University of Illinois

No. 22/August 29, 1997

Surveys of Soybeans Reveal Activity of Defoliators

In the first article in this issue of the Bulletin, Mike Gray described the surveys of soybean fields that are being conducted to determine the presence of western corn rootworm adults. During these surveys, the "team" also has kept track of numbers of some of the other defoliators that occur in soybeans at this time of year. We thought you might find these numbers useful, so we present the numbers of bean leaf beetles and Japanese beetles per 100 sweeps of a net in several counties (Table 2).

Table 2. Bean leaf beetles and Japanese beetles in Illinois, August 13 to 26, 1997.
County1DateNumber of bean
leaf beetles
per 100 sweeps
Number of
Japanese beetles
per 100 sweeps
ChampaignAugust 13420
FultonAugust 13730
LoganAugust 131390
MasonAugust 134850
McLeanAugust 13680
FultonAugust 20450
LoganAugust 20750
MasonAugust 20870
McLeanAugust 20460
ChampaignAugust 21390
LivingstonAugust 21352
ClayAugust 2590
ColesAugust 2511549
CumberlandAugust 2515
EffinghamAugust 251914
IroquoisAugust 251110
IroquoisAugust 253821
JasperAugust 25355
MoultrieAugust 25480
PiattAugust 2521225
ShelbyAugust 25600
DeWittAugust 26130
LoganAugust 26210
MaconAugust 26340
MasonAugust 2690
MenardAugust 26420
SangamonAugust 261060

1One field per county on the date indicated was sampled by taking 100 sweeps of a sweep net.

So what do these numbers mean? Well, they mean that some defoliators still are present in some soybean fields in some counties in Illinois. Definitive answer, right? The point is this: As soybeans begin to senesce, the threat posed by soybean defoliators diminishes significantly. However, if pods are still filling out in some of these fields infested with defoliators, 20 to 25 percent or more defoliation could result in economic yield loss. Some team members observed 30 to 40 percent defoliation in some of the fields they surveyed (for example, the field in Mason County infested with 485 bean leaf beetles per 100 sweeps, and the field in Piatt County infested with 225 Japanese beetles per 100 sweeps). Based upon these observations, scouting for soybean defoliators is time well spent. Also remember that some defoliators (bean leaf beetles, grasshoppers) also cause injury to pods. Many people do not continue scouting soybeans this late in the growing season, but we recommend otherwise. Refer to issue no. 21 (August 15, 1997) of this Bulletin for more specific guidelines for dealing with soybean defoliators and pod feeders.

Kevin Steffey and Mike Gray, Extension Entomology, (217)333-6652