Drought Shifts Pest Mix

Mite damage on corn.
Mite damage on corn.

Conventional wisdom says most tiny critters should have perished under last season’s parched conditions — but some actually thrived in the heat. Christian Krupke, Extension entomologist with Purdue University, would call 2012 “the year of the mites� thanks to the drought, though yield damage from both factors would be difficult to differentiate.

Although mites are present in fields every year, they moved into crops sooner and fared much better than usual, in June and July, says Erin Hodgson, Extension entomologist at Iowa State University. “This early migration made it possible to produce several generations in soybeans and corn,� she says. “In some cases, spider mite populations caused leaf discoloration, stunting and premature leaf drop.�

The drought spurred unusual spider mite infestations in Ohio as well. The mites were prevalent on soybeans but also became a problem in numerous corn fields in northern Ohio, just south of Lake Erie, with a few reports from other areas of the state.

“We expected and predicted them in soybeans because of the drought, but the infestation on corn was totally unexpected because we really had never seen them causing significant problems in the Midwest,� says Ron Hammond, Extension entomologist, The Ohio State University (OSU).

For mite problems on both soybeans and corn, Krupke says control options are limited because there is no pre-harvest interval registration data for soybeans, although there are many effective products labeled for specialty crops. Hammond reports that materials with bifenthrin appeared to work well, especially Hero, which is bifenthrin plus Mustang Max. On soybeans, where Ohio saw the most problem, bifenthrin is a relatively new product.

Jeff Bradshaw, Extension specialist at the University of Nebraska-Lincoln, says there are a few newer products available and labeled for use on corn against spider mites. The most promising ones (Onager and Zeal) cause the mites to lay sterile eggs or affect mite growth — and so do not suppress beneficial insects. Be aware, though, that these products are slower acting than more commonly used tools, such as chlorpyrifos.

Bradshaw anticipates problems with spider mites in 2013, and Hodgson expects the same if crops are drought stressed again, especially during stand establishment and early vegetative growth. She recommends proactive scouting. The mites can get out of control with persistent hot and dry weather, so they should be monitored closely during these optimal conditions.

Rootworm Issues

Overall insect pressure in corn and soybeans in the Corn Belt was somewhat light last year, says Michael Gray, professor and assistant dean at the University of Illinois. But Western corn rootworm adults did emerge a full month early, causing problems. The drought created even greater yield losses when insect damage, such as root feeding by rootworm larvae, was present.

“The other insect story was the continuing development of resistance to Bt corn with the Cry3Bb1 protein in some areas of Illinois and the Midwest,� says Gray. The problem is especially prevalent in fields of continuous corn.

Indeed, Iowa researchers have confirmed resistant populations since 2009, and problem fields have all been in continuous corn for at least three years with the same trait (specifically Cry3Bb1 and mCry3A), says Hodgson.

Gray has a number of suggestions on how to mitigate the risk of continuing or encouraging these product failures. First would be for a grower to diversity his rotation with soybeans — get out of corn on corn. Second is to use a non-Bt hybrid along with a soil insecticide. Third is to use a pyramided Bt hybrid that contains another Cry protein such as the binary protein Cry34/35Ab1, as well as the Cry3Bb1. The problem here, though, is the amount of selection pressure put on the Cry34/35Ab1, especially since pyramided Bt hybrids allow for a smaller refuge. “It’s a pivotal protein that we’d hate to lose,� says Gray. This third approach could compromise the long-term durability of this tool as well.

Indiana has not seen evidence of Bt resistance so far, and Krupke would encourage growers there to keep doing what they’re doing — rotating crops and products and utilizing refuges. “If there’s no rootworm pressure don’t use Bt products that target rootworms, so we can preserve this technology,� he says.

Newer Players

Last season was the first time Ohio soybean growers started seeing economic levels of stink bugs — with a complex including green stink bugs, red shouldered stink bugs, and in some fields, even a few brown marmorated stink bugs (though still at low levels). “Though not specifically related to the drought, we hypothesize that they are building up in Ohio because of generally warmer conditions brought about by climate change,� says OSU’s Hammond.

Another first for Ohio was the identification of the Asiatic garden beetle grub as the culprit in significant stand losses in north central and northwest Ohio corn following soybean. Though these stand losses have been occurring for the past four to five years, this was the first time this species was seen and correctly identified, Hammond notes.

