The Science

Corn Rootworm

Western and northern corn rootworms are able to overwinter across all of the corn-growing region, so their development is chiefly driven by accumulated four inch depth soil temperatures where eggs are laid during the previous growing season.  Fields primarily growing continuous corn are at the greatest risk of experiencing western or northern corn rootworm damage.  Once enough soil heat units have accumulated, larvae hatch and begin feeding on corn roots (if present).  Soils that are extremely wet or those that are more abrasive (in addition to not having any corn roots present at the same time as hatch) can limit corn rootworm development and subsequent damage.  Therefore, soils that are drier and warm more quickly in the spring stand the greatest risk of seeing increased corn rootworm numbers and earlier in the season (such as in 2012).  Later in the summer, pupation and adult emergence occurs, and a secondary threat to corn (in addition to other plants such as soybeans and alfalfa) occurs during silking.  Corn rootworm adults can clip silks and reduce pollen levels that may lead to poor pollination.  Migration events are primarily focused locally but can be intense especially between fields that show differing growth stages.  Scouting, in addition to following forecasts, provide the best method for managing this particular insect.

Source:  University of Illinois Integrated Pest Management

Corn Earworm

Accurately forecasting Corn Earworm migration is possible. It involves the daily monitoring of insect traps throughout the country and is based on the hypothesis that increases in insect trap counts correspond with increased migration risks. Weather is a critical factor in determining if the migration may or may not occur—with some uncertainty, summer weather patterns can be predicted in advance of occurrence. When a southern source region of Corn Earworm is present, insects may migrate northwards based on a number of factors:

  • Climatology (crop stage and generational aspects)
  • Strong nighttime low level jet stream
  • High and low pressure cells creating an “insect pump” effect
  • Precipitation or downward moving air serving as a “drop zone”

Source: Vegetable Insect Management, Chapter 1A (How Weather and Climate Impact Your Pest Management Decisions), pages 23-29. Authors: Mike Sandstrom (lead), co-authors David Changnon, Brian R. Flood. Image is on page 26 and was created by Tracy Flood.

Western Bean Cutworm

Western bean cutworm development is driven primarily by air temperature heat unit accumulation as this insect can overwinter successfully across the corn-growing region.  In a typical year, western bean cutworm emergence is first observed in the Plains and western Midwest where heat units accumulate more rapidly.  Emergence and subsequent flights then spread east and northeast into the upper Mississippi River valley, Great Lakes region, and southern/southeast Canada as heat unit accumulations increase in fields in these regions.  Migrations can occur especially during peak flight and when primarily strong west to east wind events occur following cold frontal passages.  This particular insect, once only a threat to the Plains states, has now become a widespread nuisance in fields across much of the Midwest, Great Lakes region, southern and southeastern Canada, and portions of the northeastern United States as it has greatly expanded its range in the last decade.

Source:  University of Illinois Integrated Pest Management

Soybean Aphid

Soybean aphids lay eggs on their primary host, buckthorn trees, in the autumn of the year. These eggs are able to withstand very cold temperatures (≥ -29°F [-34°C]) allowing them to overwinter across much of the upper Midwest and Great Lakes region (except in the coldest winters). In the spring, soybean aphid eggs hatch once accumulated heat units have reached a certain level. Newly-hatched soybean aphids can be susceptible to sub-freezing temperatures immediately after hatch. If a widespread hard or killing freeze occurs immediately after egg hatch, soybean aphid populations may be locally limited early in the growing season. Once soybean is planted and emerged, soybean aphids migrate from buckthorn to soybean. Soybean is not susceptible to damage from soybean aphids until the reproductive stages of growth.

The weather between planting and the reproductive stages is critical in determining a local potential soybean aphid population. Soybean aphids prefer temperatures between 75-85°F (24-29°C), light to moderate winds, non-beating rainfall, and soybeans that are considered to be in good to excellent condition. The presence of few natural predators, including multicolored Asian ladybeetles, parasitic wasps, and lacewings, will also keep soybean aphids populations high. Regeneration can be rapid (in a matter of a few days) when environmental conditions are favorable and populations can rapidly increase. Scouting is important from the time of crop emergence through the R6 (full seed) growth stage, but is most critical from just before R1 (beginning bloom) through 5.5 (seed development) when yield loss is most likely to occur. A soybean aphid plant population generally at or above 250 aphids per plant, and increasing, and in the critical R1-R5.5 growth stage suggests the possible need to spray the field with an insecticide. Spraying should also be considered if soybean aphid plant populations are near threshold, the weather forecast indicates that temperatures may be favorable for aphid development over the next several days to week, or natural predators are either not present or, if present, are no longer effective at keeping populations below economic threshold and soybean aphid populations appear to be rising. Soybean aphid migration is possible but is generally observed locally. Strong cold fronts and associated high wind speeds may allow longer distance flights, especially when soybean aphid populations are high. Once a field is past growth stage R6, spraying is no longer needed as yields will not be affected at a level to justify treatment. Soybean aphid populations can be quite variable, so growers are urged to scout their own fields and not rely simply on what a neighbor or the local report is saying to decide whether or not to spray.

Source: Bahlai, C.A., J.A. Welsman, A.W. Sciiaafsma, and M.K. Sears (2007). Development of Soybean Aphid (Homoptera: phididae) on Its Primary Overwintering Host, Rhamnus cathartica. Environmental Entomology, Vol, 36, No. 5, pp. 998-1006.

Iowa State University Soybean Aphid Field Guide (2010)