Acttra SWD Bait Formulation Against Drosophila Suzukii and its Compatibility with Berry Pollination by the Honey Bee, Apis Mellifera

Good production practices minimize their impact on natural ecosystems and maximize sustainability (Tilman et al., 2002). Consumers are aware of this, so they are changing their habits and acquiring commodities that are produced through sustainable agricultural practices (Vermeir & Verbeke, 2006). However, a growing human population and modern lifestyle create demand for an ever-increasing volume and variety of agricultural goods. Modern intensive agricultural technology holds the promise to fulfill current and future humanity needs, even though it commonly involves the use of conventional chemical insecticides. Entire crops are sprayed to manage insect pests that reduce crop yield (Tilman et al., 2002), and though the use of insecticides reduces the damage caused by pests, the practice usually comes with deleterious side effects on non-target organisms, especially beneficial insects such as predators, parasitoids and pollinators (Carvalho, 2006). The honey bee, Apis mellifera L. is widely used to pollinate flowering crops around the world providing growers with economic inputs both from increased crop production and honey but is increasingly affected by the indiscriminate use of insecticides (Gallai et al., 2009).

Biological control, i.e. the use of organisms or their metabolites for the control of pests, is currently considered a better, more environmentally friendly alternative to conventional synthetic insecticides (Heimpel & Mills, 2017). Spinosyns, a group of molecules that is produced through the fermentation of the actinomicete Saccharopolyspora spinosa, are a prime example of microbial metabolites used as bioinsecticides (Thompson et al., 2000). They irreversibly bind to nicotinic acetylcholine receptors, causing continuous excitation of neurons in the central nervous system, paralysis and death of the insect (Salgado, 1998). Nicotinic acetylcholine receptors are present in all insects, and because of that spinosyns are regarded as highly toxic to many non-target terrestrial arthropods (Mayes et al., 2003; Millar & Denholm, 2007; Biondi et al., 2012), and special formulations have been developed to increase their specificity to the pest (Perrin, 2000; Burns et al., 2001; Lockwood et al., 2001; Cloonan et al., 2019; Lasa et al., 2019). In the case of fruit flies (Diptera, Tephritidae) this is primarily achieved through a mixture of food bait, which attracts and phagostimulates the target species, and the killing agent, which can be a bioinsecticide or a drowning solution (El-Sayed et al., 2009). In the design of the “attract and kill” formulation, some components might also work to repel non-target species (Mangan & Moreno, 2009).

Because the “attract and kill” formulation lures the pest to the application points, it is unnecessary for the crop to be completely covered with the insecticide, and instead the formulation is applied either as large drops, bursts on the base of trees or placed inside bait stations (Gazit et al., 2013). Non-targets are not attracted to the bait, and the probability of contact between non-target insects and the insecticide formulation dramatically decreases. Therefore, the use of “attract and kill” formulations should be significantly more compatible with the management of honey bee colonies for crop pollination than conventional cover sprays of chemical pesticides for crop protection (Kirkland, 1999; Cabrera-Marín et al., 2016). GF-120 (Dow Agrosciences, CA) is a formulation that consists of NH3-releasing compounds, sugars and the bioinsecticide spinosad (a mixture of spinosyns A and D), aimed to control fruit flies in Mexico (Sánchez et al., 2012). Even though spinosad is considered to be a minor threat to wildlife including mammals, it has been shown to be toxic in contact and oral tests to pollinators such as the honey bee (Miles, 2003; Hardstone & Scott, 2010). However, given that the GF-120 formulation has shown to be mostly repellent to bee pollinators (Mangan & Moreno, 2009; Gómez-Escobar et al., 2014), actual oral toxicity to pollinators is substantially reduced. More importantly, application at an ultra low volume (ULV, 2 liters of diluted formulation per ha) with large size droplet completely eliminates the necessity of complete crop coverage, leaving most of the crop free of insecticides, which might provide refuge for pollinators (Cabrera-Marín et al., 2016).

The Spotted-Wing Drosophila, Drosophila suzuki (Matsumura) (SWD) (Diptera: Drosophilidae) is a serious pest of soft fruit and berries (e.g. various Rubus and Vaccinium species) (Asplen et al., 2015). Its populations are reduced through the application of spinosad or spinetoram (a mixture of spinosyns J and L)-based insecticides in the form of ULV micron-sized droplets over the foliage of crops (Beers et al., 2011; Bruck et al., 2011; Van Timmeren & Isaacs, 2013). Such a mode of application implies that the spinosyns enter in contact with non-target species as well, especially if applied when bees are foraging.

A new product, Acttra SWD (ISCA Technologies Inc., Riverside, CA, US), aimed for the management of D. suzukii, incorporates into a ready-to-use flowable slow release formulation a proprietary semiochemical blend that provides specificity of attraction. It also contains a blend of phagostimulant compounds, including sugars, that induces the consumption of the bait by the attracted target pest (Klick et al., 2019). When mixed with the bioinsecticide spinosad, it can be applied as a short burst in the lower part of blackberry or raspberry plants, away from inflorescences and fruits, thus reducing the contact with potential pollinators such as bees. However, it was hypothesized that the entire blend, or some compounds, in Acttra SWD attracts honey bees, so even though contact through spraying is considerably reduced, residual contact and potential ingestion by honey bees cannot be discarded. Thus here we report the results of evaluating the attraction of Acttra SWD mixed with spinosad to honey bee foragers following Cabrera-Marín et al. (2015) procedures. We aimed to determine whether honey bee foragers are able to reject Acttra + spinosad treated food sources when alternative Acttra + spinosad-free resources are available, as a strategy to make agriculture and pollination more compatible. We also conducted two other experiments to determine honey bee mortality by contact to, and by ingestion of, Acttra SWD mixed with spinosad.

