The Effect of Rainfall on the Abundance of Tarnished Plant Bug Nymphs [Lygus lineolaris (Palisot)] in Alfalfa Fields

Abstract

Rainfall has generally been overlooked as a regulator of insect population size, especially for mirids (Hemiptera). During a 13-year field study in northwestern New Jersey, total May-June rainfall ranged from 6.5 to 10.7 inches (165–272mm). The highest rainfall reduced first generation nymphs of the tarnished plant bug, Lygus lineolaris (Palisot) (Miridae), in alfalfa by 50%. This reduction was likely mechanical (raindrops knocking the mirids off the plants) rather than biotic, because infections by nematodes and a fungus disease were not significantly increased. The rainfall-reduced mirid populations also had less mortality by the introduced parasite Peristenus digoneutis Loan (Braconidae), probably a result of the lower host density (density dependence). Conversely, in low-rainfall years there were more tarnished plant bugs, and parasitism rates were doubled, so the two mortality factors were complimentary. Rainfall may also be responsible for differences in lygus numbers between regions. Lygus bugs were five times more abundant in Idaho alfalfa than in New Jersey alfalfa, despite considerably higher parasitism rates in Idaho. Perhaps the lower lygus numbers in New Jersey were caused by the nearly four times higher rainfall there.