Spring Bean Leaf Beetles

Bean leaf beetles have not been a big problem the last couple of years, but we usually have at least some areas each year that have problems with the bean leaf beetle, and early planted soybean fields always attract some beetles. Because the bean leaf beetle can be a pest of seedling soybean, we'll discuss it in some detail.

Bean leaf beetles have two generations a year in Nebraska. However, since they overwinter as adults, three periods of beetle activity are seen in the growing season: Overwintering colonizers, F1 generation (offspring of the colonizers, the true first generation) and the F2 generation.

Bean leaf beetles overwinter as adults in leaf litter (woodlots) and soybean residue. They become active fairly early in the year (April-May) and often can be found in alfalfa prior to soybean emergence. As soybeans emerge, the beetles quickly move to the seedling plants, feeding on cotyledons and expanding leaf tissue. These overwintered beetles, called colonizers, mate and begin laying eggs. Females live about forty days and lay from 125 to 250 eggs. After egg laying is complete the colonizing population dwindles as the beetles die. A new generation of beetles (F1) will begin to emerge in late June to early July. The F1 beetles mate and produce a second generation of beetles (F2) that begin to emerge in mid August and feed on leaf and pod tissues. The pod-feeding F2 beetles are most likely to cause economic damage.

Bean leaf beetles vary in color, but are usually reddish to yellowish-tan. They are about ¼ inch long and commonly have two black spots and a black border on the outside of each wing cover. These spots may be missing, but in all cases there is a small black triangle at the base of the wings near the thorax.

Because they move to soybean fields so soon after seedling emergence, early-planted fields will usually have more beetles and suffer the most injury. This has become more of a problem in recent years because planting dates seem to be getting earlier each year. Although the defoliation can appear quite severe, research in Nebraska and elsewhere has shown that it usually does not result in economic damage. Soybean plants can compensate for a large amount of early tissue loss, so it takes a considerable amount of beetle feeding to impact yield. Generally, unless insect populations are large enough to cause more than 50% to 60% defoliation of seedling soybeans, it is unlikely that treatment would be economically justified. Tables 1 and 2 show economic thresholds for bean leaf beetle on seedling soybean. Be aware that these thresholds are for defoliation of beans at VC - V1. If beetles enter the field right at or during seedling emergence, the thresholds will likely be lower because the beetles do not have leaf tissue to eat and will feed on the growing point, stem, and cotyledons. We do not have a good research base for bean leaf beetle injury to newly emerging soybean, but the thresholds are probably about 1 to 1.5 beetles lower than the VC thresholds.

Remember that early-planted soybeans are the most susceptible. If economic thresholds are reached, many insecticides are available for bean leaf beetle control, including Asana, Baythroid, Dimethoate, Lorsban, Lannate, Mustang Max, Penncap-M, Pounce, Sevin and Warrior. All will do an adequate job if applied according to label directions.

Another reason some producers treat bean leaf beetle on seedling soybeans is to reduce the pod-damaging F2 generation that emerges in August; however, UNL Extension does not recommend this practice. There are many environmental factors that can impact beetle populations throughout the growing season, making it impractical to use spring beetle numbers to accurately predict if beetle populations will reach economically damaging levels in August. Regular scouting and the use of the appropriate economic thresholds are the best way to manage late season bean leaf beetle in soybean. Late-season economic thresholds will be included later this summer.

For similar reasons, we do not at this time recommend treating bean leaf beetles to prevent the spread of bean pod mottle virus. Although the beetle is a known vector of the virus, and yield declines have been documented in other states, the relationship in Nebraska is not as well known. (TH&KJ)

Table 1. Early Season Bean Leaf Beetle Economic Thresholds (Beetles per plant).

	VC Soybeans Management costs
Crop Value	$6	$8	$10	$12
$5	3	4	4	6
$6	2	3	4	5
$7	2	3	3	5
$8	2	2	3	4
$9	2	2	3	3
$10	1	2	2	3

 

	V1 Soybeans Management costs
Crop Value	$6	$8	$10	$12
$5	4	5	7	8
$6	3	4	6	7
$7	3	4	5	6
$8	3	3	4	5
$9	2	3	4	4
$10	2	3	3	4

Ranking Weed Species Competitiveness in Soybean

Weed scientists have developed the concept of competitive indices as a scale for ranking competitiveness of different weed species. Competitive indices are usually based on the dry matter produced by weed plants. Weed competitiveness is highly influenced by the cropping practices, including crop row spacing. For example, narrower crop rows can reduce the competitiveness of weed species by 20-50% than the crops planted in wider rows. Weed competitiveness also depends on the weed emergence time relative to the crop growth stage. In general, later emerging weeds are much less competitive than earlier emerging ones.

We conducted field studies in eastern Nebraska at two locations in 2002 and 2003 to determine and compare the values for competitive indices among weed species as influenced by soybean row spacing and the weed emergence time relative to the crop's growth stage. This study is also an MS project for Shawn Hock. Soybeans were planted in 7.5 and 30-inch rows. Total of 7 broadleaf and 4 grassy species were planted at three soybean growth stages, crop planting (VP), crop emergence (VE), and 2 nd trifoliate stage (V2). The list of species included common lambsquarters, redroot pigweed, common waterhemp, common sunflower, common cocklebur, Pennsylvania smartweed, giant ragweed, yellow foxtail, giant foxtail, fall panicum, and barnyardgrass. Soybean yield data, weed biomass, and weed seed production were collected at the season end.

The most competitive weed found in this study was common sunflower, producing twice as much dry matter than any other species. Common cocklebur was the next most competitive weed followed by giant ragweed and then velvetleaf. Common waterhemp was more competitive than redroot pigweed but less competitive than velvetleaf. Common lambsquarters was the next competitive and slightly more competitive than the grassy species. Giant foxtail was the most competitive grass, followed by barnyardgrass, fall panicum and yellow foxtail. In general, competitive indices were affected by row spacing and its emergence date. Weed species growing in 30-inch wide rows were more competitive than weeds in 7.5-inch rows. Weeds were also more competitive when emerging with the crop than the ones that emerged a week or two later.

The major practical implications of this study are: (1) it is important to properly identify weed species and their composition before making weed management decisions because weed species do differ in their competitiveness, (2) planting soybean in narrower rows will reduce the competitiveness of most weed species, thus providing a competitive advantage to the crop; and (3) scout the fields to determine weed emergence times relative to the crop stage, because our data shows that weeds emerging a week or two after the crop are much less competitive than when emerging with the crop. This study was partially funded by the North Central Regional Weed Science grant. (SK)