This study revealed three important aspects about the use of Acttra SWD/Entrust in the field. First, we showed that the formulation repelled bees since most of the foragers moved from the Acttra/Entrust feeder to the feeder without it, and moreover the remaining foragers that continued visiting the Acttra/Entrust feeder did not collect it. Our data suggest that Acttra SWD was clearly discouraging foragers from collecting it, since the number of foragers in the Acttra/Entrust feeder was reduced by over 95% during the exposure phase. The GF-120 formulation elicited a similar behavior in honey bees and stingless bees (Gómez-Escobar et al., 2014; Mangan & Moreno, 2009). In this case, the protein contents in this formulation are assumed to keep bees from consuming it, despite the sugar in it. Honey bees should be able to taste in Acttra SWD invertose, sucrose, methionol and the inerts. We hypothesize that methionol discouraged our bees from collecting the formulation upon contact, since all foragers landed on the platform, but did not collect it and instead were repelled from a distance. Because this formulation constitutes proprietary technology, the composition of the vehicle is unknown to us and could also be a source of repellence, but this has to be tested. Many foragers that abandoned the Acttra/Entrust feeder during the exposure phase showed up at the sucrose feeder, which suggests that, as long as there are food alternatives available, some bees will prefer to move to these resources.

A similar result was obtained by Cabrera-Marín et al. (2015) with honey bees exposed to GF-120, when foragers abandoned the feeder that changed from offering sucrose to show the pesticide. Thus, some spaces must be left in the field untreated, for example through the application of the formulation at evenly spaced locations and that the application does not cause too much aerosolization in order to reach floral structures. In any case, our bees landed on the platform, and some had to make contact with the formulation to reject it, and some foragers still tried to collect it. Further evaluations need to be conducted in order to determine the presence of residues of spinosad in honey from honey bee colonies located nearby treated farms and to estimate actual risks.

Secondly, the low toxicity observed in Experiment 2 is in agreement with the lack of attraction of honey bees towards Acttra/Entrust observed in Experiment 1. No foragers exposed to droplets with Acttra SWD had their abdomens colored red, and they preferred to starve than to feed upon any of the Acttra treatments, which can be considered a no-choice test. This result gives further evidence on the safety of Acttra/Entrust to honey bees. However, during application it would be necessary to provide food for the bee colonies, since they could stop foraging completely and become weaker if no other food source were available. Although it is likely that a mixture Acttra/Exalt should also be repellent to honey bees, further studies must be done to confirm this.

The third important finding of our study is that the acute toxicity of spray applications aimed to completely cover the plantation with Acttra/Entrust can be as high as that observed for Entrust and Exalt formulations at the recommended field dose. Given that several studies have shown spinosyns to be highly toxic to A. mellifera, even similar to neonicotinoids, in oral and topical tests (Gómez-Escobar et al., 2018; Miles, 2003; Padilha et al., 2020; Stanley et al., 2015), it was not surprising to observe such high mortality when honeybees were experimentally covered by each spinosad treatment by directly aiming the spray on them. This, nonetheless, would be the worst-case scenario. However, the recommended application mode of Acttra/Entrust in the field can be considered safe for bees because 1) a splash of 1 m-long of product is left on the lower cover, far from the fructification structures (Klick et al., 2019) and, 2) the formulation is repellent to bees. When Acttra is not used, the application of Entrust or Exalt is carried out in such a way that the plantation is completely covered with a layer of the insecticide. This mode of application leaves no chance for bees to escape contact, particularly if it is done during periods of high foraging activity (Cabrera-Marín et al., 2016). However, if it is done during nighttime or very early in the morning, the risk to bees is greatly reduced, as shown in laboratory, semifield and field tests for spinosad (Miles et al., 2012; Schoonover & Larson, 1995). Residual deleterious effects on honeybees need to be addressed as well, since tarsal contact with dry, or even wet residues, will occur.

The environmental fate and half-life of spinosad and spinetoram is similar. They rapidly break-down on plant surfaces exposed to sunlight (2–16 d), water and soil (<1 d), but it takes longer in attenuated light conditions (9–17 d) and even longer in anaerobic conditions in water (161–250 d). It is strongly adsorbed by soil particles, so the probability of leaching is low (Cleveland, 2007). Thus, even though other routes of exposure do not seem to represent a threat, more research is needed, to discard any potential chronic intoxication by Acttra/Entrust, or that its effects (at field realistic concentrations) are not detrimental in the long term.

In conclusion, in this work we found that if Acttra/Entrust, which is formulated under the principle to “attract and kill” the pest (Klick et al., 2019), is applied according to manufacturer's instructions the probability of causing acute toxicity to honey bees is very low, making it compatible with commercial pollination in the conditions of the study site.