Suitability of the defoliating beetle Physonota maculiventris (Coleoptera: Chrysomelidae) for release against Tithonia diversifolia (Hemsl.) A. Gray (Asteraceae) in South Africa.

Abstract

This study was conducted to assess the suitability of the defoliating beetle Physonota maculiventris Boheman (Coleoptera: Chrysomelidae: Cassidinae) for release as a biological control agent against Mexican sunflower, Tithonia diversifolia (Hemsl.) A. Gray (Asteraceae), in South Africa. The biology and host range as well as the potential impact and distribution of P. maculiventris were studied under quarantine conditions to determine its safety and effectiveness. Under favourable conditions, females laid 5.3 ± 0.3 (mean ± SE) egg batches during their lifetime, with each batch consisting of approximately 33 eggs. Larvae are highly gregarious as early instars and both larvae and adults feed voraciously, often defoliating the plants completely. The life cycle of the beetle was completed in 67.5 ± 7.5 days under quarantine conditions. Among the 58 test plant species subjected to no-choice tests, P. maculiventris developed successfully on T. diversifolia but on very few non-target species. However, only minor damage was recorded on non-target species, notably the exotic weed Xanthium strumarium L. and some sunflower (Helianthus annuus L.) cultivars. Also, survival to adulthood was considerably lower on sunflower cultivars than on the target weed during these tests. During choice tests, P. maculiventris oviposited and developed successfully on T. diversifolia only, with minor feeding damage on some H. annuus cultivars, suggesting that the beetle’s field host range will be confined to the target weed. Risk analysis also showed that P. maculiventris presents an extremely low risk to non-target plant species, notably those within the tribe Heliantheae and other close relatives. The effectiveness of P. maculiventris was assessed on the basis of its impact on the growth and biomass production of the weed. Significant foliar damage by the adult and larval stages of P. maculiventris was recorded at low and high insect densities, causing a 50.2 % and 55.0 % reduction in plant biomass, respectively. Climatic modelling (CLIMEX) suggested that the beetle is likely to establish over the entire range of T. diversifolia in South Africa and neighbouring countries. The study concludes that P. maculiventris is safe for release and is likely to become widely established and cause significant damage to populations of T. diversifolia in South Africa. An application to release P. maculiventris into the field is thus being prepared for submission to the relevant South African regulatory authorities.