Hydrotimetes natans as a suitable biological control agent for the invasive weed Cabomba caroliniana.
The aquatic macrophyte Cabomba caroliniana A. Gray is a major invasive weed in Australia and several other countries. A classical biological control program was initiated in Australia in 2003 and native range explorations in Argentina that year led to the discovery of the aquatic weevil Hydrotimetes natans Kolbe feeding on C. caroliniana, making it the first, and so far, only potential biological control agent for the weed. However, the program was discontinued because the largely unknown biology had made rearing of H. natans difficult under quarantine laboratory conditions. We report here key aspects of the biology and reproductive behaviour of H. natans that provided significant insights to successfully establish a laboratory colony when the program was restarted in 2016. In addition, we studied the physiological host range of H. natans and determined its risks to 15 non-target plant species. The preoviposition period of H. natans was 6.50 ± 0.85 days, and the development times of eggs and pupae were 7.65 ± 0.86 and 14.27 ± 0.51 days, respectively. Egg to adult development time was 46.52 ± 0.82 days with a larval development time of 25-27 days. A single female laid 123.13 ± 23.03 eggs in 24 weeks under non-limiting laboratory conditions. Oviposition was intermittent and age-dependent with 75% eggs oviposited within 8.54 weeks after adult eclosion; percent viability of these eggs was 55.62 ± 4.61. Females oviposited mostly on the apical tips followed by on the first few nodes from the tip. Adults survived a maximum of 521 days with a mean longevity of 235.16 ± 21.16 days and females remained reproductive for 211.00 ± 35.05 days. Field surveys and laboratory host-specificity studies demonstrated H. natans is adequately host-specific to C. caroliniana. No non-target effects were observed on Nymphaea, Victoria and Trithuria species. Brasenia schreberi indicated the possibility of lifecycle completion by H. natans in choice and no-choice trials but did not sustain a population in continuation trials. The risks to B. schreberi were deemed negligible and H. natans was approved for release in Australia.