn rootworm larvae of various sizes and thus they are starting to feed enough to cause a little root damage.

CRW larvae

CRW larvae_sizes

CRW larva on root

CRW root damage

Seems like, despite the recent cooler, wetter conditions, the rootworm development is about where it usually is at this time.  Most western corn rootworms have completed the larval stage by the 1st of July, thus root damage will be completed later this month as the larvae begin to pupate in south central and north central Kansas.

 

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