Lethal Effects of a Silica Gel + Thyme Oil (EcoVia) Dust and Aqueous Suspensions on Amblyomma americanum (Ixodida: Ixodidae) Larvae and Nymphs

Abstract

Ixodids suck blood from host animals and transmit pathogens that cause important medical and veterinary diseases. As synthetic conventional acaricide resistance becomes increasingly common, alternative tactics are likely to enhance control efforts. Laboratory bioassays assessed the lethality of CimeXa, a commercial silica gel desiccant dust, and EcoVia, a commercial product containing silica gel + thyme oil, against lone star tick, Amblyomma americanum (L.), larvae and nymphs. Both life stages were completely killed by CimeXa by 24 h, and EcoVia achieved total larval mortality within 1 h when they were initially immersed in the dusts and when they crawled across treated substrate. Larvae were killed faster than nymphs. Temporary immersion of A. americanum larvae and nymphs in aqueous suspensions of the dusts were less effective than exposure to the dusts applied dry. Larval and nymphal mortality associated with crawling on dried aqueous suspensions of the products was also not as strong as when the immature life stages crawled across substrate treated with dry dusts, and EcoVia was not more efficient than CimeXa. CimeXa and EcoVia can likely be used to protect cattle in a prophylactic context because silica gel does not degrade, and EcoVia might be effective at eliminating ticks that are feeding on cattle as well. We suggest that the two dust products be assessed for efficacy against other ixodid species, such as the southern cattle fever tick, Rhipicephalus microplus (Canestrini), which transmits the causal agents of babesiosis to cattle.

Integrative Alternative Tactics for Ixodid Control

“…Pyrethrins, however, were found to be 100% effective against A. americanum , A. cajenennense , and A. maculatum within minutes [ 95 ]. The addition of pyrethrins to a silica gel-based desiccant dust (a commercial product, Drione) killed A. americanum larvae and nymphs substantially faster than the silica gel component alone [ 31 ], and, unlike silica gel, the product also killed A. americanum larvae and nymphs while they fed on cattle [ 29 ].…”

“…Resistance to silica gel (and other desiccant dusts), in the form of thickened cuticle, is only known to have occurred in one Australian laboratory strain of bed bugs, Cimex lectualrius L. [ 509 , 510 ]. Desiccant dusts are not likely to induce the development of resistance in ixodids at least in part because treatment of vegetation and host animals will be intermittent, and the dusts can be rotated with chemically-based acaricides, in addition to desiccant dusts combined with chemically-based toxins [ 31 , 32 ]. In addition, the dusts might be amenable for organic animal production as well as in wildlife refuges, parks, and other protected areas where application of synthetic chemical pesticides is generally discouraged.…”

“…Another advantage of inert dusts involves indefinite shelf lives, including under poor storage conditions (e.g., heat, ultraviolet radiation, light, and aging). The dusts are amenable to augmentation with toxic chemicals, including botanical substances, that are acutely lethal to ixodids [ 29 , 31 , 32 ]. Moreover, desiccant dusts can be applied to livestock by active and passive means; active treatment involves equipment, such as manual and motorized “dusters” [ 28 , 513 ], while passive application involves, for example, dust bags that are commonly used for protecting cattle against ectoparasites [ 368 , 514 , 515 ].…”

“…Potential disadvantages of inert dusts include slow action relative to conventional toxin-based acaricides [ 517 , 518 ]; the dusts, however, can result in complete mortality of larval ixodids within hours [ 28 , 29 , 30 , 31 ]. Dusts are less effective after being wetted and they are best applied to dry surfaces.…”

“…Miller et al [ 27 ] suggested that organophosphate resistance in R. microplus required high-sode treatments to overcome resistance. Research into naturally derived compounds such as desiccant dusts [ 28 , 29 , 30 , 31 , 32 ] have been showing promise as alternatives to overcome the weaknesses inherent to synthetic acaricides.…”