The ATSB, a blended formulation of fruit juice/flower nectar, a toxin, and sugar solution is a recently developed innovative strategy against mosquitoes. The ATSB approach is considered an effective, technically simple and low-cost solution to avoid the issues and concerns associated with contact insecticides [19] as the formulated bait works by competing with naturally accessible sources of plant sugar, the food and energy source for the mosquitoes.

The toxic sugar baits (TSB) comprising a combination of sugar and toxicant (malathion) have been used earlier to control Aedes aegypti [20]. The potential of TSBs containing various other insecticides (bifenthrin, cyfluthrin, deltamethrin, permethrin) have been tested against different mosquito species; Cx. quinquefasciatus, An. quadrimaculatus, Ae. taeniorhynchus, Culex nigripalpus, and Aedes albopictus [16, 21, 22]. Though laboratory trials with these TSBs were found effective, the field trials could not lure mosquitoes and give efficient results due to the presence of natural sugar sources in the environment. Therefore, the formulation of ATSBs was recommended with adding fruit juices, flower nectar, or insect honeydew [23, 24].

The present study identified an effective attractant, optimized the concentration of toxicant and formulated an effective ATSB against malaria vector, An. stephensi. The efficacy of ATSB was evaluated against the NIMR strain and the AND strain of An. stephensi. As the attractant is a significant component in ATSB in order to lure the adult mosquitoes on the bait, initially nine ASBs were prepared with different fruit juices and evaluated for their attraction potential against the two strains. The ASBs formulated with guava juice, plum juice and mango juice exhibited significantly higher attractancy against both the strains in comparison to the control (p < 0.05) and rest of the ASBs with other fruit juices. The assays ascertaining the relative attraction potential of the juices revealed the highest attractancy of guava juice-ASB in comparison to the rest of fruit juice ASBs (p < 0.05) for both the NIMR strain and the AND strain. The other two ASBs found effective were plum juice-ASB and mango juice-ASB. Similar results were obtained in earlier experiments when nine ASBs were tested against two laboratory strains (the AND strain of Ae. aegypti, and the DL10 strain of Ae. aegypti) and two field strains of Ae. aegypti (SHD-Delhi and GVD-Delhi). Against all the four strains, the guava juice-ASB exhibited the highest attractant potential followed by plum and mango juice-ASBs. However, the guava juice-ASB possessed 1.22 to 1.4-fold higher attraction potential for An. stephensi strains in comparison to Ae. aegypti [17]. The optimization of toxin dosage to be added in ATSB formulations against these Ae. aegypti strains and cage as well as field bioassays is in progress.

Similar studies were held in Bagamoyo, Tanzania to assess the attraction potential of seven ASBs on Anopheles arabiensis, banana (Muso), guava (Psidium guajava), mango (Mangifera indica), orange (Citrus sinensis), papaya (Carica papaya), tomato (Solanum lycopersicum) and watermelon (Citrullus lanatus) pulps, and showed significant attractant potential of orange juice-ASB > tomato juice-ASB > guava juice-ASB [25]. In Mali, Muller et al. [23] evaluated the attractancy potential of locally available 26 types of fruits/seedpods and 26 different flowering plants for malaria vector, Anopheles gambiae and demonstrated significant attraction potential of the 6 species of fruits and 9 species of flowering plants with Acacia macrostachya identified as the most attractive flowering plant, while guava and muskmelon (Cucumis melo) as the most attractive fruits.

The current study formulated an ATSB with Guava juice-ASB and the toxic component, deltamethrin. Nine ATSBs were prepared containing different concentrations of deltamethrin and were tested against both the strains of An. stephensi to determine their efficacy. The assays revealed a dose-dependent effect of ATSBs resulting in higher mortality of An. stephensi adults with the increasing deltamethrin concentration in the ATSB, the 0.8% deltamethrin-ATSB registered 97.96% mortality in the NIMR strain and 96.91% in the AND strain of An. stephensi. The LC50 values recorded with ATSBs were 0.061% and 0.073% against the NIMR strain and the AND strain of An. stephensi, respectively, after 24 h post introduction of different dosages of ATSB.

