EFFECTS OF HOST SWITCHING ON GYPSY-MOTH (LYMANTRIA-DISPAR (L)) UNDER FIELD CONDITIONS

Effects of various single and two species diets on the performance of gypsy moth (Lymantria dispar (L.)) were studied when this insect was r eared from hatch to pupation on intact host trees in the field. The tr ee species used for this study were red oak (Quercus rubra L.), white oak (Q. alba L.), bigtooth aspen (Populus grandidentata Michaux), and trembling aspen (P. tremuloides Michaux). These are commonly available host trees in the Lake States region. The study spanned two years and was performed at two different field sites in central Michigan. Concl usions drawn from this study include: (1) Large differences in gypsy m oth growth and survival can occur even among diet sequences composed o f favorable host species. (2) Larvae that spent their first two weeks feeding on red oak performed better during this time period than larva e on all other host species in terms of mean weight, mean relative gro wth rate (RGR), and mean level of larval development, while larvae on a first host of bigtooth aspen were ranked lowest in terms of mean wei ght, RGR, and level of larval development. (3) Combination diets do no t seem to be inherently better or worse than diets composed of only a single species; rather, insect performance was affected by the types o f host species eaten and the time during larval development that these host species were consumed instead of whether larvae ate single speci es diets or mixed species diets. (4) In diets composed of two host spe cies, measures of gypsy moth performance are affected to different ext ents in the latter part of the season by the two different hosts; larv al weights and development rates show continued effects of the first h ost fed upon while RGRs, mortality, and pupal weights are affected str ongly by the second host type eaten. (5) Of the diets investigated in this study, early feeding on red oak followed by later feeding on an a spen, particularly trembling aspen, is most beneficial to insects in t erms of attaining high levels of performance throughout their lives.