August 6, 2010

	Figure 1.  (Above) Green cloverworm and (left) southern corn rootworm beetle. (UNL Department of Entomology)

There have been numerous reports of green cloverworm, southern corn rootworm, grasshopper, first generation bean leaf beetle, and other defoliating insects in Nebraska soybean fields, and growers are becoming concerned about the increasing levels of 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 (Figure 1), and a few others. Rarely does any single species reach population levels high enough to cause economic defoliation 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. (Also see Tips for Identifying Defoliating Caterpillars in Soybeans.) 

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 the whole spectrum of species we’re 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 20 years has established that the key factor driving yield losses from defoliating insects is the degree that defoliation reduces light interception by 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. Growing conditions are another factor. When environmental conditions are very favorable for soybean development (e.g. adequate water), plants have a greater capacity for regrowth and compensation.

General Thresholds.  The following guidelines can be used for defoliating insects that lack species-specific thresholds or when two or more 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 soybean stage, 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, consider the entire canopy, not just the injured portion.

Estimating Injury

In order to predict if defoliation will reach 20% or 40%, the current injury must be estimated. The following steps are suggested:

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 2 and determine the average level of defoliation.
4. Repeat steps 1 - 3 at four or more randomly selected locations in the field.

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 found at the Department of Entomology website at http://entomology.unl.edu/instabls/soydefol.shtml. Most of our commonly used foliar insecticides are effective against most soybean defoliators.

Tom Hunt
Extension Entomologist, Haskell Agricultural Lab, Concord
Keith Jarvi
Extension Educator, Dakota, Dixon, and Thurston Counties
Robert Wright
Extension Entomologist, Lincoln