Soybean Aphid Update

As of this writing (August 24), most of the aphid populations in Nebraska are now stabilizing or decreasing, particularly in beans that are in the R5 stage or beyond (R5 is when at least one pod in the uppermost 4 nodes of the plant has an 1/8 inch bean developing inside). Lady beetles and other predators and parasites have caught up to the aphids in many fields. At this time, it appears that irrigated soybeans are safe from damage since the soybeans are maturing rapidly, have many predators, and have not shown signs of stress up to this point. The overall later maturity of the soybean crop appears to have lengthened the time the aphids have been present in the fields. Some later maturing dryland fields in northern counties may have some populations still increasing slightly, but now it is to the point where the return from a treatment is highly questionable. Near one thousand aphids or more per plant would be necessary to provide a potential return on investment. Again, should you decided to treat, please leave an untreated check area to compare with the treated area for yield. From here on as the dryland beans mature and the irrigated beans remain greener, populations will begin to drop in the dryland and linger in the irrigated beans. Moisture stressed dryland areas will not have many aphids, as they will prefer more succulent plants.

Bean Leaf Beetle - Late Season Management

The aphid frenzy has provided a lot of interest, but another soybean insect pest must be managed for the remainder of the season. We have gone through one generation of bean leaf beetles and are now experiencing renewed feeding as the true second generation of beetles is emerging and feeding on leaves and developing pods. Here is a review of the biology of the beetle and management suggestions:

There are 2 generations of bean leaf beetles that develop in Nebraska . The second generation overwinter as adults and these beetles are the ones seen early in the year feeding on seedling soybeans. These beetles feed, mate, lay eggs and die in early-mid June. There is usually a distinct period from mid June to early July when few if any beetles are present in the field, before the first generation emerges. Total developmental time from egg to adult can range from 25 to 40 days. Because of this range of development, it is common to see adults from the first generation and the second generation in the field at the same time. In other words, the generations overlap and beetles will be present at some levels from mid-July until the end of the growing season. Because of this overlap it is important to monitor beetles on a regular basis to determine shifts in population, which will aid in management decisions.

 

Bean leaf beetles will feed on soybean leaves throughout the season, but leaf feeding seldom causes yield loss. Most damage (economic yield loss) occurs when beetles feed on the developing pods. This yield loss can occur in several ways. Pods may be clipped from the plants, but it is not the primary cause of yield loss. Many flowers and pods are aborted naturally and to blame pod loss on bean leaf beetle feeding may be a costly mistake. Beetles normally injure soybean pods by feeding on the outside layer of the pod, leaving a thin layer of tissue still covering the seed. They do not usually eat into the developing seed but this may occur on very small pods. Fungal pathogens may enter the pod from the feeding sites, causing seeds to appear shrunken, discolored, and moldy, which can result in dockage. After full pods are formed and seeds begin developing, soybeans are most susceptible to yield loss from pod feeding.

The best time to sample is before significant pod feeding occurs, but after second generation beetles have emerged. Second generation bean leaf beetles are emerging and beetles numbers will be approaching their highest levels for the summer. Beetle numbers will slowly decline as beans continue to mature and beetles will move to overwintering sites.

Economic thresholds have been developed for both drop cloth (beetles per foot of row) or sweep net (beetles per sweep) sampling.

Perhaps the most accurate way to sample beetles is with a drop (or shake) cloth. A drop cloth is a 3 x 3 ft piece of muslin or plastic attached on each side to dowel rods. Hold one rod against the base of the plants and lay the cloth between the rows. Shake the plants against the cloth to knock off the insects, and count the beetles. Remember to estimate the number per row foot, so if you use a three-foot cloth divide your total by three. Also, sample through the field in several areas to get a good estimate of the population. In narrow row beans you can still sample with a drop cloth but the procedure is slightly different. Set the rod at the base of the row of plants you want to sample and lay part of the cloth on the ground and hold the rest of the cloth upright or over the opposite row to be sampled. Shake the soybeans against the upright cloth, and then count the beetles knocked down on the bottom of the cloth.

Thresholds are based on the number of beetles per foot of row, which varies according to total application cost and the crop value per bushel.

The following tables show economic thresholds for beans in 30 inch rows and 7 inch rows. To use the tables find the number that fits both crop value and application costs. For example, if you set the value of your soybeans at $6.00 per bushel and your application costs would be $9.00, you would need 6.8 or more beetles per foot of row to justify an application in 30 inch row beans, or 1.6 or more beetles per foot of row in 7 inch row beans.

Table 1. Economic thresholds in beetles per row foot for R5-R6 (beginning pod and full seed) soybeans in 30 inch rows.

Soybean Value	Pest Management Costs Per Acre
$7.00	$8.00	$9.00	$10.00	$11.00	$12.00
$4.50	7.1	8.1	9.1	10.1	11.1	12.1
$5.00	6.4	7.3	8.2	9.1	10.0	10.9
$5.50	5.8	6.6	7.4	8.3	9.1	9.9
$6.00	5.3	6.1	6.8	7.6	8.3	9.1

Table 2. Economic thresholds in beetles per row foot for R5-R6 (beginning pod and full seed) soybeans in 7 inch rows.

