With financial support from the Virginia Soybean Board and federal funding for our state IPM program, our soybean scout Ed Seymore has begun to monitor about 30 soybean fields for soybean aphid and a new invasive pest species, the brown marmorated stink bug. From Aug. 2 to mid-August 2010, surveyed fields in the following counties had little to no soybean aphids: Augusta (0), Albemarle (0), Buckingham (0), Caroline (0), Charles City (0), Clark (0), Culpeper/Fauquier (1-5 aphids/plant), Gloucester (0), Goochland (0), Henrico (0), King and Queen (0), King William (0), Lancaster (0), Mathews (0), Middlesex (0), Orange (6-39 aphids/plant), Rockingham (0), and Shenandoah (0). The economic threshold for soybean aphid is 250 aphids/plant so these low numbers of aphids do not pose any threat at present. Very low numbers (about 1 per 15 sweeps) of brown marmorated stink bugs were detected in two soybean fields in central Culpeper County. Researchers are still trying to determine the extent and nature of brown marmorated stink bug feeding injury to soybean, but it will likely pose a risk if populations become established.

Soybean looper is a common pest of our soybean crop and can be found in small numbers in most fields, most years. But it is very uncommon to have large infestations. The moths have to migrate into Virginia from the south so when we do have big problems, they occur late in the season. Our last bad soybean looper year was 2005. In 2005, they moved into several soybean fields in mid-September. Being unfamiliar with the pest, growers were not looking for them and not sampling fields. In a few cases, fields were totally destroyed with loopers eating all the leaves leaving only leaf veins, stems and stalks (see the attached images, insecticide protected vs. unprotected soybean field). This insect is capable of doing a lot of feeding in a short period of time. This is why I try to correct folks when they call green cloverworms–loopers. Although green cloverworms do ‘loop’ or ‘inch’ when they crawl, they are distinctly different from soybean loopers both in appearance and in the rate they feed. Both are light green with white longitudinal strips down the tops of their backs and sides. The differences are that green cloverworms are of equal girth from head to tail, have 3 pairs of abdominal prolegs, and wiggle vigorously if you put one in the palm of your hand (see the attached images). Loopers tend to be smaller at the head end and fatter towards the tail end, have only 2 pairs of abdominal prolegs, and they don’t thrash around in your hand. And most importantly, loppers eat much more, worm-for-worm, compared with cloverworms.

A pattern that played out in 2005 was that all the fields hit hardest by soybean looper had been previously treated with a pyrethroid. The pyrethroids had reduced the beneficial population so when the invading moths laid their eggs, most survived. Large numbers of soybean looper moths have now been spotted in several fields in North Carolina. Almost all of our soybean fields have been treated with a pyrethroid. We could be set up for a huge problem. Loopers are also fairly hard to control with pyrethroids which provide only about 50% control, at best. Products with pyrethroid/neonicotinoid mixes will not help. Field trials in 2005 showed that only Lannate, Larvin, Steward and Tracer provided acceptable levels of control. Belt and Intrepid could likely be added to that list, but we have not evaluated those.

Bottom line, stay alert to these late season pests and keep a watch on your soybeans, especially any that have been treated with a pyrethroid.
Additional information: looper-vs-gcw-herbert-2010-ppt

To date, this has been one for the records in terms of corn earworm pressure in soybeans. Many of our fields have been treated, although most are still in the flowering/late flowering stage. We do not have thresholds for flowering beans but know that high numbers of worms can do too much damage by eating all the flowers which can result in delayed maturity. What’s next for these fields? Will earworms recycle to present another threat to podding fields? My guess is, yes, at least some percentage of fields may need to be retreated. The decision to retreat should be made based not on rumor, seeing spray rigs running in the neighbor’s field, or conversations at the diner, but based on knowledge that thresholds have been met. Growers cannot afford to make unnecessary sprays, especially with low yield potential and having already paid for one application. And, if a pyrethroid was used the first time we would strongly recommend that these second applications be non-pyrethroids. If you have treated with Karate, Baythroid, Endigo, Birgade, Bifenthrin, Tombstone, Sniper, Mustang Max, Hero, Proaxis, or Leverage—you have treated with a pyrethroid. In our field trials we are having good success controlling worms with relatively low rates of these non-pyrethroids (e.g., Steward at 4.6 oz and Larvin at 10 oz). But even at these rates, the cost is more than the pyrethroids.