Objective This paper explores the effects of arbuscular mycorrhizal fungi (AMF) on the resistance of Qingshan poplar (Populus pseudo-cathayana × P. deltoides) to the gypsy moth (Lymantria dispar) larvae and the adaptability of L. dispar larvae to the AMF colonization, so as to provide a scientific basis for the study of insect resistance of Qingshan poplar colonized by AMF.

Method The spongy moth larvae were fed with fresh leaves of Qingshan poplar seedlings, which were treated with Glomus intraradices or G. mosseae or untreated (CK: control). The body mass, length, head capsule width and food utilization of the 3rd, 4th and 5th instars of L. dispar larvae were measured, and the activities of acid phosphatase (ACP) and alkaline phosphatase (AKP) of the 4th and 5th instars of larvae were determined.

Result The body mass of the 3rd/4th instar larvae in GI group was significantly higher than that in the control group, but there was no significant difference in the body mass of the 5th instar larvae between the GI group and the control group. The body length, mass, head capsule width and food utilization of the 5th instar larvae in the GM group were significantly higher than those in the control group. Food intake of the 3rd instar larvae in the GI group was significantly higher than that in the control group, but the reverse was true for the 4th and 5th instars; the food intake of the 5th instar larvae in the GM group was significantly higher than that in the control group, but there was no significant difference in food intake of the 3rd and 4th instars between the GM group and the control group. The ACP activity of the 4th and 5th instars in the GI group was significantly lower than that in the control group, and AKP activity of the 4th instar in the GI group was significantly higher than that in the control group, whereas the reverse was true for the 5th instar. On the other hand, the activities of these two detoxifying enzymes in the GM group were significantly higher than those in the control group.

Conclusion The GI treatment enhances the growth of younger (3rd and 4th instars) L. dispar larvae, but not that of the mature (5th instar) larvae. The GM treatment increases the growth and development of L. dispar larvae, improves their food utilization efficiency, and stimulates their phosphatase activity, indicating that the spongy moth larvae might have a better adaptability to the GM-colonized Qingshan poplar trees. The effects of AMF on trees and herbivorous insects seem to be species-specific. GI colonization doesn’t affect Qingshan poplar’s resistance to the insect, while GM colonization reduces Qingshan poplar’s resistance to the insect.