Bryan Jensen, Dept. of Entomology and Integrated Pest Management Program, University of Wisconsin-Madison
As people are getting out to monitor corn health and emergence, I thought I would take a few minutes to review some general damage characteristics of seed corn maggot, wireworm and white grub. Frankly, it isn’t easy to diagnose the problem(s) because damage isn’t always “text book”. Decisions need to be made on a range of symptoms form several plants, field histories, degree day accumulations and often times your instinct.
Seed corn maggots have multiple generations each year. This growing season, I think the first generation will be the only generation which will cause emergence issues. First generation peak occurred approximately May 7 in southern Wisconsin. Obviously, the peak will be sometime after that as you move north. Knowing the timing of this flight period may help with seed corn maggot diagnosis because damage should be concentrated on fields planted within a relatively narrow time frame for your area. Not a wide range of planting dates. Field histories may help with diagnosis as well. The adult is mobile and tend to seek out fields with green and/or livestock manure applied as well as fields which are recently tilled prior to planting to lay eggs. The immature maggot will feed on either the germinating corn seed (and soybean) or the emerging shoot. All damage is below ground. Above ground symptoms include skips in the row and cotyledons with small holes. Soybean injury may include “snakeheads”, feeding scars on the cotyledons and holes in the unifoliate leaves. Dig up areas with poor germination to verify that seed was planted. Expect to see a range of these symptoms. Not just one or the other. Also, seed corn maggot injury is usually random within a field. Assuming the entire field was treated the same. If not? Use that information to help confirm your diagnosis.
True white grubs are a complex of several species with similar damage symptoms and a three-year life cycles. One year as the adult June (or May) beetle and two growing seasons as the immature grubs. White grubs do not feed on seed but will feed on corn roots and perhaps the underground portion of the stalk. Above ground symptoms are stunted plants which commonly exhibit nutrient deficiency symptoms because of the root feeding. Also, newly emerged leaves may be wilted as a result of below ground feeding. White grub feeding is usually clumped within a field. These areas may be where there were previous grass weed escapes or along field edges where females dropped in from surrounding food sources to lay eggs. Grubs can usually be found in the soil around damaged plants.
Wireworm larvae will feed on ungerminated seed (similar to seed corn maggot) but also on the underground portion of the stalk. Above ground symptoms will depend where the larvae are feeding. If feeding is at the growing point the whorl leaves will look wilted. If feeding is above the growing point there will be holes in the leaves. These foliar symptoms can be confused with hop vine borer, common stalk borer, black cutworm, sandhill cutworm and bill bug. Verify wireworm feeding by digging up seed and or injured plant. Unfortunately, wireworms will move down in the soil profile if conditions are dry and/or summer temperatures return.
Expect to see a range of symptoms from wireworm feeding that include skips in the row, holes in the leaves and wilted whorl leaves. Feeding will also be spotty. Areas with a greater chance of injury include where there was previous grass weed escapes. Rotation may also help determining if wireworms are to blame. Corn planted after alfalfa or sod are likely fields to have injury. Because of wireworm’s multiple year life cycle, damage may be as severe in second or third year corn.
For more background information on these and other insect pests please review our Field Crops Scouting Training Manual.