Insect communities associated with three submerged aquatic macrophytes, with emphasis on the phytophagous species

• Main Author: Schutz, Luke
• Format: Others
• Language: en
• Published: 2008
• Online Access: http://hdl.handle.net/10539/4969

The implementation of classical biocontrol in managing exotic aquatic weeds, such as Lagarosiphon major Ridley (Hydrocharitaceae) (coarse oxygen-weed) in New Zealand (N.Z.) and Europe, and augmentation biocontrol in managing native weeds, such as L. major, L. muscoides Harvey (Hydrocharitaceae) (fine oxygen-weed) and Potamogeton pectinatus L.(Potamogetonaceae) (sago pondweed) in South Africa (S.A.) is being hampered by the dearth of knowledge surrounding submerged macrophytes and their associated insect assemblages. In addition, this knowledge gap hinders the accurate evaluation of any biocontrol initiatives that are implemented. These facts are even more sobering when considering the global number of invasive species is ever increasing, for example the recent invasion of S.A. by the submerged weed, Hydrilla verticillata. To begin addressing these issues, this study surveyed the invertebrates associated with three of S.A.’s indigenous submerged macrophytes, L. major, L. muscoides and P. pectinatus, while investigating the role certain environmental variables (site, plant species, nutrient levels, light penetration, water depth and distance from the shore) played in shaping these insect assemblages. In surveying these plant species on two dams within the Mooi River district of S.A., the following herbivorous insects were identified: Athripsodes harrisoni (Trichoptera: Leptoceridae), Leptocerus sp. (Trichoptera: Leptoceridae), Parapoynx fluctuosalis (Lepidoptera: Pyralidae), Micronectus scutellaris (Hemiptera: Corixidae), M. dorothea (Hemiptera: Corixidae) and species from the Chironominae (Diptera: Chironomidae) and Orthocladiinae (Diptera: Chironomidae). Of these, A. harrisoni and Leptocerus sp. showed the most promise of being destructive, host-specific agents to manage L. major in N.Z. and Europe; consequently, they require further investigation. Concerning the use of augmentation control in managing the three plant species within S.A., all the herbivorous insects have potential as future agents. However, nitrate to phosphate ratios of below ten in the sampled dams have resulted in excess nutrients being available to all three plant species; resulting in their growth and spread throughout the dams not being hampered by the feeding of the phytophagous insects. Again, this highlights the need to address eutrophication in the management of all S.A.’s aquatic weeds. The herbivorous fauna found in this study may also have the potential to be important in the management of H. verticillata in S.A., given the fact that this weed and Lagarosiphon are from the Hydrocharitaceae family. In addition to the herbivorous insects, 18 morphospecies from 10 insect families were found associated with L. major, 14 morphospecies from nine families with L. muscoides, and 19 morphospecies from 11 families with P. pectinatus. The evenness of the insect communities associated with all three plant species was very similar; however, their species richness was significantly different. Consequently, the community of P. pectinatus was the most diverse (Fisher’s alpha of 4.74), while that of L. muscoides was the least (Fisher’s alpha of 2.95). Of the environmental variables investigated at the 10 replicate sites for each plant species, different sites, different plant species and surrounding land use were important in shaping the communities. In addition, this study highlights the feasibility of local research institutes surveying the insect assemblages associated with their indigenous macrophyte species, which are weeds or potential weeds elsewhere. This could be a very cost effective procedure of refining global biocontrol.