HOST PLANT ADAPTATIONS AMONG GEOGRAPHIC POPULATIONS OF THE COLORADO POTATO BEETLE

Intraspecific variation in host plant adaptation of an oligophagous insect is interesting because it signifies adaptive changes among populations of a given species and may play a key role in speciation. The Colorado potato beetle, Leptinotarsa decemlineata (Say), is an oligophagous feeder indigenous to the Americas, where it is widely distributed and infests some ten native and exotic solanaceous plants. The possible existence of host-adapted populations was investigated with four geographically isolated beetle populations collected from Solanum tuberosum and two native hosts, S. rostratum and S. elaeagnifolium. Host plant affinity was compared by rearing progeny from field adults on seven solanaceous hosts to assess growth performance and reproductive success of each population. Considerable differences were recorded among these populations on several hosts. The Arizona beetle is uniquely adapted to its natural host, S. elaeagnifolium, which is a typical unsuitable host for the Utah, New Mexico, and Texas beetles. This population also ranked most adaptable to the seven hosts tested. The Utah beetles were intermediate and the New Mexico and Texas beetles were the least adaptable. The evidence obtained so far strongly suggests that host-adapted populations are developing in North America. The finding implies that the formation of host races or biotypes among oligophagous insects must be preceded initially by geographic isolation of the preferred and non-preferred hosts so as to allow sufficient time for the insect population to develop adaptation to the less preferred hosts. The mechanism differs radically from that of some polyphagous species, in which host race or biotype formation occurs sympatrically without any evidence of geographic isolation. RESUME ADAPTATIONS A LA PLANTE-HOTE CHEZ DES POPULATIONS GEOGRAPHIQUES DE DORYPHORES Une variation intraspecifique dans l'adaptation a la plante-hote d'un insecte oligophage represente des changements adaptatifs chez des populations d'une espece donnee et peut jouer un role essentiel dans la speciation. Le doryphore, Leptinotarsa decemlineata (Say), est un insecte oligophage indigene dans les Ameriques ou il est largement distribue et ou il infeste quelques dix solanacees indigenes ou exotiques. On a etudie l'existence possible de populations adaptees a la plante-hote chez quatre populations geographiquement isolees sur Solanum tuberosum et sur deux hotes indigenes, S. rostratum et S. elaeagnifolium. L'affinite pour la plante-hote a ete comparee en elevant la descendance d'adultes pris sur le terrain sur sept solanacees-hotes pour evaluer la croissance et le succes de la reproduction de chaque population. On a enregistre des differences considerables entre ces populations sur divers hotes. Le doryphore de l'Arizona est adapte uniquement a son hote naturel, S. elaeagnifolium, qui est un hote typiquement inacceptable pour les doryphores de l'Utah, du Nouveau Mexique et du Texas. Cette population etait aussi la plus adaptable de toutes aux sept hotes examines. Les doryphores de l'Utah avaient une position moyenne, tandis que ceux du Nouveau Mexique et du Texas etaient les moins adaptables. Les resultats obtenus jusqu'ici font fortement penser que des populations adaptees a l'hote sont en train de se developper en Amerique du Nord, l'implication etant que la formation de races ou de biotypes adaptes a l'hote chez les insectes oligophages doit etre precedee a l'origine par l'isolement geographique des hotes preferes et non-preferes. Ce mecanisme differe radicalement de celui que l'on trouve chez certaines especes polyphages, chez lesquelles la formation de races ou de biotypes adaptes aux hotes se produit sympatriquement sans qu'il y ait de preuve d'un isolement geographique.