Tithonia rotundifolia (Mill.) S.F. Blake (Asteraceae) is among the three Tithonia species from Mexico that are invasive in South Africa. To curb its invasiveness and negative impact in South Africa, two chrysomelid beetles, Zygogramma signatipennis (Stål) and Zygogramma piceicollis (Stål), from Mexico were investigated as candidate biological control agents. The life histories and host ranges of these beetles were studied under laboratory conditions to determine their suitability for release. The two beetle species displayed very similar life histories, including a short pre-oviposition period (13-14 days), incubation period (4-5 days) and lifecycle (40-45 days). The longevity of Z. signatipennis and Z. piceicollis was 113 and 125 days, while their fecundities were 1146 and 1133 eggs per female, respectively. Feeding, oviposition and development of both beetle species were confined within the tribe Heliantheae, but showed a very strong preference for the invasive T. rotundifolia. The only non-target species that supported development to adulthood was the exotic weed Tithonia diversifolia (Hemsl.) A. Gray, itself a target for biocontrol. Although some sunflower (Helianthus annuus L.) cultivars were partially utilized during host-specificity tests, none supported complete development, suggesting that both Zygogramma species are suitable for release in South Africa.
Keywords: Zygogramma piceicollis; Zygogramma signatipennis; biology; fecundity; host specificity; weed biocontrol.