Aggregative oviposition behavior of viburnum leaf beetle and plant defenses of North American and European viburnums

Viburnum leaf beetle [Pyrrhalta viburni], is a relatively new invasive chrysomelid causing extensive loss of several Viburnum species in the Northeast U.S and Canada. P. viburni females prefer to oviposit on twigs previously infested by other females. We hypothesized that this aggregative oviposition behavior helped P. viburni overcome plant wound response to oviposition (=plant defenses). We investigated 1) the impact of plant defenses on P. viburni larval emergence and 2) the relation between aggregative oviposition and plant defenses, comparing several North American (V. dentatum, V. rafinesquianum, V. trilobum) and European (V. tinus) viburnums: 1) There was a significant negative correlation between plant defenses and larval emergence for all species tested. 2) We found a positive correlation between number of egg masses and twig mortality and a negative correlation between number of egg masses and amount of plant defenses for all the North American viburnum species tested. On V. tinus, we found no correlation between number of egg masses and twig mortality, and a positive correlation between number of egg masses and plant defenses. We have shown that plant defenses against oviposition have a negative effect on egg survival but that viburnum leaf beetles can overcome plant defenses due to their aggregative oviposition behavior. Induced increased defenses to oviposition observed in V. tinus suggests that European viburnums might have evolved to counteract P. viburni aggregative oviposition behavior.