Effects of fertilizer and low rates of imidacloprid on hemlock woolly adelgid, Adelges tsugae (Adelgidae)

The main focus of this study was to understand how to maintain healthy trees and adelgid, Adelges tsugae Annand (Adelgidae) populations so predators, Laricobius nigrinus Fender (Derodontidae) and Sasajiscymnus tsugae (Sasaji & McClure) (Coccinellidae) have sufficient food to maintain population growth. Sixty eastern hemlock trees between 15.2 and 38.1 cm diameter at breast height were selected in the Chattahoochee National Forest. Trees were treated with 0, 10 or 25% of 1.5g imidacloprid (Merit 75WP) per 2.5 cm of tree dbh and were either fertilized or not, in a factorial design. After two years, more ovisacs and eggs were found on trees which received imidacloprid in the order 0% > 10% > 25%. Fertilized trees had greater adelgid fecundity and significant positive correlation with the total nitrogen in both the winter generations (February 2008 and 2009). Although, higher total nitrogen was observed on fertilized trees receiving 10% pesticide compared to unfertilized ones, adelgid fecundity was not higher in February 2009 samples. In addition, nitrogen content was higher in the pesticide treated trees than in untreated trees regardless of fertilizer in June 2009 samples. Trees treated with low rates of imidacloprid were healthier but only trees treated with the lowest rate had sufficient adelgids to possibly sustain predators over extended periods of time.