Similar assays with various guava juice-ASBs combined with 0.5% chlorfenapyr, 2% boric acid, or 1% tolfenpyrad resulted in > 90% mortality in pyrethroid-susceptible population of An. gambiae, as well as pyrethroid-resistant population of An. arabiensis and Cx. quinquefasciatus. However, the hut trials with these ATSBs could cause just 41–48% mortality in An. arabiensis and 36–43% mortality in Cx. quinquefasciatus [26]. Likewise, ATSB formulated with mango juice, guava juice, brown sugar and boric acid resulted in 100% mortality of Ae. albopictus in laboratory trials, while 95% and 58% mortality under semi-field and field trials, respectively [27].

The bioassay with ATSB containing guava juice-ASB and 0.2–2% boric acid or 0.05–0.5% chlorfenapyr against An. gambiae showed 100% mortality at 2% boric acid and 0.5% chlorfenapyr against both the susceptible (Kisumu) and resistant (M’bé) strains [28]. In Mali, ATSB containing guava and honey melon juice (1:1), sugar and boric acid caused 83.78% population reduction of An. gambiae within a month after its application [23], while in Israel, same formulation reduced nearly 90% An. gambiae population just after 1 week [29]. Another study in Israel held with ATSB (75% juice of Opuntia ficus-indica, 5% wine, 20% brown sugar, 1% BaitStab™ and 1% boric acid) reduced daily survival rates of Anopheles species [19].

Current study investigated a contact insecticide, deltamethrin, in the ATSB, against An. stephensi, which was found effective in controlling mosquito population in the field. Till date, limited studies have been conducted with contact insecticides-ATSBs. Most of the ATSB studies have been carried out with baits containing oral toxicants, such as dinotefuran, spinosad, chlorfenapyr and boric acid. The efficacy of three ATSBs, two containing oral toxicants—1.0% boric acid, 0.5% dinotefuran; and one with contact toxicant—0.1% deltamethrin, was assessed against both susceptible and deltamethrin-resistant strains of Cx. quinquefasciatus [30]. The results showed higher efficacy of all the ATSBs against resistant populations than the susceptible ones, probably due to the lower survival fitness of resistant population in the fields. In comparison to ATSBs containing boric acid and dinotefuran, the efficacy of deltamethrin-containing ATSB was lower against the deltamethrin-resistant population. It was suggested that the resistant population was more susceptible to the boric acid and dinotefuran than deltamethrin because of the different mechanisms of action and absence of cross-resistance to deltamethrin [30].

Presently, malaria vector management is reliant on the pyrethroids used in IRS and LLINs which has resulted in the development of resistance in mosquitoes [5]. Evidences have shown that pyrethroid-resistant adult mosquitoes have developed cross-resistance to other insecticides with same mechanism of action because of metabolic detoxification or insensitivity of the target site [31]. Such studies indicate that mosquitoes have ability to develop resistance to ATSBs because of use of toxicants with same mechanism of action as that of pyrethroids. However, no such studies have been carried out till date. It is believed and recommended that rotation of toxicants with different mechanisms of action in ATSBs, can not only mitigate the problems associated with the additional pressure selecting for the development of pyrethroid resistance but also can cause reversion of resistance in the field as suggested for other interventions.

ATSB methods have been suggested as effective tools for mosquito management in the fields. However, very few reports have assessed the environmental concerns associated with their use and thus needs to be investigated extensively for their effects on the non-targets. The available reports have suggested their safer use in non-flowering areas in comparison to the flowering areas. Eugenol containing-ATSB sprayed to control Ae. albopictus impacted 5.5% of the non-target insects investigated, when applied in the flowering vegetation while only 0.6% insects fed on the ATSB in non-flowering conditions indicating safety of the bait in the field [32]. Likewise, garlic oil-containing ATSB against Anopheles sergentii population showed minimal effects on non-target insects when applied to foliage of non-flowering plants as compared to the flowering plants [33]. Reports regarding non-target impact of ATSB-Pyrethroids are still lacking. However, considering their known safety against non-target insects, however, make them plausible tools for mosquito control in the field.

Based on the results obtained in current laboratory study, an extensive work is proposed to be carried out to setup trials for the assessment of the developed guava + deltamethrin ATSB formulation in the field conditions to assess the feasibility of use of this approach in mosquito management.