Usually considered a minor turf pest, the Asiatic garden beetle grub was introduced on the East Coast in the 1920s and has continued its march westward ever since. In 2006 it was found in northeast Indiana and southwest Michigan corn, always in sandier soils and following soybeans.

Ohio also saw low corn rootworm populations, so Bt efficacy was not an issue there. But growers and researchers did discover that transgenic corn with both borer and rootworm protection were not effective against the new beetle grub or the spider mites – and seed treatments didn’t help against the grubs either.

Aphids posed little threat in the states that shared input with CropLife. The news was not surprising as the pests don’t like hot, dry conditions. Scouts in Ohio and elsewhere have found no aphids in buckthorn this past fall, which is unusual after a low year. “We are not offering a prediction this year for aphids, which is the first time since the aphid showed up that we cannot,� says Hammond. Retailers and growers in other Midwest states may want to see if extension staff found similar low fall counts.

Disease Roundup

Dry conditions played a huge role in the incidence and spectrum of diseases growers saw in Illinois, says Carl Bradley, Extension specialist with the University of Illinois. Two problems that appeared: Charcoal rot in both corn and soybean and Aspergillus ear rot in corn.

Iowa fields experienced elevated levels of soybean cyst nematode and a newly identified virus called soybean vein necrosis virus (SVNV) and to a lesser degree, charcoal rot, reports Daren Mueller, Extension program specialist at Iowa State University. Corn in certain parts of the state had some trouble with seedling diseases such as Pythium and Fusarium, southern rust, Fusarium stalk rot and some mycotoxins — aflatoxin.

“Drought did play a role by minimizing some of the more common diseases, but overall even the ‘drought diseases’ were lower than expected,� says Mueller. He feels solid genetics can be credited in great part for the resiliency of corn and soybean production in a year like 2012.

He was surprised, however, by the level of SCN reproduction on certain resistant varieties, and it’s a problem “we should be concerned about again in another dry year and keep an eye on.� Under dry conditions, growers should also be on the lookout for charcoal rot on both corn and soybeans, as well as aflatoxin.

In fact, Mueller was also surprised Iowa did not see more aflatoxin in 2012, saying, “somehow we dodged a bullet.�

Indiana fields showed some Aspergillus ear rot as well as the new soybean vein necrosis virus, reports Kiersten Wise, plant disease specialist, Purdue University. Symptoms include light green patches or mottled green and brown speckled areas near veins. These symptoms appear during mid to late summer and as the season progresses, affected tissue may die — resulting in a scorched look on hard-hit plants. She points out this damage may actually be confused with drought stress or ACCase herbicide injury.

In fact, these symptoms have been spotted in mid-South and Midwest fields for several years, but the new disease wasn’t officially identified until 2009. So far Wise does not recommend any changes in production practices, but she will be watching SVNV and assessing its impact.

Bradley in Illinois will be doing the same. “We don’t really know yet how much this disease affects soybean yield, but it definitely was prevalent here in 2012,� he says.

Since weather impacts plant pathogens and resulting diseases so much, it’s impossible to make any disease predictions, says Bradley. But one thing he cautions growers and retailers to be on the lookout for is stobilurin fungicide-resistant strains of Cercospora sojina, the pathogen that causes frogeye leafspot in soybeans. His lab documented several occurrences of resistant strains in fields from several states and in new areas last year, including new areas of Illinois.

Seedling diseases made an appearance in corn and soybean in some locations, and Bradley has found that fungicide seed treatments seem to work “pretty well overall.� They can be particularly helpful when planting into cooler soils.

Opinion Corner

CropLife asked pest specialists if they thought chemical and seed companies and retail firms were doing enough to help growers. Most were satisfied, but a few raised red flags.

Companies are doing too much, says OSU’s Hammond. “There is a lot of marketing going on for seed treatments and soil insecticides — and over-the-top marketing of transgenic corn — plus adding foliar insecticides to fungicides or herbicide sprays when not needed,� he explains. The thinking behind the sales is that a grower is already going across the field anyway, and these products are cheap insurance.

In fact, Hammond appreciates a term his colleague Michael Gray in Illinois has used recently: “There is way too much Insurance Pest Management (IPM) and not enough Integrated Pest Management (true IPM),� he says.

Nebraska’s Bradshaw believes company representatives should consider attending regional workshops held by universities to help them learn, share and coordinate knowledge with researchers on the best practices for pest suppression.

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