Induced plant defenses, host-pathogen interactions, and forest insect outbreaks

Area of Science:

• Forest ecology
• Insect population dynamics
• Chemical ecology

Background:

• Cyclic outbreaks of forest defoliating insects are poorly understood.
• Natural enemy models are commonly used but fail to explain certain outbreak patterns, like those of gypsy moths in North America.
• Induced defenses are often overlooked due to weak toxic effects and limited explanatory power in simple models.

Purpose of the Study:

• To investigate the role of induced defenses in explaining variable gypsy moth outbreak severity in North American forests.
• To test the hypothesis that an interaction between induced defenses and natural enemies drives observed outbreak patterns.
• To develop and validate a model incorporating spatial variability in induced defenses.

Main Methods:

• Experimental induction of hydrolyzable-tannin defenses in red oak trees.
• Assessing the impact of induced defenses on gypsy moth baculovirus infection risk.
• Developing an extended natural-enemy model that incorporates spatial variation in tree inducibility.

Main Results:

• Experimental induction of oak defenses reduced variability in gypsy moth baculovirus infection risk.
• The extended model accurately reproduced observed patterns of alternating severe and mild outbreaks in oak-rich forests and uniform moderate outbreaks in forests with fewer oaks.
• Oaks were identified as the only gypsy moth host genus capable of induced defenses.

Conclusions:

• The interaction between induced plant defenses and natural enemies is crucial for understanding complex insect outbreak dynamics.
• Induced defenses in oak trees can modulate pathogen transmission, influencing the severity and pattern of gypsy moth outbreaks.
• Future research should combine experimental and modeling approaches to detect the effects of induced defenses on insect cycles.