Impact of cogongrass (Imperata cylindrica) presence and management strategies on arthropod natural enemy populations in longleaf pine stands
Type of Degreethesis
Entomology and Plant Pathology
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Cogongrass (Imperata cylindrica Beav.) is an aggressive, invasive weed that is threatening the integrity of longleaf pine (Pinus palustris Mill.) ecosystems in southeast Alabama. Various management strategies are implemented to suppress cogongrass. In Objective 1, populations of natural enemies and bark beetles (Hylastes spp.) were monitored over 2 years in response to four different management strategies; herbicide, seeding with native species, seeding plus herbicide, and control (no treatment) in burned and unburned longleaf pine plots. In Objective 2, the impacts of cogongrass on the abundance of arthropods and soil dwelling microfauna in longleaf pine were investigated. Insects were collected via pitfall traps and sweep samples, then sorted and counted in the lab. For Objective 1, sentinel species (mainly generalist predators) were monitored. For Objective 2, all arthropods were sorted to family level and counted. In spring and fall 2011, earthworms were sampled using a disclosing solution in longleaf pine with or without cogongrass. Soil cores were taken in May 2011 to determine the relative abundance and diversity of nematodes. Cogongrass management strategies had minimal impacts on natural enemies and no impact on Hylastes populations. The seeding plus herbicide treatment had significantly fewer spiders than other treatments but this was the only affected group of natural enemies. Thirty of the >100 families collected were significantly impacted by cogongrass but not all impacts were negative. For example, Curculionidae, Tettigoniidae, Acrididae, and Cercopidae were more abundant in longleaf pine infested with cogongrass. Similarly, earthworm abundance and weight were significantly greater in longleaf pine with cogongrass present. Plant parasitic nematodes were significantly greater in longleaf pine with cogongrass present, the opposite being true for free-living nematodes. These results suggest that cogongrass management does not significantly affect natural enemy and bark beetle populations, but its presence does. The families that were impacted, particularly Scarabaeidae (and their relatives) or Gryllidae, can be used in future evaluations of ecosystem impacts of cogongrass invasion. Also, families of herbivores that increased in the longleaf pine stands where cogongrass was present may provide insight for future studies seeking herbivores that may be able to utilize this invasive grass.