Fipronil was tested against the eastern subterranean termite, Reticulitermes flavipes (Kollar) to determine toxicological effects of various exposure methods and possible mechanisms of horizontal transmission between topically-treated donors and untreated recipient nestmates.
In laboratory assays, individual termite workers were treated topically with acute doses of fipronil which resulted in a 24 and 48 h LD50 of 1.2 and 0.76 ng per insect, respectively. Topical concentrations of fipronil at doses 2.5 and 5.0 ng per insect resulted in LT50s of 10.3 and 3 h, respectively. Symptomatic termites exhibited uncoordinated movement, tightly curled tarsi, and paralysis, which always lead to death. Reaction to fipronil-treated paper resulted in similar, yet delayed responses for termites allowed to walk on 12 concentrations. After 24 h exposure, LC50 and LC90 were 3.5 and 7 ppm (wt AI: wt filter paper), respectively. After 48 h of continuous exposure to treated filter paper, any concentration greater than 2 ppm resulted in 100% mortality. Termites suffered quick and complete mortality when exposed to a 1:1 mixture of fipronil-treated sand: vermiculite substrate. No repellency was observed as termites readily tunneled into 4 cm of each concentration, which ranged from 0.5 - 75 ppm (wt AI: wt substrate). Substrate concentrations greater than 6 ppm achieved 100 % mortality within 24 h. As expected, there was a dose-response effect, where higher concentrations of fipronil resulted in faster onset of symptoms and rapid moribundity and mortality.
Fipronil and three of its metabolites were tested with topical applications. Fipronil and fipronil-desulfinyl had identical toxicological effects at a dose of 10 ng per insect. Fipronil-sulfide had the greatest mortality of the four compounds tested at 3 and 6 h observation time points. Fipronil-sulfone was the slowest to result in symptoms of poisoning and mortality when compared to the other three compounds. Using GC analysis, fipronil was found to metabolize via biological pathways in termite bodies to fipronil-sulfone. In tests with fipronil and fipronil-sulfone at low doses (1.0 - 5.0 ng per insect), LD50s were similar and LD-P graphs had similar slopes. Topical LD50s were 1.3 and 2.3 ng per termite for fipronil and fipronil-sulfone, respectively.
Horizontal transfer studies, using 14C-radiolabeled fipronil, were performed to monitor mortality of recipients and the distribution of fipronil within a closed system. Blue-stained donors were treated topically with 5 ng per insect and allowed to interact with 15 untreated, unstained recipients. Donor-Recipient ratios were 1:15, 1:5, and 1:3. Within 24 h, donors were able to transfer more than 50% of fipronil to the immediate environment. For all treatments, approximately 10 % fipronil was recovered from the filter paper, 25 -40% remained in the donor, and 50 - 66% was recovered from recipients. Increasing the number of donors treated with 5 ng per termite increased recipient mortality, and increasing the surface are of the Petri-dish arena increased recipient mortality as well. Using GC Analysis fipronil-sulfone was recovered in donors and recipients from transfer studies. Fipronil was transferred from donors to recipients by allogrooming, however, fipronil-sulfone, which has greater water solubility than the parent material, may be transferred among nestmates via trophallaxis.