As corn and soybean planting are underway soon, a view ahead on the calendar to May is warranted. What early season insect pests are County Agents, growers and consultants likely to encounter in seedling soybeans and corn fields?

 

Although occasional and site-specific, one early season field crop insect pest to be aware of is the seedcorn maggot. Although seedcorn maggot can be a threat to corn, damage is not usually as severe in corn as when found on soybeans, peas and succulent beans.

 

Seedcorn maggot overwinters as a pupa (e.g., cocoon-like resting stage) in the soil. This insect is a fly (Insect Order: Diptera), and adult emergence from overwintering pupae will occur typically from early to mid-May in Wisconsin. Adults mate within 2-3 days of emergence. Females prefer to lay eggs in soils containing high organic matter and may be seen flying in large numbers over recently-tilled fields. Larval feeding, development and pupation all occur below ground. Larvae are yellowish-white, 1/5-inch long when fully grown, legless and wedge shaped. Pupa are brown, 1/5-inch length and cylindrical. Adults resemble miniature houseflies; dark grey, 1/5-inch long and wings held overlapped on top of the body when at rest. Seedcorn maggot flies are smaller than cabbage and onion maggot flies, with which they may be confused.

 

Female flies are more attracted to lay eggs in fields where application of livestock manure and/or incorporation of vegetation have recently occurred (decaying weeds and newly tilled crop residue). Populations are generally higher following incorporation of a legume crop into the soil than when a grass crop is tilled in.

 

Conditions that delay emergence, such as crusted, dry topsoil or cold, wet weather will increase the likelihood of damage because seedlings take longer to emerge allowing more time during which seed is susceptible below ground to seedcorn maggot feeding injury.

 

On soybeans, maggots burrow into the germinating seed. If the seed germinates, maggots feed in and on the cotyledons. As a result soybean seed leaves may have brown feeding scars when they appear above ground. Shoots may also emerge with cotyledons absent, “snakehead” seedlings. On corn, all parts of the sprouting seed can be fed upon by seedcorn maggot larvae resulting in poor germination rates. When it occurs (on soybeans or corn), seedcorn maggot damage is usually more widespread within a field than “spotty”.

 

Cultural Control: Because adult females are attracted to lay eggs in fields with decaying organic matter, fields where animal or green manure (live cover crops) have been recently applied or incorporated in the spring are more susceptible to this early season pest. Planting procedures that promote fast germination and seedling growth will help reduce chances of seedcorn maggot damage. For example, planting seeds as shallow as feasible, or delayed planting date in high organic matter fields situated in low, poorly drained areas or under cold, wet spring conditions to allow soil to dry and warm up.

 

Seedcorn maggot typically has 3 generations per year, with the first generation causing the most damage. With extra monitoring effort and knowledge of seedcorn maggot life cycle and identification, it is possible to minimize damage by planting the crop during “fly free” periods that occur between generations of flies. One option is to set out 3 to 4 yellow dishpans of soapy water at 100-foot intervals along the field edge in April, emptying dishpans every 5 days and refilling with soapy water. Keeping a record of the number of seedcorn maggot flies caught up to the early to mid-May period should indicate when populations have peaked and entered a subsequent decline. Another option is to forecast the appearance of generations by accumulating degree days after the ground has thawed. For seedcorn maggot, degree days are accumulated each day using the formula ((minimum temperature + maximum temperature)/2)-39. Peak emergence of the first three generations of adult flies will occur after totals of 200, 600 and 1000 degree days, respectively, have been reached. Planting at the tail end of first generation, or in between first and second generation flights will help reduce damage.

 

Chemical Control: There are no thresholds established because seedcorn maggot damage cannot be detected until it is too late to take control action. Therefore, control practices are preventative and applied as a planter box seed treatment (e.g., Kernel Guard Supreme, a.i., permethrin), or commercial seed treatment (Cruiser, a.i., thiamethoxam, Gaucho, a.i., imidacloprid, Poncho, a.i. clothianidin). Refer to seed applied insecticide labels for a full range of products available on corn and soybeans. More information is available through University of Wisconsin Extension Publication A3646, Pest Management in Wisconsin Field Crops. A3646 can be downloaded from the UW Extension Cooperative Extension Publications Website at http://cecommerce.uwex.edu/

From the home page, click on “Agriculture”, then select “Crops and Forages” and select Pest Management in Wisconsin Field Crops 2006 (A3646) from the list.