Neurophysiological and behavioral responses of gypsy moth larvae to insect repellents: DEET, IR3535, and picaridin

Abstract

The interactions between insect repellents and the olfactory system have been widely studied, however relatively little is known about the effects of repellents on the gustatory system of insects. In this study, we show that the gustatory receptor neuron (GRN) located in the medial styloconic sensilla on the maxillary palps of gypsy moth larvae, and known to be sensitive to feeding deterrents, also responds to the insect repellents DEET, IR3535, and picaridin. These repellents did not elicit responses in the lateral styloconic sensilla. Moreover, behavioral studies demonstrated that each repellent deterred feeding. This is the first study to show perception of insect repellents by the gustatory system of a lepidopteran larva and suggests that detection of a range of bitter or aversive compounds may be a broadly conserved feature among insects.

Figure 1. Dose response curves (in percent mean consumption) to increasing concentrations of (A) IR3535, (B) picaridin, and (C) DEET.

Asterisks denote the concentration of repellent that significantly decreased feeding relative to the control. Vertical bars represent standard errors.

Figure 2. Representative traces of responses elicited from gustatory receptor neurons contained in the medial and lateral styloconic sensilla of Lymantria dispar larvae to various stimuli.

(A) 30 mM KCl (lateral), (B) 10 mM Picaridin (lateral), (C) 30 mM KCl (medial), (D) 10 mM DEET (Medial), (E) 10 mM IR3535 (Medial), (F) 10 mM Picaridin (Medial) (G) a mixture of 10 mM DEET, IR3535, and picaridin, (H) 10 mM caffeine, and (I) a mixture of 10 mM caffeine and 10 mM each of DEET, IR3535, and picaridin. Stimulations of lateral styloconic sensilla with the three repellents, picaridin (shown above in B), IR3535, or DEET did not elicit responses from the gustatory receptor neurons. Up-arrowheads represent the response of the large-amplitude deterrent-sensitive neuron and bars represent the response of the small-amplitude KCl-sensitive neuron.

Figure 3. Total number of impulses per second produced in the interval 0.05–1.05 s after initial stimulation of the medial styloconic sensilla in L. dispar by the deterrent-sensitive neuron to 10 mM DEET, IR3535, and picaridin compared with the control (30 mM KCl in 10% ethanol).

Different letters represent significant differences between groups. Vertical bars represent standard errors.

Figure 4

A. Representative traces of responses elicited by the deterrent-sensitive neuron in the medial styloconic sensilla to increasing concentrations of (A) IR3535 (B) Picaridin and (C) DEET. Up-arrowheads represent the response of the large-amplitude deterrent-sensitive neuron.

Figure 5. Dose-response curves constructed from numbers of impulses per second in the interval 0.05–1.05 s after initial stimulation by the deterrent-sensitive neuron elicited by increasing concentrations of (A) IR3535, (B) picaridin, and (C) DEET.

An asterisk indicates the concentration at which a significant increase in the number of impulses occurred.

Figure 6. Temporal dynamics of the response of the deterrent-sensitive neuron to 10 mM concentrations of (A) IR3535, (B) picaridin, and (C) DEET.

All repellents elicited a phasic-tonic firing pattern which peaked within the first 100–200 ms following stimulus onset and gradually decreased over the next 800 ms for picaridin and 1000 ms for both DEET and IR3535. A tonic rate of activity (marked by an asterisk) occurred after 1200 ms for picaridin and 1300 ms for DEET and IR3535.