Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 8, 1998
Publication Date: N/A
Interpretive Summary: Waterlettuce is an aquatic weed that constitutes the major impediment to water flow in the southeastern US. This manuscript describes research results that investigated methods of improving control of this weed with biological methods, namely the use of insect caterpillars that feed on the waterlettuce leaves and thereby reduce its importance. These caterpillars were originally introduced from Thailand. After intensive testing they are presently being released throughout the southeastern US. However, considerable difficulty has been encountered establishing a field colony. One major cause of this difficulty may be the quality of the host plant upon which they feed. Their impact on the weed populations may be compromised by their variable ability to feed and utilize the weed when grown under different environmental conditions. This report indicates that in some field conditions the plant consists of relatively low quality and the weevils that feed on these plants have higher mortality, slower growth and reduced size. This information is useful to improve methods to culture plants that are most nutritious for the production of large numbers of caterpillars for massive releases and to predict the best sites for further redistribution of caterpillars . Furthermore, additional insect species are needed that either utilize waterlettuce of lower nutritional quality or species that are not as sensitive to its nutritional variability.
Technical Abstract: Performance of the specialist herbivore Spodoptera pectinicornis (Hampson)(Lepidoptera: Noctuidae) was studied when fed the floating aquatic plant waterlettuce Pistia stratiotes L. (Araceae). Plants were either collected from six populations in south Florida or from plants grown with low or high fertilizer levels. Consumption of leaves with increasing toughness resulted in increased larval mortality to more than 80 percent; most mortality occurred during the first instar. Larvae fed low nitrogen leaves required up to twice the time and typically an extra instar to complete development. Larvae compensated for low nitrogen leaves by increasing fresh weight consumption 3 fold. Despite these dietary challenges, the biomass gained by the larvae was similar regardless of the quality of leaves consumed. These results are useful in understanding the adaptations and limitations of specialist herbivores to low nutrient foods. Additionally, they assisted in the selection of suitable release sites for this weed biological control agent. Finally, they improve our mass rearing techniques for augmentative releases of this biological control species.