Performance and impact of the biological control agent Xubida infusella (Lepidoptera; Pyralidae) on the target weed Eichhornia crassipes (waterhyacinth) and on a non-target plant, Pontederia cordata (pickerelweed) in two nutrient regimes

Xubida infusella (Walker) (Lepidoptera: Pyralidae) is potentially a useful biological control agent targeting Eichhornia crassipes (waterhyacinth) in the USA but many regions infested with waterhyacinth are also inhabited by an alternative native host, Pontederia cordata (pickerelweed). Experiments were conducted in Australia to assess the impact of X. infusella on pickerelweed compared to waterhyacinth where both these plants were available and X. infusella had already been released. Overall X. infusella had a greater impact on pickerelweed than on waterhyacinth. More than one larva per plant was required to reduce the total shoot dry weight of waterhyacinth but only one larva per plant reduced the total shoot dry weight of pickerelweed. Insect feeding caused the number of secondary shoots (daughter plants) of pickerelweed to double whereas the number of daughter plants produced by waterhyacinth remained unchanged. We suggest this indicates a considerable impact on pickerelweed rather than effective compensation for insect damage because the shoots produced were very small. Waterhyacinth produced a constant number of daughter plants when fed on by up to three larvae per plant. Higher nitrogen status of both species of host plant increased the rate of larval development and pupal weight of X. infusella. The weight and fecundity of X. infusella reared on pickerelweed were lower than those reared on waterhyacinth but large numbers of progeny were produced on both plant species. This experiment demonstrates a considerable impact of X. infusella on pickerelweed suggesting this plant is at risk from this agent if released in the USA where pickerelweed is present. The considerable impact on waterhyacinth demonstrates the potential for this insect to contribute to waterhyacinth control in countries where risk assessment favours release.