Substrate vibrations elicit defensive behaviour in leafminer pupae

Late instar larvae and pupae of the spotted tentiform leafminer Phyllonorycter malella (Ger.) (Lepidoptera: Gracillariidae) react with defensive behaviour when attacked by one of their parasitoids, the eulophid wasp Sympiesis sericeicornis Nees (Hymenoptera: Eulophidae). Vibrations produced during the insertion of the ovipositor into the mine are known to be important cues by which larvae detect the presence of their enemies. The aim of this study was to investigate which frequency components elicit defensive reactions in leafminer pupae using synthetic vibrations. Sine vibrations and bandlimited noise stimuli were offered to both free pupae and pupae concealed in their leafmines. Using Laser vibrometry we measured the vibrations experienced by pupae inside their mines and assessed the influence of the mine. Pupae were shown to react to substrate vibrations, and do so over a broad range of frequencies. Behavioural reactions to noise stimuli were stronger than to pure sine stimuli. Mine tissue attenuated vibration amplitudes of the input signal from 5.1 to 22.6dB. However, as response thresholds of concealed pupae were only twice as high as thresholds of free pupae (which is adequate to 3dB) pupae inside their mine were more sensitive than expected. This discrepancy is discussed both in terms of the conditions of pupae and in terms of mine structure. The results indicate that broadbandedness of vibrations produced by hunting parasitoids during ovipositor insertion into the mine may be a major criterion used by leafminers to perceive parasitoid presence and to escape ovipositor stings.