Oviposition by Pyrrhalta Viburni (Paykull) on Dead Plant Material: Successful Reproductive Strategy Or Maladaptive Behavior?1
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Oviposition by Pyrrhalta viburni (Paykull) on dead plant material: successful reproductive strategy or maladaptive behavior?1 Gaylord A. Desurmont2, Charissa M. Fritzen3, and Paul A. Weston4 Abstract. Viburnum leaf beetle, Pyrrhalta viburni (Paykull, 1799), is a Eurasian chrysomelid recently introduced to North America, where it has become a major landscape pest. P. v ibur ni deposits eggs in the terminal twigs of infested viburnum shrubs, in small cavities that are then covered by the female with a frass-like secre- tion. In the fi eld, fresh P. v ibur ni egg masses are sometimes laid on dead plant mate- rial, prompting the current study to investigate the frequency and proximate causes of this behavior. In the fi eld, P. v ibur ni females were found to lay signifi cantly more eggs on live twigs than on dead twigs, and to lay more eggs on dead twigs that had been infested the previous growing season and contained remains of old egg masses than on non-infested dead twigs. In laboratory choice-tests, females laid signifi cantly more eggs on dead twigs if they contained remains of old egg masses, but did not show preferences between young and old infested twigs. We conclude that the pres- ence of remains of egg masses deposited the previous growing season is stimulatory to P. v ibur ni females and triggers oviposition on dead plant material. Keywords. Pyrrhalta viburni, oviposition behavior, insect-plant interactions, repro- ductive strategy, site selection. 1. Introduction Viburnum leaf beetle, Pyrrhalta viburni (Paykull, 1799), is a chrysomelid belonging to the subfamily Galerucinae and the tribe Galerucini. Native to Eurasia, P. v ibur ni 1 Paper presented to the 7th International Symposium on the Chrysomelidae, held at Durban (South Africa), July 9, 2008. 2 Department of Entomology, Cornell University, Ithaca, NY 14850 (USA). gd53@cornell. edu, [email protected] 3 Current address: 155 Plymouth Drive Syracuse, NY 13206. [email protected] 4 Department of Entomology, Cornell University, Ithaca, NY 14850 (USA), current address: School of Agricultural and Wine Sciences, Charles Sturt University, Wagga Wagga, NSW 2650 (Australia). [email protected] Research on Chrysomelidae, Volume 2, pp. 119–129, © Koninklijke Brill, Leiden, 2009 120 Gaylord A. Desurmont, Charissa M. Fritzen, and Paul A. Weston was accidentally introduced to North America where it has become a major pest of native and exotic viburnum shrubs in nurseries, managed landscapes, and natural habitats. Although early records indicate that the beetle was present in Canada as early as 1924 (Majka & LeSage 2007), it was not observed causing serious damage until 1978 when it was found in Ottawa, Ontario and Hull, Quebec (Canada), de- foliating Viburnum opulus (L.) shrubs (Becker 1979). Observed in the USA for the fi rst time in Maine in 1994, P. v ibur ni has spread in all New England states, New York, Pennsylvania, and Ohio (Weston & Hoebeke 2003). A separate introduction has occurred on the ywest coast of North America, where the beetle has been re- ported in British Columbia (Canada), and the state of Washington (USA) (Weston et al. 2008). Despite a recent rise in interest due to its pest status in North America, very little has been published on the biology of P. v ibur ni . Th e most extensive studies existing on the subject are the works of Zorin (1931), Lühmann (1934), Balachowsky (1963), and Weston et al (2007). One of the most remarkable features of the biol- ogy of P. v ibur ni is its distinctive oviposition behavior. Females lay their eggs in cavities that they excavate in the terminal twigs of susceptible shrubs. Th e female chews away the bark and hollows out the egg cavity in the pith (Hilker 1992) be- fore laying an average of 8 eggs per egg cavity (Weston et al. 2008). Th e egg cavity is then closed by a ‘cap’ made of a frass-like secretion produced by the female. It can take up to a few hours for a female to make a complete egg mass (egg cavity + eggs + cap), chewing the cavity being the most time-consuming step of the proc- ess (G.A.D, personal observation). P. v ibur ni females can produce 1–3 egg masses per day (G.A.D, unpublished), and 75 on average over their lifetime (Weston et al. 2008). By concealing their eggs inside twigs, P. v ibur ni females protect them from most potential natural enemies. Th e cap also sponges and stores moisture, thereby pro- viding humidity for the eggs. Th is is especially important since P. v ibur ni over- winters as eggs, and the eggs have to go through an obligate period of diapause of several months in the fi eld before hatching in the spring (Weston & Diaz 2005). It was recently discovered that P. v ibur ni females prefer to lay eggs on twigs already infested with egg masses of conspecifi cs, and position their egg masses ad- jacent to existing masses (G.A.D, unpublished). Th is aggregation behavior leads to the formation of clusters of egg masses linearly arranged along terminal twigs of in- fested shrubs. Th e main benefi t of this behavior is thought to be overcoming plant defenses. Indeed, it has been observed that plants can respond to the wounding caused by P. v ibur ni oviposition by producing undiff erentiated tissue at the wound- ing site, forming a small mass of wound tissue that either overgrows the egg mass or squeezes the eggs out of the egg cavity. When multiple egg masses are laid along a twig, vascular tissues are eventually disrupted and the twig dies, disabling its capac- ity to produce wound tissue. Twigs killed by repeated oviposition are commonly observed in the fi eld on most viburnum species attacked by P. v ibur ni ..