The response of the Asian gypsy moth Lymantria dispar (L.) (Lepidoptera: Lymantriidae) to a fusion gene consisting of the spider, Atrax robustus Simon (Araneae: Hexanthelidae) &amp;#969;-ACTXAr1 sequence coding for an &amp;#969;-atracotoxin and a sequence coding for the Bt-toxin C-peptide, expressed in transgenic poplar Populus simonii x P. nigra L. (Malphigiales: Salicaceae) was investigated. Individual performance, feeding selection, midgut proteinase activity and nutrition utilization were monitored. The growth and development of L. dispar were significantly affected by continually feeding on the transgenic poplar, with the larval instars displaying significantly shorter developmental times than those fed on nontransgenic poplar, but pupation was delayed. Mortality was higher in populations fed transgenic poplar leaves, than for larvae fed nontransgenic poplar leaves. The cumulative mortality during all stages of larvae fed transgenic leaves was 92% compared to 16.7% of larvae on nontransgenic leaves. The highest mortality observed was 71.7% in the last larval instar stage. A two-choice test showed that fifth-instar larvae preferred to feed on nontransgenic leaves at a ratio of 1:1.4. Feeding on transgenic leaves had highly significant negative effects on relative growth of larvae, and the efficiency of conversion of ingested and digested food. Activity of major midgut proteinases was measured using substrates TAME and BTEE showed significant increases in tryptase and chymotrypsinlike activity (9.2- and 9.0-fold, respectively) in fifth-instar larvae fed on transgenic leaves over control. The results suggest that transgenic poplar is resistant to L. dispar, and that the mature L. dispar may be weakened by the transgenic plants due to Bt protoxins activated by elevated midgut proteinase activity. The transgenic poplar expressing fusion protein genes of Bt and a spider insecticidal peptide are good candidates for managing the gypsy moth.