Entomopathogenic nematodes as a potential biological control method for ticks.

Entomopathogenic nematodes have been used for biological control of certain insect pests. In these studies the nematodes were tested as a possible biological control agent for engorged female ticks. Five species of infective juveniles (IJs) were tested initially for their ability to penetrate and kill ticks, including Steinernema glaseri (SG), S. riobravus (SR), S. carpocapsae (DT), S. feltiae (SF) and Heterorhabiditis bacteriophora (HP88). Infective juveniles (IJs) of SRs and SFs appeared to be the most effective in killing ticks and invaded and killed 30 to 100% of replete females. These two nematode species were tested on several tick species including Amblyomma americanum, A. cajennense, A. maculatum, Dermacentor variabilis and Rhipicephalus sanguineus. Although the killing rate of each tick species varied, the nematodes did not appear to be host specific and were able to kill ticks of all species tested. Egg mass weights of exposed ticks of each species were significantly lower than those of the controls. Ticks were examined with microscopy to determine whether nematodes entered and multiplied inside ticks. Partially fed female Amblyomma americanum and Dermacentor variabilis exposed to 5000 IJs in petri dishes were collected at 8, 24, 48 and 96 hrs (Trial 1) and 1, 2, 3, 4, 7 and 9 days (Trial 2) post-exposure, and fixed, processed and embedded in resin for microscopy studies. Only a few nematodes were seen in the hemocoel and tissues and they were surrounded by a clear space. Bacteria, released from the nematodes, were present in the exposed ticks and appeared to increase daily causing a generalized infection. Degeneration of tick tissues and death of the ticks appeared to result from bacterial proliferation. Nematodes did not multiply within ticks as they do in insect larvae. In these controlled laboratory studies, exposure of ticks to nematodes resulted in tick mortality and reduced egg production. Entomopathogenic nematodes appear to have potential as a biological control agent of ticks, but future studies will be required to determine whether nematode/tick interactions will occur in the field.