• Early season insect pests such as wireworms, seedcorn maggots and white grubs can damage soybean seeds and seedlings.
• Soybean seeds or cotyledons may be attacked by seedcorn maggots when cool, moist conditions prevail, and germination and early growth are slowed.
• When peak emergence of overwintering bean leaf beetle adults coincides with soybean emergence/ early growth, heavy feeding on soybean seedlings may result.

SEEDCORN MAGGOTS (SCM):

Seed corn maggot damage has been reported in some soybean fields in Illinois and adult flies have been observed in areas of Iowa. Soybean seeds or cotyledons may be attacked by SCM when cool, moist conditions prevail, and germination and early growth are slowed. SCM tends to be more of a problem in soils with manure, green vegetation or reduced tillage. The maggot (larva) is white- pale yellow, legless with no heads (Image 1). They burrow into seeds preventing them from emerging and even when damaged seedlings do emerge they usually have poor development. The flies, which are the adult stage of the maggots, are attracted to decaying organic matter, manured and freshly tilled soils.They lay eggs at or just below the soil surface and the newly hatched maggots immediately seek out organic matter or seeds.It is uncommon for the maggots to attack germinating seed in well drained or low organic matter soils where manure has not been applied. If damage is suspected after plant emergence, dig 2 linear foot of row in five suspicious areas in the field and examine the seeds for damage.Unfortunately, no rescue insecticides are available for SCM and if the stand damage is severe, replanting is the only option. Planting in conditions that favor timely soybean emergence and early growth or delayed planting (which will be the case in some areas this year) will help avoid damage by SCM. Insecticide seed treatments are available to protect the soybean seed and seedlings from SCM damage and are recommended when planting in high risk conditions.

Image 1 (below). Soybean seed damaged by seedcorn maggots. Notice the white maggots inside the seed.

BEAN LEAF BEETLE (BLB):

BLB adults are ¼” long with color variations from yellow, tan or red. Some will have two black spots on each wing cover but others will have no spots at all. The one distinguishing characteristic of BLB adults is a black triangular mark at the top of their wing covers (Image 2). BLB’s have two generations per year in IA, IL and IN (summer and early fall). The first beetles seen early in the spring are the overwintering adults which, when the timing is right, can cause damage to emerging soybean seedlings (Image 3). A recent report by Iowa State has predicted very high mortality rates (73-99%) for these overwintering beetles during the 2018-2019 winter across the state. However areas with milder winters may still have significant numbers of BLB’s adults and scouting is the only way to determine the potential for damage. Early in the season, walk newly emerged soybean fields looking for the presence of beetles or feeding damage. If damage is noted, evaluate 10 plants in at least 5 areas of the field and determine the average percent defoliation. Also, record the number of beetles found in each of those areas to get an estimate of the number of beetles per plant. If 3-5 beetles are found per plant from VE-VC or 30% defoliation is noted from V1-V7, a treatment is recommended.

Image 2 (below): Color variation of bean leaf beetles. (Note the black triangular spot behind thorax) (Source: GROWMARK, Inc.)

Image 3 (below): Early bean leaf beetle damage on soybean seedlings. (Source: GROWMARK, Inc.)

Diagnosing early season soybean damage can be somewhat challenging. Early BLB damage can be mistaken for seed corn maggot or slug damage. Seed corn maggots can leave tunnels along the cotyledons. If you dig the damaged seedlings, you may find some mining along the stem. BLB’s can also feed on cotyledons but they will start by gently scraping off the surface leaving circular spots of eaten tissue. Under heavy infestations these circular feeding signs/holes will coalesce and may resemble slug damage. If BLB’s are present you might see the adults moving while scouting the field. Slugs will be more “aggressive” in their feeding potentially skeletonizing the leaves while leaving slime trails on the leaf surface. Neonicotinoid seed treatments can protect soybeans from seed corn maggot and BLB damage but slugs will be unaffected by these seed treatments.