Biological control of the larger black flour beetle, cynaeus angustus leconte (coleoptera: tenebrionidae) in cotton gin trash using entomopathogenic nematode
In the Southern High Plains of Texas, the larger black flour beetle, Cynaeus angustus (Leconte) (Coleoptera: Tenebrionidae), is a nuisance pest to homes and businesses near cotton gin trash piles. When complaints of infestations occur by communities near gin trash piles, there is no management practice other than relocating gin trash piles, and users of gin trash are worried about the legal ramifications of potentially producing large numbers of this nuisance pest. In this project, entomopathogenic nematodes were researched to determine their potential as biological control agents to reduce C. angustus populations in gin trash piles. Three entomopathogenic nematodes [Heterorhabditis bacteriophora (Poinar)(Nematoda: Heterorhabditidae)(Laboratory strain), Steinernema riobrave (Cabanillas Poinar and Raulston) (Nematoda: Steinernematidae)(Laboratory strain) and Steinernema Carpocapsae (Weiser) (Nematoda: Steinernematidae)(Laboratory strain)] were examined, and data was collected from two laboratory experiments and a field experiment. In the first laboratory experiment, change in population density of two entomopathogenic nematodes over time (expressed in days after treatment, or DAT), and their virulence (ability to kill) the larval stage of C. angustus across a range of soil textures. Results showed the population density of infective juveniles (IJs) of H. bacteriophora and S. riobrave, at DAT 0 were not affected by soil texture, but their population density decreased at each sampling point thereafter. H. bacteriophora differed in mortality to C. angustus larvae only at DAT 0 as it was not effective in the sand%/clay% of 29/55%. S. riobrave population density increased as clay content increased at 3 and 7 DAT, but its virulence was not affected by soil texture and virulence decreased as time after inoculation increased. In the second laboratory experiment of soil moisture content, H. bacteriophora population density showed a negative curvilinear response to the increase in soil moisture at DAT 0, and a negative linear response to soil moisture content at DAT 3 and 7. S. carpocapsae virulence in general increased with increasing soil moisture content, with the highest virulence occurring at >15% soil moisture content. , In the 2011 field study using experimental plots of cotton gin trash inoculated separately with S. carpocapsae, S. riobrave, and H. bacteriophora observed the change in nematode population density and virulence on Galleria mellonella F. (Lepidoptera: Pyralidae) larvae, and C. angustus adults.. Soil taken from under the gin trash piles was used in bioassays with G. mellonella larvae and C. angustus adults to demonstrate continued virulence of the entomopathogenic nematodes. S. carpocapsae caused significant mortality to G. mellonella for the first 35 days after inoculation and maintained a similar density of approximately 151 IJs/200cm3 soil for the first 21 DAT. S. riobrave population density after inoculation was approximately 67 IJ/200 cm3, and this density fluctuated at various time points throughout the experiment. S. riobrave killed more G. mellonella (relative to the untreated check) on 0, 14, 21, and 63 DAT. S. riobrave was the only nematode species to cause significant mortality to G. mellonella as late as 63 days after inoculation which may indicate a new generation of nematode had emerged. The recovery of H. bacteriophora was significantly different among field sites. H. bacteriophora population density at the Wilson site was approximately 80-120 IJ/200cm3 soil for the first 21 DAT. There was no change in population density at the field sites of Acuff and Station and was approximately10-90 IJs/200cm3. H. bacteriophora virulence on G. mellonella larvae was significant on the initial inoculation day only compared to the control treatment. When using C. angustus adults as a bioassay to indicate viable entomopathogenic nematodes from the gin trash field experiments, S. carpocapsae was successful at causing mortality on initial day of inoculation, and H. bacteriophora was successful only at 14 DAT. The findings suggest that soils from the Southern High Plains of Texas do not affect virulence of entomopathogenic nematodes, but that soil moisture content should be ≥15% for optimal virulence. The findings also suggest that field application of entomopathogenic nematodes in soil underneath gin trash is a suitable habitat. Specifically, when exposed to a highly susceptible host, S. carpocapsae was virulent for 35 DAT and S. riobrave being virulent for 63 DAT. However, differences in virulence of entomopathogenic nematodes to G. mellonella larvae compared to C. angustus adults suggest future research of exposing entomopathogenic nematodes to the larval stage of C. angustus in field trials.
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