NUTRITIONAL INDEXES IN THE GYPSY-MOTH (LYMANTRIA-DISPAR (L)) UNDER FIELD CONDITIONS AND HOST SWITCHING SITUATIONS

A large proportion of gypsy moths (Lymantria dispar (L.)) are likely t o experience multiple species diets in the field due to natural wander ing and host switching which occurs with these insects. Nutritional in dices in fourth and fifth instar gypsy moth larvae were studied in the field for insects that were switched to a second host species when th ey were fourth instars. The tree species used as hosts were northern p in oak (Quercus ellipsoidalis E. J. Hill), white oak (Q. alba L.), big -tooth aspen (Populus grandidentata Michx.), and trembling aspen (P. t remuloides Michx.). Conclusions of this study include: 1) Insects whic h fed before the host switch on northern pin oak performed better afte r the host switch than did insects with other types of early dietary e xperience. While the northern pin oak-started insects had very low rel ative food consumption rates on their second host species immediately after the switch, one instar later they had the highest ranked consump tion rates. During both instars they had the second highest efficienci es of converting ingested and digested food to body mass. High food co nsumption rates and relatively high efficiency of food conversion help ed these insects to obtain the highest ranked mean relative growth rat es in the fifth instar compared to the relative growth rates obtained by insects from any of the other first host species. 2) Among the four host species examined, a second host of trembling aspen was most adva ntageous for the insects. Feeding on this species after the switch led to higher larval weights and higher relative growth rates for insects than did any of the other second host species. The insects on trembli ng aspen attained excellent growth despite only mediocre to low food c onversion efficiencies. The low efficiencies were offset by high relat ive food consumption rates. 3) Low food consumption rates often tend t o be paired with high efficiency of conversion and vice versa. 4) Ther e is no discernable tendency for the first plant species eaten to caus e long-term inductions which affect the ability of gypsy moths to util ize subsequent host plants. Insects did not tend to consume more, grow faster, or be more efficient if their second host plant was either th e same as their rearing plant or congeneric to it. Methods are delinea ted which allow values of nutritional indices to be obtained for insec ts on intact host plants under field conditions. These methods are use ful for the purpose of answering questions about the relative effects that different diet treatments have on insect response.