This study demonstrates that nylon strips can act as a sustainable matrix for dispensing synthetic attractants of host-seeking An. gambiae mosquitoes, performing much better than low density polyethylene (LDPE) sachets. It was remarkable that attractant-treated nylon strips continued to attract mosquitoes without re-application and remained consistently more attractive than LDPE sachets filled with the same attractants over a period of 40 nights post-treatment. The higher catches of mosquitoes associated with nylon strips were apparently not due to smaller surface area, uneven spread of the attractant on inner surfaces or LDPE sheet thickness.
The baseline experiments reported herein confirm findings of our previous studies in which nylon strips were found to provide a better release matrix for delivering synthetic attractants of host-seeking An. gambiae mosquitoes than did LDPE sachets or open glass vials . LDPE and nylon differ in physico-chemical characteristics such as porosity and chemical binding affinity that may explain the observed differences in mosquito catches through their effects on the release rate of odorant volatiles [1, 10, 11]. Although the use of LDPE sachets allows the adjustment of attractant release rates, release rates from nylon have yet to be determined e.g. through headspace sampling at the trap outlet.
That IB1-treated nylon strips remained consistently more attractive to host-seeking An. gambiae mosquitoes than LDPE sachets filled with the same attractants for a period of up to 40 days post-treatment is definitive proof of inherent residual activity. This finding corroborates that of related studies where nylon stockings impregnated with human emanations remained attractive to An. gambiae mosquitoes for several weeks [12–14]. Blend IB1 impregnated on nylon strips may have been subject to bacterial degradation over the prolonged experimental time. This may have resulted in the release of additional components than were originally present on the nylon strips [15–17]. However, the present study did not investigate the presence of microbes or additional attractant compounds on aging IB1-treated nylon strips.
The current study shows that, attractant-treated nylon strips can be re-used for at least 40 consecutive days as baits for host-seeking An. gambiae mosquitoes, thereby reducing costs of odorants and nylon strips, time and labour used to prepare fresh baits. These attributes are consistent with those associated with the long-lasting fabric materials impregnated with mosquito repellents or insecticides [18, 19]. The availability of long-lasting mosquito-attractant fabrics is interesting as these can potentially be combined with mosquito pathogens such as entomopathogenic fungi or bacteria . Thus, a cheap and effective tool for intercepting and eliminating host-seeking mosquitoes can be exploited for vector-borne disease control. However, further testing is needed to examine the maximal duration of residual activity of the attractant-treated strips.
Contrary to our expectations, LDPE sachets optimized for release rates and surface area caught fewer mosquitoes than nylon strips. The release rate of some compounds (propanoic, pentanoic, heptanoic, lactic acid and water) was significantly increased when uniformly thinner-sheeted sachets were utilized. Because sheet thickness of LDPE sachets is a determinant of volatile release rate the composition of the volatile blend released may have changed so as to negatively affect attractiveness to An. gambiae mosquitoes [1, 21]. We conclude that, blend ratio and concentration affects orientation and capture rates of insect vectors with odour-baited systems [22, 23].
Although LDPE sachets have been effectively used to release attractants for tsetse flies and other insect pests [1, 2], they attracted fewer mosquitoes compared to nylon strips when both were treated or filled with the same blend of attractants. This could be explained by differences in optimized sheet thicknesses of LDPE sachets and physical and chemical characteristics of the odorants used for attraction of tsetse flies versus those used for mosquitoes . Moreover, trap designs used for collection of both insect vectors were also different [3, 25, 26]. Delivery of synthetic attractant components through sachets with standardized sheet thickness and surface area have demonstrated consistent mosquito catches under laboratory and semi-field conditions .Whereas nylon strips were associated with higher mosquito catches, we currently lack information on the release rates of the odorants dispensed. Accurate release rates have been established for odorants delivered through LDPE sachets , and such chemical measurements should also be done for nylon, as this allows for a direct comparison of the active aerial odorant concentration that host-seeking mosquitoes encounter.