No. 9/May 22, 1998
Bean Leaf Beetles Are Swarming into Soybean Fields
Not surprisingly, the soybean fields in which plants are emerging or have emerged are catching the brunt of the bean leaf beetles' hunger. As you may recall from a discussion in a previous issue of this Bulletin, bean leaf beetles overwinter as adults under debris in noncrop areas such as woodlands. They come out of "hibernation" early in the spring, often in early April, and move into alfalfa and clover fields, where they feed a little and lay some eggs. However, as soon as soybean plants become available, the bean leaf beetles leave their early spring habitats and move into fields in which soybeans have emerged. Consequently, growers who got their beans into the ground relatively early have invited the bean leaf beetles in their area into their fields.
Most folks who find beetles in soybeans at this time of year correctly identify the insects as bean leaf beetles. However, due to the scare of western corn rootworms' laying eggs in soybeans, some people have identified bean leaf beetles incorrectly as western corn rootworms. Although these two insects are in the same family (Chrysomelidae) of beetles, they are different species with different life cycles and behaviors. Therefore, it's probably appropriate to compare these two insects at this time.
Western corn rootworms currently are in the egg stage, possibly in the early larval stage if eggs have begun to hatch. The adults will not emerge from the soil until about one month after the eggs hatch. We usually don't see our first western corn rootworm adults until the latter part of June at the earliest. As stated in the first paragraph of this article, bean leaf beetles overwinter as adults, and they are flying around right now. Although adult bean leaf beetles are about the same size (1/4 inch) and shape as western corn rootworm adults, the markings of these two beetles are characteristically different. Western corn rootworm females are yellow and have three distinct stripes on their wing covers, one on the outside of each wing cover and one along the middle, where the wing covers meet.
Bean leaf beetle adults vary in color (usually yellow or tan, but sometimes green or red) and usually have a distinct black band around the edge of the wing covers and two black spots on each wing cover. However, although occasionally both the black band and the spots are absent, one characteristic is evident on every bean leaf beetle adult--a black triangle behind the head at the base of the wing covers. This characteristic is lacking in western corn rootworm adults. A typical bean leaf beetle adult is shown in Figure 3.
Figure 3. Bean leaf beetle adult; from Practical Insect Management, Vol. 2; Insects of Livestock & Agronomic Crops, Waveland Press, Inc., Prospect Heights, Illinois.
We have received reports from virtually every area of the state of bean leaf beetles feeding on seedling soybeans. The types of injury reported range from the beetles' clipping off seedlings just breaking through the soil surface, to defoliation of unifoliate and trifoliate leaves, depending upon the plants' stage of development. As could be expected, the primary question is "Are these bean leaf beetles causing any economic damage to my seedling soybeans?" Obviously our answer to this question varies, depending upon the extent of injury, the numbers of beetles present, and the growth stage of the seedlings. However, in general, people tend to overreact to bean leaf beetle injury. For most of the instances of injury that have been reported to us, we suspect that the injury was not economic and that no treatment was necessary. In fact, results from research conducted by some entomologists at the University of Nebraska a few years ago caused us to change our economic thresholds. The researchers indicated that the amount of economic loss attributed to bean leaf beetles feeding on seedling soybeans had been overestimated. Following is a comparison of our old and new thresholds for bean leaf beetles feeding on seedling soybeans.
Our old threshold indicated that treatment for bean leaf beetles in seedling soybeans was warranted if "20% of the plants are cut and the stand has gaps of 1 foot or more, or if at least 1 seedling per foot of row is destroyed. This level of damage usually requires 5 or more beetles per foot of row." Our more recently published thresholds are based upon the research from the University of Nebraska: "An insecticide treatment for seedling soybeans is rarely justified. Densities of 16 beetles per foot row in the early seedling stage or 39 per foot of row at stage V2+ are necessary for economic injury." This rather dramatic change reflects the outstanding ability of soybeans to compensate for early losses of both plants and leaf material.
Information regarding hail damage to seedling soybeans might lend additional insight. A little pamphlet entitled Pocket Guide to Evaluating Hail-Damaged Soybeans, Extension Bulletin 455-1980, from the University of Minnesota, discusses the importance of hail injury at different stages of growth of soybeans, and the ability of soybeans to compensate for stand loss. This type of information may be used somewhat comparably for making decisions related to bean leaf beetle injury in soybeans. Regarding bean leaf beetle injury to seedling soybeans, plants cut off below the cotyledons will not recover. If seedlings are cut off just above the cotyledons, some may die, but most will regrow from axillary buds. Defoliation of unifoliate or trifoliate leaves has to be excessive (very likely greater than 50 percent) before soybean seedlings have difficulty recovering.
If bean leaf beetles are cutting off seedlings below the cotyledons, how much of this type of injury can a soybean stand tolerate? The previously mentioned publication indicates that soybean plants compensate for skips within rows by producing additional branches. As plant density within the row decreases, distribution of the remaining plants within the row becomes important in producing yields comparable to those from fields with higher plant densities. The information in the pamphlet illustrates this with a table, Table 1 in this Bulletin. So before you decide to "pull the trigger" on treating for bean leaf beetles, make certain you understand that soybeans likely will recover from the damage. There's no reason to spend money needlessly if the return on investment is poor. Kevin Steffey (firstname.lastname@example.org) and Mike Gray (email@example.com), Extension Entomology, (217)333-6652