Agronomic Crop Insects – September 17, 2010

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Soybeans
We continue to get an occasional report and question regarding the need to treat fields for stinkbugs, soybean loopers and beet armyworms. Last week (Sept 9), Dr. Ames Herbert from Virginia Tech had a great article in their Ag Pest Advisory regarding the need to treat for soybean loopers: (http://www.sripmc.org/Virginia/)

“When helping growers make the decision on whether to treat a field for loopers we have to take the time to consider several components that influence the decision: the maturity of the crop, the health of the leaf canopy, and the number of loopers present. Let’s take them one at the time.

“In terms of crop susceptibility, after some long discussions with soybean agronomists (David Holshouser at VT and Jim Dumphy at NCSU), we came up with a rule-of-thumb as to when fields are safe, that is, worms can be left untreated with no fear of lost yields. We suggest that fields will need to be protected as long as the pods are still green and until the lower leaves are just beginning to yellow. This should correspond, more or less, with the R6.5 stage (10 days after R6.0 = full green seed). If leaves are beginning to yellow up the stem, not from drought but from the maturity process, and there are any pods on the plant that are beginning to yellow, the field is safe, no need to treat.

“Next we have to determine the health of the leaf canopy, robust, average, or thin. Each can tolerate different amounts of leaf loss before reducing yield potential. Robust fields (mid chest or higher) can tolerate a lot of feeding. Average fields (upper thigh to mid chest) can tolerate normal amounts of feeding. Thin canopy fields (mid thigh or below) cannot tolerate additional leaf loss. Also in this canopy assessment, we need to take a stab at estimating the current percent defoliation. This is not an exact measure, but your best estimate looking over the entire canopy top to bottom. The eyes tend to focus on those badly defoliated top leaves. Look beyond those and try to come up with an overall average. One thing we (and others) have noticed about soybean loopers is that their feeding is often in mid-canopy, not at the top like most other defoliators. All the more reason to inspect the entire canopy.

“Finally, how many loopers are present? There is not a single threshold because of all the factors we have just discussed, but a very general rule of thumb is that 15-20 or greater/15 sweeps constitutes a potential threat, depending on the maturity and canopy health. Thresholds vary quite a bit from state to state but this one falls pretty well in line with the other states. In considering these components, some fields will be no-brainers. Mature fields (late R6 or older) or fields with robust canopies and just a few loopers (10 or less/15 sweeps) can be left alone. On the other end of the spectrum, early R6 stage or younger fields with stressed, thin canopies and 15-20 or more loopers/15 sweeps need to be treated. This week in Virginia, there are a lot of fields in the grey zone (plants are in the mid to late R5 to early R6 stage, the canopy is average, the looper number is in the 12-18 range, and defoliation is less than 20%). I tell folks, if they can do this, to take a close look at these fields and make a mental image of the extent of defoliation. Revisit in 2-3 days (no longer) to see if it has greatly increased. If the percent defoliation has increased and loopers are still present at or near the threshold, treat it. If the level of defoliation has not increased much and/or the looper numbers have decreased, don’t treat. It all sounds pretty complicated and it is but taking the time to consider these components should help determine if a field needs to be treated some will and some won’t and allow growers to protect at risk fields but save money on safe fields.”

For those who are keeping track of the spread of brown marmorated stink bug, it has now been found in soybeans in all 3 counties in Delaware.

Small Grains
Be sure to sample fields at emergence for aphids, true armyworm and fall armyworm feeding. In past years, we have seen economic damage from all three insect pests.

With the heavy late summer worm pressure in soybeans and vegetable crops, it will be important to watch for “worm” damage to emerging plants. We have seen fields destroyed by armyworms in past years, especially in no-till situations. In many cases it has been true armyworm, although fall armyworm can also cause damage. Although there is no threshold available, you will need to watch for larvae feeding on small plants.

As you make plans for small grain planting, you should consider the following factors when making a treatment decision for aphids. In general, cooler summer temperatures with adequate rainfall followed by a warm, dry fall are conditions that favor aphid development in small grains, especially in early planted fields. Early fall infestations of the greenbug aphid (which cause direct damage to small grains as well as vector BYDV) are favored by cool, late summer conditions.

The main reason one would consider aphid control in the fall (except for greenbug aphid that causes direct damage) is the potential for Barley Yellow Dwarf Viral (BYDV) transmission. Plant pathologists in our area still do not feel that we are seeing a significant increase in the incidence of BYDV. However, in areas where you have seen BYDV in the past, where you are planting early, or you have seen direct damage by green bug aphids, a seed treatment that control aphids (i.e. Cruiser and Gaucho) would be a good control option. Information from Kentucky indicates that planting date is the most important factor determining the intensity of an aphid infestation. If you have a history of aphids transmitting viruses in the fall and you plan to scout for aphids, data from the south indicates that the most important time for controlling aphids to prevent BYDV is the first 30 days following emergence. The second most important time is the second 30 days following emergence. The following link to a fact sheet from Kentucky provides more information on aphids and BYDV in wheat (http://www.ca.uky.edu/entomology/entfacts/ef121.asp).

Orchard Grass
Although billbugs have been a significant problem in Virginia and West Virginia fields for the last few years, this is the first season that we have documented significant damage from this insect in the Delaware/Maryland Eastern Shore region. Dr. Rod Youngman from Virginia Tech has taken the lead in developing sampling and treatment timings for this insect. He has just posted a presentation on his website that gives good information on the biology of this pest, sampling methods, treatment timing and control options. (http://connect.ag.vt.edu/billbugipm2)

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