Passive transfer of Steinernema riobrave entomopathogenic nematodes with implications for treatment of cattle fever tick-infested nilgai

Authors: Goolsby, John; Shapiro Ilan, David

Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2020
Publication Date: 9/13/2020
Citation: Goolsby, J., Shapiro Ilan, D.I. 2020. Passive transfer of Steinernema riobrave entomopathogenic nematodes with implications for treatment of cattle fever tick-infested nilgai. Biocontrol Science and Technology. 12:1330-1339. https://doi.org/10.1080/09583157.2020.1817332.
DOI: https://doi.org/10.1080/09583157.2020.1817332

Interpretive Summary: Cattle fever ticks (CFT) Rhipicephalus (=Boophilus) microplus and Rhipicephalus annulatus are invasive livestock pests that are endemic to Mexico and invasive along the Texas – Mexico border. Acaricide resistance, alternate wildlife hosts, and pathogenic landscape forming weeds present challenges for sustainable eradication of this pest in the U.S. CFT are the vectors of the microbes causing bovine babesiosis, which is a lethal disease causing high mortality particularly in susceptible European breeds and severely affecting the beef cattle industry. Efforts to eradicate CFT from the United States have been successful; however, a permanent quarantine zone (PQZ) is maintained between Texas and Mexico to prevent incursions of CFT from wildlife and stray cattle. In recent years, there has been an increase in CFT infestations outside of the PQZ in Texas. One of the alternate wildlife hosts for CFT in South Texas are nilgai (Boselaphus tragocamelus), an exotic Asian antelope. Nilgai are highly mobile with large home ranges and are implicated in the spread of CFT, through the landscape. Currently, there are no methods for control of CFT on nilgai other than culling. Insect and tick killing parasitic round worms (entomopathogenic nematodes) are under evaluation for eradication of CFT on nilgai. Nematodes would be applied as a water-based spray to nilgai as they move through fence crossings and use latrines. A remotely activated field sprayer was developed for application of nematodes on CFT infested nilgai. This paper reviews the potential for nematodes to be transferred to tick-infested nilgai and they move through fence crossings and brush against vegetation and soil that has been wetted by the nematode suspension. Using wax moth larvae as a substitute for cattle fever ticks we found that nematodes could be transferred via wet leaves of Guineagrass and from soil. The nematodes remained viable for up to 3 hours in water droplets of Guineagrass. This paper reviews the potential for nematodes to be transferred to tick-infested nilgai as they move through fence crossings.

Technical Abstract: The entomopathogenic nematode, Steinernema riobrave is under evaluation for eradication of the southern cattle fever tick, Rhipicephalus microplus (Canestrini) on free-ranging nilgai antelope (Boselaphus tragocamelus Pallas) in South Texas. Previous lab, barn and pen trials have shown that S. riobrave is effective against R. microplus on cattle. Remotely operated field sprayers have been developed to directly treat infested nilgai as they transit fence crossings. Mist from the sprayers also wets the surrounding foliage and soil in the nilgai crossing pathways. Field and lab studies were conducted to determine the potential for passive transfer of the nematodes from wetted foliage/soil and length of time nematodes remained viable in water droplets on leaves of Guineagrass (Megathyrsus maximus). Wax moth larvae, Galleria mellonella were used as a facsimile organism for R. microplus. Wax moth larvae exposed to the entomopathogenic nematode, S. riobrave under field conditions showed significant differences in levels of survival (0-42%) at 5 days post application. In contrast, wax moth larvae not exposed to S. riobrave showed high levels of survival (84-93%). Both the hand-misted in Petri dish and misted by the field sprayer treatments showed very low levels of survival (2-0)%. The three treatments which represented passive transfer of nematodes (nematode misted soil, drenched soil, and misted leaves) also showed low levels of survival at 42, 22, and 15% respectively. In lab studies, Steinernema riobrave remained viable in water droplets on Guineagrass leaves for up 180 minutes, which is the length of time between automatic 15 second applications of the nematode water suspension. These results indicate that foliage and soil is likely to stay wet with water droplets of the nematode suspension for most of the night and early morning while RH remains high. This study shows the potential for direct and passive application of entomopathogenic nematodes to nilgai for control of cattle fever ticks as they transit the area in around fence crossings.