ARMY CUTWORM MOTHS (“MILLERS”) FLYING

We have received numerous calls of “millers” flying in large numbers and being nuisances around houses, garages and buildings.  The majority of these moths are army cutworms.  The moths are generally gray to light brown with a wingspan of 1 ½ to 2 inches, each forewing being marked with spots, wavy lines, and other dark and light markings. Questions are coming in as to whether these moths will develop into a crop problem later this year.  The answer is no.

Army cutworm moths are emerging from area alfalfa and grass fields and are currently on their way to the Rocky Mountains where they will spend the summer before returning in the fall.  They are one of the few local insects that migrate (the monarch butterfly is another).  When the moths return in the fall, they will lay eggs in sparsely vegetated fields; winter wheat, grasslands, or alfalfa.  The eggs will hatch and the larvae will feed and overwinter.  Next spring the larvae will become active and feed before they transform into the moths and begin the cycle again.  Army cutworms have been serious pests of alfalfa and wheat, particularly as you move further west in the region, although we have seen them prevent greenup of alfalfa in the spring in northeast Nebraska.

The moths themselves cause no harm but are just a nuisance.  Eventually (1-2 weeks) they will move out of the area as they “head for the Rockies”. (KJ)

Army cutworm moths ("millers").
http://ianrpubs.unl.edu/insects/nf526.htm

ALFALFA INSECTS TO WATCH FOR

Now that first cutting is nearing completion, remember to scout your alfalfa 3 to 4 days after greenup to
make sure there are no insects holding back the regrowth.  Insects to watch for are alfalfa weevils (both larvae
and adults), variegated cutworms, and potato leafhoppers.  More on potato leafhoppers n our next issue. (KJ)

This spring we have been seeing high numbers of bean leaf beetles in Nebraska, and the calls have been coming in.  Because seedling plants are small, the beetles and the defoliation they produce are easily seen, leading to questions about how many beetles or how much damage justifies treatment.

Bean leaf beetles have two generations a year in Nebraska; however, since they over-winter 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 over-winter as adults in protected sites such as grassy field edges, leaf litter, and crop residue. They become active fairly early in the year 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 over-wintered 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 1/4 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. Although the defoliation the beetles cause 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, it is unlikely that treatment would be economically justified. This point is illustrated by the economic thresholds for bean leaf beetle on seedling soybean given in Tables 1 and 2. For example, if the value of soybeans is $5/bushel and the management costs are $6/acre, it takes three beetles per soybean seedling (stage VC) before treatment is justified. It is rare to see beetle numbers this high.

Although we seldom experience economic damaging populations of bean leaf beetles early in the season, they can occur. Remember that early-planted soybeans are the most susceptible. If economic thresholds are reached, many insecticides are available for bean leaf beetle control. All will do an adequate job if applied according to label directions.

Table 1. Soybean seedling (VC) economic thresholds (beetles per plant)
Crop value $/bu	Pest management cost $/acre
	$6	$8	$10	$12
$5	3	4	5	6
$6	2	3	4	5
$7	2	3	3	4
$8	2	2	3	4
Table 2. V1 Economic thresholds (beetles per plant)
Crop value $/bu	Pest management cost $/acre
	$6	$8	$10	$12
$5	4	5	7	8
$6	3	4	6	7
$7	3	4	5	6
$8	2	3	4	5

Another reason some producers treat bean leaf beetle on seedling soybeans is to reduce the pod damaging F2 generation that emerges in August. NU 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.

 Finally, questions have been coming in concerning treating bean leaf beetles to control bean pod mottle virus. The bean leaf beetle is a vector of the bean pod mottle virus (BPMV), and it appears that as the beetles move around and feed on seedling soybean, they transfer the virus from plant to plant.   However, much about the relationship between the beetle, virus, and soybean remains unknown.  BPMV is present in many areas of Nebraska, but its impact on yield is unclear.  In many instances the presence of BPMV in a field does not translate into yield loss.  Research is currently underway to determine the extent and impact of BPMV in Nebraska. Unless you know you have had BPMV on your farm in the recent past and also know it resulted in significant yield loss, we do not recommend spraying beetles to manage a problem you may not have.  (TH)