During the past three summers, bean leaf beetles have reached historically high populations. At the same time, we learned that the insects are transmitting a disease called bean pod mottle virus. The occurrence of both of these pests in Iowa soybean has dramatically affected yields. Because the beetles are spreading the virus in soybean, we must rethink how we are going to manage these two pests.
We found our first bean leaf beetle of the year on April 23 on the Iowa State University campus, which means that bean leaf beetles are beginning to leave overwintering sites in search of food. This year, we expect the populations to be higher than last year because a high percentage of beetles (average of 48 percent across the state) are predicted to survive the winter.
|The bean leaf beetle often has four rectangular spots in the center of the wing covers.|
Bean leaf beetles prefer to feed on legumes. Before soybean seedlings emerge, beetles feed on alfalfa, red clover, and prairie legumes. However, as soon as soybean seedlings emerge, most beetles move to soybean fields to feed and lay eggs. Often, the earliest planted soybean fields are the hardest hit by bean leaf beetles. Later planted fields (second or third week of May) typically have fewer problems with bean leaf beetles.
We expect that many growers will be wondering whether management of these insects is needed. Management of bean leaf beetles in early season depends on two factors: 1) presence of bean leaf beetles, and 2) history of bean pod mottle virus.
Bean pod mottle virus is transmitted by bean leaf beetles. The disease can cause reductions in soybean yield and seed quality. Some symptoms of the disease include a blistered appearance to soybean leaves, green stems at the end of the season when pods are mature, and a mottled appearance (bleeding hilum) on the seed coat of harvested soybean. The only way to confirm that a plant is infected with bean pod mottle virus is to have plant tissue tested. If bean pod mottle virus has not been present in your fields, i.e., no green stem at harvest or seeds with bleeding hylums then the only reason to consider early-season bean leaf beetle management is if populations reach extremely high levels (more than 2.5 beetles per plant).
|A green streak, running left to right across this soybean field in early September shows symptoms of "green stem" from bean pod mottle virus. The rest of the field is maturing normally and was sprayed in July for control of first-generation bean leaf beetles..|
If bean pod mottle virus is a concern then early-season bean leaf beetle management is necessary to manage the disease. In 2000 and 2001, as part of a research project funded by the Iowa Soybean Promotion Board and the North Central Soybean Research Program, we explored management tactics for bean pod mottle virus. We documented bean pod mottle infected soybean plants as early as the VC plant stage. We evaluated three different insecticide spray schedules: 1) spray once after soybean emergence, 2) spray twice after soybean emergence, and 3) spray after soybean emergence and a second time in early July. Our results suggest that the third treatment, spraying once early season and once mid-season, is the best option for reducing incidence of both the beetle and virus, while protecting yield and seed quality. This strategy helps to stop the beetles from transmitting bean pod mottle virus because it stops two beetle populations (overwintered and first generation) at key times in the season. The details of bean leaf beetle and bean pod mottle virus management using insecticides are as follows:
- Scout fields when soybean seedlings start to emerge. When the first bean leaf beetle is found, spray the field with a long-lasting insecticide. The goal is to reduce the initial inoculation of bean pod mottle virus in the soybean. The earlier the plants are infected, the greater the yield and quality losses.
- In early July, begin scouting fields for first-generation bean leaf beetles. When bean leaf beetle adults first emerge from the soil they are gray, their bodies are very soft, and the wing covers are not hardened. The appearance of these beetles signals the beginning of the first generation. Spray the field again with a long-lasting insecticide.
Insecticides to stop bean leaf beetles provide a short-term answer to managing bean pod mottle virus. Research to explore more long-term management options, such as host plant resistance, are ongoing. As in any pest management program, monitoring for insect pests and diseases is the starting point for making any management decisions. Updates on bean leaf beetle management will continue throughout the season.
Insecticides for control of bean leaf beetles in soybean.
|Ambush 2EC*||3.2-6.4 ounces||60|
|Asana XL*||5.8-9.6 ounces||21|
|Dimethoate*||See label||See label|
|Lorsban 4E*||1-2 pints||28|
|Pounce 3.2EC*||2-4 ounces||60|
|Sevin XLR Plus||1-2 pints||0|
This article originally appeared on pages 57-58 of the IC-488 (7) -- May 6, 2002 issue.