In Delaware, Dectes stem borer has been a pest of soybeans since the early 1980s. From all of the work done in Delaware, Kansas and Nebraska over the last 30 plus years, we do know that the most severe losses occur in fields that are not rotated i.e. continuous soybean production. Since the fall of 2013, we have seen an increase in lodging losses from Dectes stem borer larvae (Figure 1). Although some have asked if we are seeing an expansion in the range of infestation in the Mid-Atlantic, we feel it is more related to a combination of the following factors: (a) an increase in full season soybean acres, (b) producer’s inability to harvest in a timely manner, (c) a return to continuous soybeans, and (d) no-till crop production. In many cases, adult beetles can be found at varying levels in many fields but in certain years, especially under drought stress and with delayed fall harvest, we see and hear about more lodging losses. Although losses have been most common and severe in full season soybeans, we can see lodging in double crop soybeans, especially in soybeans planted behind barley.
Since the early 1980s, there has been a great deal of work in Delaware and Kansas on the management of this insect pest. In Delaware, we have worked on numerous management strategies including at planting insecticides, timing of foliar insecticides, evaluation of row spacing, and timing of harvest and variety selection. Due to the long egg laying period of this insect (late June through August), timing a single foliar spray has been difficult and has not always resulted in a reduction in lodging losses. The only developmental stage that can be targeted for control is the adult beetle. Reductions in beetle populations can be achieved with insecticide applications; however, because adult emergence occurs over a large window, fields can be quickly re-infested. Although multiple applications have looked promising in some years, research results in Delaware and other areas of the US have shown mixed results with multiple insecticide applications. Research from Kansas has indicated that adult control is not always highly correlated with levels of larval infestation at the end of the season.
Figure 1. Stem Lodging
In 2015, Delaware’s IPM program worked with one grower to time insecticide applications and using two application approach we did see some promising results. In 2016, the Delaware Soybean Board is again supporting work on Dectes stem borer. As a team, we are once again looking at the use of foliar sprays. We are evaluating 3 timings reported in replicated research trials and the popular literature so we will have more information this winter. We have surveyed 15 fields in Kent and Sussex counties, some with treated and untreated areas. As is typical with this insect, we saw different patterns of adult emergence and will begin stem sampling in late September so we will keep you posted on the results.
So what is currently happening with Dectes stem borer in soybeans?
We have seen in some of our survey fields and have been receiving reports of flagging of trifoliate leaves/petioles (Figures 2 and 3). This is an indication that female beetles have already laid their eggs into the petiole and larvae hatching from these eggs have already damaged the petiole (Figure 4) and started to move to the main stems (Figure 5). Therefore, insecticide applications at this time will not be effective. Your main option for control at this time would be early harvest. It will be important to start checking your fields for plants with stem infestations (Figure 5) and if you find larvae in the stems, plan to harvest those fields first.
Figure 2. Wilted Leaves/Petiole
Figure 3. Wilted Leaves/Petiole
Figure 4. Damaged Petiole
Figure 5. Infested Stem
For more information on the identification and biology of Dectes Stem Borer as well as possible cultural control strategies please see the following links: