Adaptation to different host plant ages facilitates insect divergence without a host shift

Area of Science:

• Ecology
• Evolutionary Biology
• Entomology

Background:

• Host shifts drive speciation in phytophagous insects.
• Host plant-mediated speciation without host shifts is less understood.

Purpose of the Study:

• Investigate divergence in two sympatric sister elm leaf beetles, Pyrrhalta maculicollis and P. aenescens.
• Examine specialization on different age classes (seedling vs. adult) of the elm host plant, Ulmus pumila.
• Identify mechanisms driving this divergence, such as host plant chemistry and divergent selection.

Main Methods:

• Conducted a field survey of sympatric beetle populations.
• Performed preference and performance trials to assess feeding and oviposition.
• Utilized artificial leaf discs with leaf surface wax extracts to test chemical cues.
• Assessed beetle survival and fecundity on natal host plant age classes.

Main Results:

• Pyrrhalta maculicollis and P. aenescens are highly divergent in feeding and oviposition preferences.
• Beetles specialize on either seedling or adult elm stages, influenced by host plant chemistry.
• Evidence of divergent selection was found, with higher beetle survival and fecundity on their natal host age class.

Conclusions:

• Identified the first probable example of insect divergence driven by host plant age, not host shift.
• Demonstrated that host plant chemistry and divergent selection contribute to specialization.
• Expanded understanding of phytophagous insect diversification mechanisms.