This experiment allowed gaining new insights into bait preference and the effects of bait flavoring, bait color, pre-baiting, baiting device and habituation on bait consumption and bait selectivity when targeting wild boar of different age classes. As initially hypothesized, we found that adult wild boar preferred flavored baits (IREC and Flavor); that pre-baiting with pig feed increased bait consumption (at the cost of decreasing bait selectivity); and that baiting device improved bait selectivity and enabled targeting specific age-classes. However, we found no habituation to the baits, meaning that previous exposure to baits did not improve bait consumption.
Bait preference towards the IREC bait, which was originally designed for wild boar (Ballesteros et al. 2009b) is no surprise. It is however important to note that, on the video footage taken at the pre-baited sites, wild boar always took the feed first and explored the baits only after most of the feed had been taken. This implies that there is still space for bait palatability improvement, even for the IREC bait.
The choice experiments demonstrated that adult wild boar (those able to lift the pavel stones) preferred flavored baits, since the rather flavor- and taste-neutral puffed leguminous baits were more consumed and more often chosen first (43.75%) when they contained the artificial flavoring. Again, other flavorings as well as the attractiveness of different bait components (sugar, wheat) deserve attention in further research.
Easily accessible and attractive food, such as cracked corn, is often used to lure wild boar or feral pigs to baiting sites (Bengsen et al. 2011a). In Texas (USA) approximately 18 days of pre-baiting allowed feral pigs to habituate to using pig-specific bait stations and consume a novel bait type (Snow et al. 2019). As expected, in our experiment pre-feeding, or deploying the baits at already used wild boar feeding sites, increased bait consumption. This occurred consistently for all bait types, although at the cost of also losing more baits to birds. It would be interesting to compare sites with no pre-feeding with sites with pre-feeding, but without using permanent feeding sites. Field experience with tuberculosis vaccine deployment suggests that uptake rates are higher when permanent feeding sites are used (Díez-Delgado et al. 2018).
We used two types of bait-delivery devices to improve bait selectivity and enable targeting specific age-classes. Selective piglet feeders (Ballesteros et al. 2009a, b) have the advantage of facilitating efficient bait deployment to piglets, which otherwise will get outcompeted by adults (Brauer et al. 2006; Díez-Delgado et al. 2018). However, this comes at the cost of higher bait losses to birds. Thus, we tested green-colored baits to try to minimize these losses, proving that this is a useful improvement. In turn, heavy stones almost ensure bait delivery to larger juvenile and adult wild boar, with limited losses to other animals, but at the cost of not targeting piglets and potentially compromising the bait uptake rate in this important age class. Therefore, we postulate that the ideal approach for vaccine deployment is one taking advantage of both kinds of devices, although this needs to be confirmed with further experiments, probably including bait markers. For toxicants however, neither option is truly suitable for operational use since non-target species accessed both baits frequently.
In this study we found no habituation to the baits: exposure to baits in week 1 did not improve bait consumption in week 2. This was rather unexpected since habituation and social interactions are likely to overcome neophobia in pigs (Figueroa et al. 2013; Snow et al. 2017). This is relevant because, at least with the bait types used in this experiment, there was apparently no neophobia, so the baits can be used rapidly in new sites without previous habituation.
The baits used in this study did not contain markers. Hence, uptake rates (i.e., the % of wild boar consuming at least one bait) were not assessed. It is well established that increasing baiting intensity and bait station density increases bait uptake in feral pigs (Cowled et al. 2008), and good uptake rates have been reported in piglets (50–92%), which were the target age group of previous trials (Diez-Delgado et al. 2018). Since adult wild boar will be as much a target as piglets in the specific case of ASF, assessing uptake rates in adults in a range of density and management situations remains an important pending issue. Furthermore, IREC baits and other larger baits targeting wild boar can contain a blister or a plastic tube carrying any pharmacological drug. This would not significantly change bait smell or taste. However, if the drug was mixed directly into the bait matrix, this could eventually affect uptake rates.
The ideal bait would be cheap and easy to produce, stable against water and extreme temperatures, hard enough to allow aerial deployment, species-selective and suitable for all age-classes. In addition to the pending issues already mentioned, seasonality (McIlroy et al. 1993; Bengsen et al. 2011a; Ferretti et al. 2014), and bait suitability for aerial deployment remain to be studied regarding the IREC bait.