Plant-Mediated Behavioural Avoidance of a Weevil Towards Its Biological Control Agent

Area of Science:

• Agricultural Entomology
• Plant-Insect Interactions
• Evolutionary Biology

Background:

• New Zealand's valuable ryegrass pastures face significant pest damage from the Argentine stem weevil (Listronotus bonariensis).
• Biological control using the parasitoid Microctonus hyperodae has declined, prompting investigation into contributing factors.
• Host plant (grass) influence on weevil behavior and parasitism rates is hypothesized as a key factor.

Purpose of the Study:

• To test the hypothesis that ryegrass species influence Argentine stem weevil avoidance behavior towards its parasitoid, Microctonus hyperodae.
• To quantify weevil on-plant presence and feeding behavior on different ryegrass cultivars in the presence and absence of the parasitoid.
• To correlate observed weevil behavior with reported declines in parasitism rates.

Main Methods:

• Laboratory-based microcosm experiments were conducted using three ryegrass cultivars: diploid hybrid (cv. Manawa), tetraploid Italian (cv. Tama), and diploid perennial (cv. Samson).
• Weevil avoidance behavior was assessed by measuring reduced on-plant presence and feeding in the presence of the parasitoid compared to control groups.
• Behavioral responses were analyzed across different grass types and ploidy levels.

Main Results:

• In the absence of the parasitoid, weevils showed similar high feeding preference for hybrid cv. Manawa and tetraploid cv. Tama, with less feeding on diploid cv. Samson.
• In the presence of the parasitoid, weevils on tetraploid cv. Tama exhibited minimal avoidance, suggesting benefits of remaining on the plant outweighed parasitism risk.
• Conversely, weevils on diploid cv. Manawa displayed strong avoidance behavior when the parasitoid was present, similar to less-preferred diploid cv. Samson.

Conclusions:

• Ryegrass ploidy and cultivar significantly influence Argentine stem weevil avoidance behavior in response to parasitoid pressure.
• The decline in parasitism rates, particularly on diploid ryegrasses, is likely driven by evolved weevil avoidance behaviors.
• Plant nutritional quality and parasitoid-induced selection pressure interact, leading to rapid evolution in pest-parasitoid dynamics and impacting biological control efficacy.