# Pine Response to Sawfly Pheromones: Effects on Sawfly’s Oviposition and Larval Growth

## TL;DR

Pine trees exposed to pine sawfly pheromones are less likely to be chosen for egg-laying, but this does not affect larval growth.

## Contribution

The study reveals that pine sawfly females avoid oviposition on pheromone-exposed trees, but larval performance remains unaffected.

## Key findings

- Pine sawfly females avoid oviposition on pheromone-exposed pine needles when given tactile access.
- Pheromone exposure does not affect larval performance or pupation success on pine trees.
- Pheromone-exposed trees show enhanced defenses against sawfly eggs but not larvae.

## Abstract

Mass outbreaks of the pine sawfly Diprion pini can cause severe damage to pine forests. The larvae of this herbivorous insect feed selectively on the needles of pine trees, notably Pinus sylvestris. During mass outbreak periods, the females release large amounts of sex pheromones. A prior study revealed that the survival rate of sawfly eggs laid on pheromone-exposed pine needles was lower than that of eggs on unexposed pine. In our study, we found that D. pini females avoided oviposition on pheromone-exposed pine, possibly as a counter-adaptation to the enhanced defenses of previously pheromone-exposed trees against sawfly eggs. The females only discriminated between pheromone-exposed and unexposed trees when they had the chance to touch the needles, but not when exposed to the odor of these types of trees. However, the performance of larvae did not significantly differ on pheromone-exposed and unexposed trees. These results underscore the complexity of the chemical ecology of sawfly–pine interactions and highlight the nuanced roles that pheromones play in shaping the relationships between herbivores and their host plants.

Insect pheromones have been intensively studied with respect to their role in insect communication. However, scarce knowledge is available on the impact of pheromones on plant responses, and how these in turn affect herbivorous insects. A previous study showed that exposure of pine (Pinus sylvestris) to the sex pheromones of the pine sawfly Diprion pini results in enhanced defenses against the eggs of this sawfly; the egg survival rate on pheromone-exposed pine needles was lower than that on unexposed pine. The long-lasting common evolutionary history of D. pini and P. sylvestris suggests that D. pini has developed counter-adaptations to these pine responses. Here, we investigated by behavioral assays how D. pini copes with the defenses of pheromone-exposed pine. The sawfly females did not discriminate between the odor of pheromone-exposed and unexposed pine. However, when they had the chance to contact the trees, more unexposed than pheromone-exposed trees received eggs. The exposure of pine to the pheromones did not affect the performance of larvae and their pupation success. Our findings indicate that the effects that responses of pine to D. pini sex pheromones exert on the sawfly eggs and sawfly oviposition behavior do not extend to effects on the larvae.