The host-plant relationships of Paropsis charybdis stal (Coleoptera: Chrysomelidae)

Abstract

Paropsis charybdis Stal, the eucalypt tortoise beetle, is an Australian chrysomelid accidentally introduced into New Zealand early this century. Since then it has caused considerable damage to various species of Eucalyptus growing in this country. Both the larvae and adults of this insect species are defoliators, feeding on the young leaves and shoots of the host plant. Paropsis was at first of less concern than the scale Eriococcus coriaceus Maskell and the weevil Gonipterus scutellatus Bois-Duval but, with the introduction of parasites to suppress these two and the gradual spread of Paropsis throughout New Zealand, it became the chief pest of eucalypts in the country. The eucalypts are not at present a major component of forests in New Zealand although with the necessity to diversify away from a virtual Pinus monoculture and to provide for specialist products derived from hardwood and hardwood pulp, they are a group of trees with considerable potential value. In the Canterbury region where the present work was undertaken there are no large forest plantings of any Eucalyptus species but there are many of these trees in amenity plantings which range from fairly large farm forestry areas (especially shelterbelts) down to single trees used for shade and shelter or as ornamentals. Many of those trees in agricultural areas have resulted from early plantings, being survivors, seedlings or regrowths. However, in order to determine the effectiveness of the various silvicultural methods proposed to alleviate the damage caused by Paropsis, more information was required about the host-plant relationships of this insect. The data on host range present in the literature tended to be vague and confusing because they were derived solely from observations of damage, but they did indicate that this was a line of inquiry worth pursuing. The present research was undertaken in response to this need to know more about the interactions between Paropsis charybdis and its host plants. A twofold approach has been adopted; the presence of antibiotic or antifeeding factors in various Eucalyptus species has been investigated in two series of laboratory rearings, while in several series of feeding trials the ability of the larvae to discriminate between different eucalypt foliages has been checked. An attempt was also made to determine whether any of the chemicals known to be present in eucalypts could be used as an attractant for adult Paropsis.