Soybean Value	Pest Management Costs Per Acre
$7.00	$8.00	$9.00	$10.00	$11.00	$12.00
$4.50	1.7	1.9	2.1	2.4	2.6	2.8
$5.00	1.5	1.7	1.9	2.1	2.3	2.5
$5.50	1.4	1.5	1.7	1.9	2.1	2.3
$6.00	1.2	1.4	1.6	1.8	1.9	2.1

The following tables are the economic thresholds in beetles per sweep for bean leaf beetles on stage R6 Soybeans in 30 inch rows. Numbers in parenthesis are for beans drilled in 7 inch rows.

Table 3. R6 Economic Thresholds (beetles per sweep). Numbers in parenthesis are for drilled soybeans with 7 inch row spacing.

Soybean Value	Pest Management Costs Per Acre
$6.00	$8.00	$10.00	$12.00
$5.00	4 (3)	5 (4)	6 (5)	8 (5)
$6.00	3 (2)	4 (3)	5 (4)	6 (5)
$7.00	3 (2)	4 (3)	4 (3)	5 (4)

Sweep at least 5 randomly selected sites. Walk through the field at an even pace, performing about 25 sweeping arcs. The best sweeping action for bean leaf beetle is a consistent upward motion through the foliage, using as much force as needed to move the net smoothly through the foliage. Bean leaf beetle activity varies during the day. Activity patterns suggest the best times to sample are around mid-morning or in the afternoon. Try to maintain a similar sampling time in each field to eliminate variability.

Economic thresholds for reproductive stage soybeans other than R6 are probably higher (more beetles are needed to justify a treatment). This is because pods on plants past R6 are maturing and there is less green pod tissue available for beetle feeding, and plants in earlier reproductive stages have greater yield compensation potential than those in R6 or older.

Insecticides Registered for Bean Leaf Beetles
Bean leaf beetles can be controlled by several insecticides. Be aware that most have 14 day or more pre harvest intervals (phi). Here is a table of insecticides for bean leaf beetle control.
Restricted Use	Product Name	Rate (formulation/acre or lb/acre)	Pre-harvest interval (days)
Yes	Asana XL	5.8 - 9.6 oz	21
Yes	Baythroid	1.6 - 2.8 oz	45
No	dimethoate	1 pt	21
No	Lorsban 4E	1-2 pts	28
Yes	Lannate WSP	.25 - .50 lb	14
Yes	Lannate LV	3/4 - 1 lb	14
No	Larvin 3.2F	18 - 30 oz	28
Yes	Mustang Max	2.8 - 4.0 oz	21
Yes	Penncap-M	2 - 3 pts	20
Yes	Pounce 3.2 EC	2 - 4 oz	60
Yes	Pounce 25 WP	3.2 - 6.4 oz	60
No	Sevin SLR Plus	.5 - 1 qt	21
No	Sevin 80 S	5/8 - 1 1/4 lb	21
Yes	Warrior	1.92 - 3.2 oz	45

Blister Beetles in Alfalfa

Alfalfa growers need to be on guard this summer and early fall for blister beetles. Since blister beetle larvae feed on grasshopper eggs, the last few years' buildup of grasshoppers may be causing blister beetle populations to reach toxic levels in alfalfa through mid-September. This is of particular concern if the hay will be fed to horses. Although blister beetles appear in Nebraska alfalfa fields every summer, populations usually aren't high enough to cause problems.

Blister beetles feed on alfalfa leaves and flowers and occasionally on soybeans, goldenrod and musk thistle. Animals are poisoned by ingesting dead beetles in cured hay. Blister beetles produce a chemical -- cantharidin -- to which horses are especially susceptible. Cantharidin remains toxic in dead beetles. Even a small amount can cause discomfort and colic in horses. Small doses also can reduce milk production and weight gains of cattle and sheep. In people a crushed beetle causes blistering of the skin – giving the insect its name.

In horses cantharidin can cause inflammation, frequent urination, straining, elevated temperature, depression, increased heart rate and respiration, dehydration, sweating and diarrhea. Animals can die within 72 hours, and it is imperative to contact a veterinarian as soon as blister beetle poisoning is suspected.

Cantharidin concentration varies with the beetle species and sex. The chemical is produced by males and some is passed to females during mating. In Nebraska , the three striped, gray and black blister beetles are the most common. The striped blister beetle contains five times more cantharidin than the black blister beetle. It takes about 40 striped blister beetles to kill a 275-pound horse and about 120 to kill an 825 pound horse. It would take 1700 of the less toxic black blister beetles to be fatal to the 825 pound horse. Timely harvest can reduce beetle numbers. Since blister beetles are attracted to flowers, alfalfa needs to be cut on a schedule that keeps it and weeds from producing flowers. Growers need to check fields for blister beetles before harvest. A sweep net, which is used to sample for other insects, may not work with blister beetles because large groups of swarming beetles can concentrate in small areas of a field.

The worst thing that can be done is to crimp or crush hay if beetles are present. Since blister beetles tend to swarm, crushing can deposit many dead beetles into a single flake of hay. Sickle bar mowers and some disk mowers can lay the hay down without crushing it. The beetles will then crawl out of the hay and leave as it dries.

Other management tips include: Do not drive on hay shortly after cutting because tires can crush the beetles. Avoid using outer edges of fields for horse hay, especially if adjacent to weedy strips. Use a short residue insecticide around field margins; follow directions on harvest intervals. Don't use a hay conditioner when harvesting blister beetle-infested alfalfa.