Seasonal abundance and host-plant affinities of Longitarsus basutoensis and related flea beetles associated with the invasive Senecio madagascariensis in their native range in KwaZulu-Natal, South Africa

Abstract

Senecio madagascariensis Poiret (Asteraceae) is a target for biological control in Australia and Hawaii, with several candidate insect agents recorded in KwaZulu-Natal, South Africa, the confirmed origin of the invasive populations. Although the root-feeding Longitarsus basutoensis Bechyné (Chrysomelidae: Galerucinae: Alticini) was prioritised for assessment, similar flea beetle taxa were also considered. We first investigated the seasonal abundance of L. basutoensis and related taxa on S. madagascariensis populations in the KwaZulu-Natal Midlands. Second, we assessed their host-plant affinities by surveying 17 non-target native Senecio species in the region. We used DNA CO1 barcoding to differentiate flea beetle taxa and their root-feeding larvae in both studies. Monthly sampling revealed three root-feeding taxa associated with S. madagascariensis, with L. basutoensis the most abundant. Longitarsus basutoensis was recorded throughout the year, with no clear patterns in seasonal abundance but significantly higher numbers at sites with deep, well drained soils that have high water holding capacity. Two unidentified flea beetle taxa, represented by larval specimens only, were recorded infrequently and in low numbers. The host-range study revealed 18 flea beetle taxa on 11 Senecio species, with L. basutoensis the most abundant species and recorded mostly on S. madagascariensis. Longitarsus basutoensis was also detected on four non-target Senecio species which, together with the results of preliminary host-specificity tests, indicated insufficient host-range restriction for countries like Australia that host a diverse Senecio flora. However, L basutoensis may have potential for release in Hawaii, due to a lack of native Senecio species on any of its islands.