Bottom-up and top-down pressures mediate competition between two generalist insects.

Area of Science:

• Ecology
• Insect Ecology
• Community Ecology

Background:

• Dietary generalist insects are often predicted to experience weaker competition due to broad host plant use.
• However, indirect competition can be significant and difficult to avoid, even for generalists.
• Competition among generalist insects, especially when temporally separated, is understudied.

Purpose of the Study:

• To investigate indirect competitive interactions between two temporally separated generalist insects: western tent caterpillar (Malacosoma californicum) and fall webworm (Hyphantria cunea).
• To assess the impact of these interactions on insect fitness through bottom-up (leaf quality) and top-down (natural enemies) effects.
• To determine if competition is as strong in generalists as in specialists, challenging previous assumptions.

Main Methods:

• Laboratory-rearing trials using chokecherry host plants with varying levels of prior damage from either insect species.
• Measurement of key fitness parameters: development time, pupal mass, and survival.
• Field trials assessing fall webworm fitness on previously damaged host plants, with and without predator exclusion.

Main Results:

• Western tent caterpillars exhibited reduced pupal mass when reared on plants previously damaged by fall webworms (bottom-up effect).
• Fall webworms showed reduced pupal mass and increased development time when reared on plants damaged by western tent caterpillars (bottom-up effect).
• In field trials, fall webworms experienced lower survival and pupal mass on plants damaged by western tent caterpillars, indicating both bottom-up and top-down effects.

Conclusions:

• Indirect competition occurs between temporally separated generalist insects, western tent caterpillars and fall webworms.
• Competition impacts insect fitness via changes in host plant quality (bottom-up) and predator interactions (top-down).
• These findings highlight the significant ecological consequences of indirect competition, even among species with broad dietary niches.