Update on Insect Pests
Posted: July 20, 2016
Two-spotted spider mites
The extended period of dry weather that many parts of Pennsylvania have endured has led to some local outbreaks of two-spotted spider mite in soybeans, and occasionally corn. Spider mites become a almost yearly mid-summer problem when conditions get hot and dry. When mites are present and rain continues not to come, it is often time to act. While there are many insecticides available in corn or soybeans, few are labeled and effective against spider mites; the choices are limited to products containing chlorpyrifos, dimethoate, or bifenthrin. Unfortunately, spider-mite populations vary in their susceptibility to these toxins, particularly dimethoate and chlorpyrifos. Some populations are easily killed by these products whereas others are more difficult to manage. This variation has been recognized in some parts of the country for a while, but may also apply to Pennsylvania. In past years, we have heard of dimethoate applications in Southeastern PA that seemed to have no effect on two-spotted spider mites. We would be curious to hear from readers of other instances where they feel that these products were not effective, so please drop us an email or give us a call. If you encounter a population that does not respond to dimethoate, you will need to consider one of the two other effective products.
Potato leafhopper populations have been large in some locations and damage to alfalfa fields (i.e., hopperburn) can be found without too much trouble. As a result, we have heard of many instances of growers protecting their fields with insecticides. Growers would be wise to continue to keep an eye on their fields in the coming weeks, particularly in fields that were recently cut and the crop is beginning to re-grow. Recall that potato leafhopper populations can be very spotty, so get out your sweep nets and scout those fields to check your local populations. The Penn State Extension potato leaf hopper factsheet provides more details on this pest, scouting protocols, and economic thresholds.
Soybean aphid is active in Pennsylvania soybean fields, but their numbers appear to be mostly low. In commercial fields in Centre and Union County, we are seeing small, scattered populations. These populations are insignificant and certainly not worth managing, but we have heard of much larger populations in New York. The economic threshold for soybean aphid is 250 aphids per plant across an entire field, so if populations do not reach this level, there is no need to control their populations. Nevertheless, farmers can get pressure from various people to manage even these small populations. A group of Extension entomologists in Northern states (including me!) recently collaborated to write a detailed article explaining the 250 aphid per plant threshold, how it relates to soybean and soybean aphid biology, and why treating below this level usually wastes time and resources. University of Minnesota article is currently posted on their website.
Western Bean Cutworm
For the past seven years, Penn State has led an effort in the state to trap for Western bean cutworm, a pest of corn and dry beans. Fortunately for growers in Pennsylvania, populations of caterpillars have been very weak across most of the state (we have only found a handful), so we choose not to continue our trapping effort for the moths into 2016. Nevertheless, if you are a corn grower in the northern part of PA, we encourage you to scout your fields for this pest, and tune into pest updates from Ohio State and New York State IPM to learn about their populations and experience with this pest species.