SOYBEAN APHID UPDATE

Most of the aphid populations in northeast Nebraska are now stabilizing or decreasing. Hot weather, which may have reduced the aphids’ reproduction, and lady beetles and other predators and parasites have overcome 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.  Some late maturing dryland fields may have benefited from treatment last week, but now it is to the point where the return from a treatment is highly questionable.  The rapid population growth and expanse westward of the soybean aphid will give us a lot to look forward to in 2004.

BEAN LEAF BEETLE - LATE SEASON MANAGEMENT

We have gone through one generation of 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.

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

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. Because the price of beans is so low, you may need to raise the thresholds by one beetle per sweep. For example if management costs were 10 dollars an acre, it would take 7 beetles per sweep in 30-inch rows or 6 per sweep for 7-inch rows. R6 is defined as a green seed filling the pod cavity in one of the four uppermost nodes with a fully developed leaf (seeds touching).

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

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	3.0 – 4.3 oz	21
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 XLR Plus	.5 - 1 qt	21
No	Sevin 80 S	5/8 - 11/4 lb	21
Yes	Warrior T	1.92 - 3.2 oz	45