Soybean looper calls are pouring in from all over the eastern half of the state. It is adding ‘insult to injury’—having to treat fields for the second, third, or even a fourth time in the most extreme cases and on top of that, many fields are so drought stressed they are struggling to stay alive.

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.

When will this looper flight end and what will stop it? I have been asked. I wish I knew. We need rain, badly, which will move fields to maturity and get us out of this mess.

A couple of new things to report from our statewide soybean survey: soybean aphid has greatly exceeded the threshold (250 aphids/plant) in at least two fields in Gloucester County. Our scout has been watching this area closely for several weeks as it has been the ‘hot spot’ in the state. As of this week, the population has jumped. As the beans are still in the R4-R5 growth stage, we are recommending a pyrethroid treatment. Pyrethroids are effective in controlling this aphid species.

The scout also brought in samples of brown marmorated stink bug (BMSB) nymphs collected from soybean fields in three counties, Culpeper, Augusta and Clark. These are in the upper- central and northern Virginia. The counts were low in these fields, only about 1/15 sweeps, which is not alarming, but definitely warrants watching. Reports of huge masses of BMSB are starting to come in from places like Fairfax, VA with bugs massing on trees, especially maples, and vegetables of all kinds.

Earworm moths are still flying. Trap catches are high in many areas, and windshields are spattered. One fellow commented that the other night coming home from a meeting, he had to stop twice to clean the moth gunk off his windshield. One observation, and I hope this holds, is that in the past this late season moth flight did not result in new worm infestations. A lot of insect behavior is relegated by environmental cues. Days are definitely getting shorter. Is this reduced photoperiod (shorter day length) causing moths not to lay as many eggs? Not sure, but for whatever reason, in my experience it has been rare to see worm infestations initiate in September. We are hearing of many fields with a few earworms, maybe left over from the earlier flight, but none with the huge numbers we had a few weeks ago. Let’s hope we are finally seeing the end of this pest, for this season at least.

Soybean loopers are now being reported in soybean fields across much of the eastern side of the state. In past years they were mostly confined to southeastern counties. This infestation has broken all the records in terms of intensity (as many as 100+/15 sweeps in some fields) and geography (now being reported from Middle Peninsula and Northern Neck counties). This must be due to the persistent hot, dry weather, and the high percentage of fields previously treated with insecticides.

Loopers are easy to identify but there is some confusion about their color. Soybean loopers have both a ‘green’ and a ‘black’ color phase (see the attached image). We are seeing both color phases in our samples. There may be some cabbage loopers (a completely different species) mixed in, but we have not verified this. There are no exact thresholds for loopers in soybean so treatment should be based on the amount of leaf feeding in relation to the size of the total canopy. But, a very loose rule of thumb could be that 20 or more per 15 sweep net sweeps may constitute a threat. Fewer than that, especially in tall, full canopy fields probably does not constitute a threat. Some fields are getting close to maturity with pods and leaves beginning to yellow. Loopers are not a threat to those fields. Pyrethroids should not be figured into a looper treatment decision. They are less than effective. If stink bugs are present in looper infested fields, I would consider adding Orthene to the looper insecticide. Orthene will provide good control of both green and brown stink bugs where the pyrethroid would miss some of the browns. Oddly, we are seeing more brown stink bugs in some fields than greens, which is opposite of what we are used to seeing.

We have initiated 5 different field trials to evaluate the effectiveness of many different insecticides/rates for loopers. Results will be presented at our annual Pre-Harvest Field Tour scheduled for Sept. 14.
Additional information: soybean-looper-color-phases-herbert-2010-ppt