Other factors also enter in to potential for economic damage

Generally, the lack of extremely cold weather this winter will favor the survival of many insects that overwinter in Nebraska. However, this information alone is not enough to predict whether individual fields will experience economic loss or to make a recommendation to use planting time controls. We will review some of the likely effects of winter weather on several Nebraska field crop insect pests and discuss what other factors may influence the occurrence of economic insect problems on your farm.

First, not all Nebraska field crop insect pests overwinter in Nebraska. Several overwinter in states south of Nebraska and migrate up each spring. Some of these insects include black cutworm, potato leafhopper, corn earworm, fall armyworm, green cloverworm, and painted lady (thistle caterpillar).

For those insects that do overwinter in Nebraska, the generally mild winter temperatures suggest that there will be good survival of those that reached the end of the growing season.

Factors affecting insect survival

Several factors will influence whether individual fields will experience high insect damage. One factor is the weather from now until the growing season. Temperature and rainfall in the spring, as well as overwintering conditions, influence insect survival.

Rainy spring weather may encourage development of certain parasitic fungi or other microorganisms that may attack insects and kill them. Some of the insects that may be susceptible to these diseases include clover leaf weevil, alfalfa weevil, European corn borers and chinch bugs. Cool, wet weather as grasshopper nymphs emerge may greatly reduce their survival by limiting their ability to feed. Saturated soil conditions at the time of rootworm egg hatch may result in the newly hatched rootworm larvae drowning in low portions of the field or areas with finer textured soils due to lack of air in the soil. Spring rainfall also may reduce survival of western bean cutworm larvae in the soil, particularly in finer textured soils.

Practices affecting insect survival

Other factors that can influence whether we see economic damage from overwintered insects include agronomic practices used in 2006. Planting dates and weed control can influence whether individual fields experience insect damage.

In soybeans, bean leaf beetles will be attracted to the earliest emerging soybeans in an area. Avoiding early planting of beans, relative to others in your area, will reduce the likelihood of injury from bean leaf beetles on seedling soybeans, even if overwinter survival was above normal. If early planting is necessary, make sure to scout those fields starting at crop emergence and use economic thresholds to determine if early season treatment is needed. You also might consider the use of an insecticidal seed treatment such as Cruiser or Gaucho, which will help protect seedling soybeans from early season feeding by bean leaf beetles.

In corn, control of winter annual weeds can reduce a field’s attractiveness to black cutworm moths as they fly into Nebraska from further south in April and May. Again, scouting for early signs of injury starting at crop emergence can allow you to respond to economic injury with a post-emergence insecticide application if needed.

Post-emergence control of weeds in corn may force common stalk borer larvae out of alternate hosts such as grassy weeds, ragweed or cocklebur, and encourage their movement into seedling corn.

Certain situations are more likely to be associated with economic losses, and a planting time control may be useful in 2006. If you had high populations of larval or adult rootworms in 2005, it is likely there will be an economic problem in 2006 if you plant that field back to corn. If corn were silking later in one field than in surrounding fields, this would encourage rootworm beetles to move into that field and lay eggs. In these situations, crop rotation, use of a Bt corn with activity against rootworms, or the full-labelled rate of a granular or liquid insecticide would be good options to provide rootworm control. Remember that the insecticide seed treatments labeled for corn rootworm control (Poncho 1250 and Cruiser) are suggested for use against low to moderate populations of rootworms. UNL research and field experience indicate that these products may not provide adequate root protection against high populations of rootworms.

Bob Wright
Extension Entomologist, Lincoln
Tom Hunt
Extension Entomologist
Haskell Ag Lab, NEREC
Keith Jarvi
Integrated Pest Management, NEREC