Figure 1. Imported longhorned weevil adult. Credit: A. Varenhorst
Imported Longhorned Weevils
While at the Southeast Research Farm last week we noticed small populations of the imported longhorned weevil. Although these beetles are fairly small at approximately ¼” long they can cause serious defoliation to soybean if large enough populations are present within a field. Although these have been a common issue in other states such as Iowa since the early 1990’s, there really aren’t any reports from South Dakota. While these beetles have only been observed in the southeast portion of the state, they are worth watching out for.
As mentioned, the imported longhorned weevils are a small (1/4” long) gray beetles that have pear-shaped bodies (Figure 1). The adult weevils have a mottled tan and white appearance due to short hairs that cover their bodies. Their antennae are fairly long when compared to their small bodies, and are elbowed. Adult weevils can be observed starting in early July and will be present in fields throughout August. The adult beetles do not fly, and are typically more of an issue near the boarders of fields. The imported longhorned weevil larvae are soil dwellers that are C-shaped and legless. The larvae feed on a wide range of plant roots.
The adult imported longhorn weevils feed on the edges of leaves and leave small notches (Figure 2), but are capable of severely defoliating plants. Although they only have a single generation per year, populations can reach very high levels. As mentioned, these populations are generally concentrated near the borders of fields. While scouting for other insects it is possible to come across imported longhorned weevils. With the other defoliators that are being observed in South Dakota, we recommend evaluating the percent of defoliation that is occurring to soybean. Most of the soybean in South Dakota have reached the early reproductive stages and are flowering. At these growth stages that threshold for defoliation is 20%.
Figure 2. Leaf notching defoliation caused by imported longhorned weevils. Credit: A. Varenhorst