Tarnished plant bugs are the #1 insect pest of cotton in Tennessee. In the fall of the year, tarnished plant bug (TPB) nymphs develop into adults on weedy hosts, and these adults overwinter on these hosts or plant debris. Previous studies have found that overwintered populations of tarnished plant bugs emerge from diapause in late winter or early spring, triggered by a combination of cues including a good food source and warm temperatures. In Tennessee, we have found tarnished plant bugs during the early spring on shepherd’s-purse, henbit, purple deadnettle, daisy fleabane, ranunculus, and other weedy hosts. The earlier it warms up, the faster these hosts have blooms or fruit for plant bugs to feed on, and the earlier they will feed, mate, and produce the first generation of the year.
In the spring of 2015, we began to participate in a regional protocol developed by Dr. Katharine Parys (USDA-ARS in Stoneville, MS). The purpose of this study is to develop degree day predictions as to when tarnished plant bugs will break winter dormancy and begin reproduction in the spring. We do this by taking sweep net samples on weedy hosts and observing when we begin to find overwintering TPB adults and nymphs. TPB have 5 instar (nymph or immature) stages, and we document what instar are present in our samples. When they reach the fifth and final instar, we know that overwintering adults have reproduced and the offspring have developed through all immature stages.
In 2015, we began finding a few overwintered adult TPB in late March and small nymphs by April 13. Last year, we found overwintering adults on February 29 and a fifth instar nymph on April 7. Thus, we found large nymphs in 2016 on dates when only overwintering adults were found in 2015. If you recall the late winter of 2016 was warm, with the daytime temperatures in February and March 2016 averaged 11 and 5.7 degrees warmer, respectively, than 2015.
This year, tarnished plant bugs are off to an even earlier start. We found overwintering adults February 17, and several fifth instar nymphs yesterday (March 29). This means that the first batch of offspring have passed through all their immature life stages and the first generation is almost complete. On average, January and February 2017 were substantially warmer than those months in 2016 by 8.8 and 7.7 degrees, respectively, although March temperatures were a couple degrees cooler. These warmer temperatures allowed many winter weeds to flower, and overwintering TPB broke their dormancy even earlier this spring. Depending on future weather and the number of cotton acres planted, this may have implications for how many plant bugs we see in our 2017 cotton crop.
Snodgrass, G. 2014. Highlights of 30 years of Research on TPB in the Mississippi Delta. Midsouth Entomologist. 7(2)38-49.