Enemy-free space maintains swallowtail butterfly host shift

Area of Science:

• Ecology
• Evolutionary Biology
• Insect Behavior

Background:

• Natural enemies drive herbivore mortality and natural selection.
• Herbivores may evolve to utilize enemy-free space, but empirical evidence is limited.
• No studies have documented enemy-free space in natural, non-agricultural host shifts.

Purpose of the Study:

• To investigate the role of enemy-free space in a natural host shift of the Alaskan swallowtail butterfly (Papilio machaon aliaska).
• To determine if predator avoidance influences host plant selection and larval performance.

Main Methods:

• Larval growth and survival of P. m. aliaska were analyzed in field experiments.
• Experiments were conducted on ancestral (Apiaceae) and novel (Asteraceae) host plants.
• Treatments included exposure or protection from predators.

Main Results:

• Larval survival was higher on novel Asteraceae hosts than the ancestral Apiaceae host when predators were present.
• Larval survival and growth were greater on the ancestral Apiaceae host in the absence of predators.
• These findings support the hypothesis of enemy-free space driving host shifts.

Conclusions:

• Enemy-free space is a significant mechanism maintaining naturally occurring host shifts in Papilio machaon aliaska.
• Predator-mediated selection favors the use of novel host plants, even if less optimal in terms of nutrition.
• This study provides empirical support for enemy-free space in a natural ecological context.