July 24, 2009

Decision-Making for Soybean Defoliating Insects

There have been numerous reports of grasshopper populations building along field edges and moving into corn and soybean fields, and bean leaf beetle populations are building in some areas (mainly in the south). In the July 10, 2009 CropWatch

was discussed. As we see bean leaf beetle populations build in some soybean fields, growers will likely begin to notice soybean defoliation.

Figure 1. Graphic representations of various levels of soybean leaf defoliation.

Defoliation is the most common type of insect injury observed by Nebraska soybean producers and can occur from emergence to harvest. A complex of insects defoliate soybeans in Nebraska including bean leaf beetle, imported longhorned weevil, grasshopper, woollybear caterpillar, thistle caterpillar, green cloverworm, and a few others. Rarely does any single species reach population levels that defoliate soybean enough to cause economic damage, but the combined injury of two or more defoliating insects can result in economic damage.

We observed this in 1997 when the combined defoliation from grasshoppers and bean leaf beetles reached 50% in reproductive stage soybean near Mead. When this occurs we must consult multiple-species recommendations. These can be multiple-species economic threshold tables, or more general "catch-all" defoliation level recommendations. Because our multiple-species economic threshold tables do not cover all of the different species of insect defoliators we have been seeing this year, we would like to discuss the more general soybean defoliation thresholds.

Insect Defoliation and Yield Loss

Soybean plants have a great capacity to compensate for defoliation by insects. Research over the last 15 years has established that the key factor driving yield losses from defoliating insects is the degree that defoliation reduces light interception of the soybean canopy. Soybean can lose tremendous leaf area without yield loss if the remaining leaves are still intercepting at least 90% of the incident light. So, in making pest management decisions about defoliating insects, a crucial consideration is the size of the remaining soybean canopy. Small canopies cannot tolerate as much leaf loss as large canopies. Another consideration is when defoliation occurs. Unless severe, defoliation in vegetative stages usually doesn't cause yield loss. Reproductive stages are more sensitive. A final factor is growing conditions. When environmental conditions are very favorable for soybean development(e.g. adequate water), plants have a greater capacity for regrowth and compensation.

General guidelines can be used for defoliating insects that lack species-specific thresholds or when two or more different defoliating species are present. In vegetative (pre-flowering) stages consider treatment if the insects are present and feeding and defoliation will reach 40%. In pod-forming or pod-filling stages consider treatment if the insects are present and defoliation will reach 20%. These percentages can vary 5% to 10% according to the stage or type of insect(s) present, environmental conditions, the specific stage of the soybean, and the size and condition of the canopy. Experience will have to be your guide when using these thresholds.

It should be pointed out that defoliation is notoriously difficult to estimate. It is almost always overestimated. This is because the injury is so dramatic and all parts of the canopy often are not considered when making defoliation estimates. Some insect species primarily feed in the upper part of the canopy (e.g. bean leaf beetle), and some feed lower in the canopy. Different portions of the canopy will suffer different levels of injury. Therefore, when estimating defoliation the entire canopy, not just the injured portion, must be considered.

In order to predict if defoliation will reach 20% or 40%, the current injury must be estimated. To estimate current injury:

1. Remove a trifoliolate leaf from the top, middle, and lower third of 10 randomly selected plants.
2. Discard the most and least damaged leaflet from each trifoliolate leaf. This will leave you with 30 leaflets.
3. Compare the 30 leaflets with the leaflets in Figure 1 and determine the average level of defoliation.
4. Repeat steps 1-3 at four or more randomly selected locations in the field.

Threshold. If defoliation approaches 40% (vegetative soybean) or 20% (pod-forming or pod-filling soybean), treatment may be warranted if the insects are actively feeding and defoliation is expected to increase.

If treatment is warranted, identify the defoliating insect(s) and use the insecticide guides on the UNL Department of Entomology Web site. Most of our commonly used foliar insecticides are effective against most soybean defoliators.

Tom Hunt, Extension Entomolgist, Haskell Ag Lab, Concord
Keith Jarvi, Extension Educator,