Soybean Pests: the last round-up?

We are still hearing reports of Green Cloverworm in soybeans in NW MN. These are light green caterpillars with a white stripe down the side (figure 1), usually between 1/4" - 1.25" long and cause defoliation that starts as shot-holing. Thresholds for these insects are based on the level of defoliation and stage of the plant. Treat only if defoliation reaches 40% in prebloom, 20% during bloom, 15% during early pod-fill (this can be raised as pod fill progresses), and 30% post pod-fill to harvest. Be sure to check if plants are still filling pods prior to making treatment decisions. 

Right now, most of the soybeans in NW MN are probably somewhere between R5 and R6, early in R5 there is still significant pod fill occurring, steadily decreasing to R6 (pods filled). So later planted soybeans are more likely to suffer yield impact if they have Green Cloverworm exceeding thresholds. When estimating damage from Green Cloverworm, it's important to evaluate defoliation over the whole plant; Green Cloverworm feeds preferentially at the top of the plant so defoliation will be worse there and make it appear as if there's more defoliation than there really is. 
It is sometimes challenging to estimate defoliation in soybean, there's a tendency to overestimate the damage - 10% defoliation is very noticeable. Estimating defoliation isn't something we have to do every year in soybeans and so it's easy to over-estimate when faced with significant feeding. And, even that 10% of leaf material missing can strain nerves. See Issue 11 or follow this link for defoliation estimates:

http://www.nwroc.umn.edu/Cropping_issues/2007/Issue11/sybn_defol_est.jpg

Remember, at this time of year, Pre Harvest Interval (PHI)is important in your choice of chemistry.  Be aware that the PHI’s on many Organophosphates, such as Lorsban, have been increased and now are in the same neighborhood as the synthetic pyrethroids. For example, in soybean the PHI for Lorsban 4E  is now 28 days and for Dimethoate it's 21 days. 

Soybean aphid populations continue to be tolerable in most fields. Keep monitoring fields until the soybeans reach the R6 stage. Remember that as we advance through the R5 stage, yield response from treating threshold populations of 250 aphids per plant declines. Yield response guidelines mentioned over the years are a response of about 4 to 5 bushels/A in early R5 decreasing to 0 to 2 bushels/A in late R5.

A quick update on a story from Issue 10, July 24, 2007 about the release of the soybean aphid parasite, Binodoxys communis. We received specimens last week and they were released into cages containing soybean plants and lots of soybean aphids. We are monitoring this limited release site to see how well the parasites accept or aphids, looking for mummies as an indication of success (figure 2). We are particular curious about how many generations we can complete at this time. Another aspect of the release will be to move parasites to aphid infested buckthorn to monitor parasitism and survival for fall and winter. Bridging the winter will be crucial to the future success of this beneficial insect.

Soybean Rust continues to spread in Texas and Oklahoma, but any impact these infections could have on our region are becoming a mute point. These infections and their spore dispersal may give insight to long distance dispersal from a more western source of spores.