Cool soils promote seed/seedling pests

This week, several soil-dwelling pests have been reported throughout the state. Typically early-season seed/seeding pests can be problematic with slowly developing plants growing in cool soils. Examples of these pests include wireworms, millipedes, slugs and corn cornseed maggot. 

Millipedes are generally beneficial to have in soil because they break down organic matter. However they can become pests when planting into cool, wet soils because they feed on seeds and seedlings. They are especially prevalent in reduced tillage fields of corn and soybean. Sometimes they can chew  through the soybean hypocotyl and kill the plant. There are no registered pesticides for millipede control. In some severe cases, grower may need to replant in order to get a desired plant population. 

 Millipedes are insect relatives with two pairs of legs per segment. 

Photo credit Marlin E. Rice.

Seedcorn maggots have been reported in southeastern Iowa.. This isn’t a big surprise considering the cool, moist soil conditions this year. Like millipedes, the maggots typically feed on organic matter. But they will feed on corn, soybean and many horticultural plants The maggots feed on seed and often just leave the shell behind. There are no rescue foliar treatments available for seed corn maggot control.

Seedcorn maggot are immature flies that can destroy seeds/seedlings.

Photo credit Purdue Extension Service.

Whenever feasible for persistently infested fields, avoid planting in cool, wet soils. In some cases, insecticidal seed treatments can help suppress feeding, but will not provide complete control. Shallow planting and late planting will encourage quick germination to avoid seed/seedling feeding. 

holiday weekend

Looks like a rainy weekend here in Iowa. Saturated soils are not ideal for insect survival, especially as corn rootworm eggs are close to hatching. Stay tuned for predicted CRW egg hatch dates coming up soon.

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Have you seen corn flea beetle?

Recently, Chad Patterson from the Webster City area reported finding corn flea beetles feeding on seedling corn in central Iowa. Corn flea beetles are small, shiny and black, and about 1/16 inches long (Photo by Mike Quinn, TexasEnto.net). 

The adults feed by scraping parallel to the veins along upper or lower leaf surface. The scratches create a “windowpane” effect where the tissue looks bleached or silvery. Sometimes the feeding scars resemble foliar diseases such as gray leaf spot or Northern corn leaf blight (Photo by University of Illinois-Urbana archives, www.ipmimages.org). 

The feeding rarely causes economic damage to established corn, but can sometimes kill corn seedlings with high beetle populations. Growers should be aware that corn leaf beetle can vector a bacterium called Pantoea stewartii, that causes Stewart’s wilt bacterial disease. Susceptible varieties of sweet corn hybrids and seed corn inbreds may be infected during the seedling stage but not show any symptoms until later in the summer. Infected plants will be covered in leaf lesions, not produce an ear, or die. 

Scouting for corn flea beetle as corn emerges is recommended. Special attention to scouting and feeding damage should be made for seed corn and susceptible varieties. Adults move into fields from grassy overwintering areas, and infestations typically start at field edges. Begin looking at 20 plants at five locations in each field and determine the number of adults per plant. Use the following thresholds for rescue treatments in corn:

• Field corn--prior to stage V5, 50 percent of plants with severe feeding injury and five or more beetles per plant.
• Seed corn--on susceptible inbreds, 10 percent of the plants with severe feeding injury and two or more beetles per plant.

There are several management considerations for corn flea beetle in the future including seed selection and cultural practices. Incorporating host plant resistant hybrids will help prevent feeding and disease transmission. Systemic seed treatments provide early season control of corn flea beetle and Stewart’s wilt in corn. Keep fields and surrounding areas weed-free to minimize overwintering habitat and food sources for larvae and adults. For susceptible varieties, plant later to avoid the spring migration of adults.

Finding soybean aphids on buckthorn

Soybean aphids overwinter as eggs on buckthorn and typically hatch right around bud burst. There are several wingless generations on buckthorn before they migrate to soybean each year. Every spring, Dr. Dave Voegtlin (University of Illinois) tours around the midwest looking for soybean aphids on buckthorn. The suction trapping network collected very few aphids last summer and fall, and so he wasn't expecting to find many aphids during his inspections.

General locations he visits every year include Rome City area in N.E. Indiana, Toledo area, Irish Hills in S. E. Michigan, Kellogg Forest near Battle Creek, Michigan, Calumet area S. of Chicago, Joliet, IL and Quad Cities area.  All these have abundant buckthorn available for overwintering. He found active aphid colonies at about half of these locations. He was surprised to see aphids because of the cool weather in the midwest so far. This is one of his pictures from a couple years ago, showing a spring colony on buckthorn being tended by aphids.

It won't be long before migratory aphids are moving to soybean. Even though it was an extremely low aphid year in 2010, we don't know what will happen this year if conditions are conducive to their growth potential. I'll keep you posted as regional information is shared.

Black cutworms are here!

For the last two days, I have received lots of calls and emails about significant black cutworm damage in southern and central Iowa. The predicted cutting dates for black cutworm larvae are pretty close to our degree-day map published on 5 May. Mark Carlton, ISU Field Agronomist in SW Iowa, had a client with fields approaching 7% infestation. That density is clearly above a treatment threshold and was treated with an insecticide to protect yield. Mark Licht, ISU Field Agronomist in central Iowa, suspected black cutworm damage in this young corn plant (and I agree).

It is important to scout for larvae and damage through the V5 stage, especially after weeds or nearby grasses have been killed. Remember, if larvae found in the field are smaller than ¾ inch, then a threshold of 2 to 3 percent wilted or cut plants indicates an insecticide application is warranted.

Soybean seeds are up!

I stopped by ISU FEEL (Field Extension Education Laboratory) today before the rain. I was checking out a couple of my planned field tour demos later this summer. I was surprised to see some soybean already emerging, with a couple already putting out the unifoliate leaves. It was great to see these plots and a few other projects progressing. My lab is flagging out plots up at the ISU Northeast Farm today with hopes of planting our soybean aphid efficacy evaluation next week.

Changing Bt corn options

Corn seed genetics is an important industry, and is moving ahead at a fast rate. Hybrid options are changing every year, and sometimes the choices can seem overwhelming. Along with the many considerations a growers must make for selecting seed, pest protection is at the top of the list. There has been a huge adoption for PIP's (plant incorporated protectants) in field crops, such as Bt to control above/below ground pests. There are many PIPs developed that target persistent pests like European corn borer and corn rootworms. Corn seed can have one protein or a combination to control several species. Some newly-approved changes in refuge requirements from the EPA have further complicated PIP options for some people.

Chris DiFonzo (Michigan State University) and Eileen Cullen (University of Wisconsin) have created a factsheet on available traits and refuge requirements. I found it very useful and thought to pass it along. Also, the National Corn Growers Association have developed a refuge calculator that can be customized for individual farms.

Lastly, there has been quite a bit of discussion about the sustainability of "refuge in a bag." Mike Gray (University of Illinois) summarizes a recent article that says this type of production is actually moving away from IPM because of the constant exposure of Bt to target pests. Christian Krupke (Purdue University) agrees that widespread adoption of Bt with a decreasing refuge can be risky for long term management.

It is important to understand the options for corn seed selection in the future, which includes knowing your actual pest pressure, overall hybrid performance, and seed costs. Also remember incorporating cultural options (e.g., crop rotation, weed control) can help discourage insects from being successful.