Appraisal of Pheromone bioassays

It is not easy to devise bioassays to determine the function of the components of the Nasonov pheromone when they probably interact in a complex way and perhaps have different functions in different behavioural situations.

It is possible that some components are especially effective in stimulating alight- ing. Nearly all methods used are open to some criticism; field experiments on foraging using different odours or combinations of odours at adjacent dishes must inevitably lead to some contamination and mixing with unknown influences on the results. The ‘roundabout’ used by Ferguson et al. (1979) and Free et al. (l98la, 1982a) must lead to a complex mixture of components in the air in the pathway of the revolving chemicals, with quite unpredictable consequences. In the clustering and hive entrance bioassays (but not the foraging) bees responding to Nasonov pheromone are induced to release it themselves.

Even slight differences in the methods used could have significant influences on the results. It is particularly difficult to perform experiments that are free from diversions and chemical contamination, especially when the observer needs to be in close proximity as in the foraging bioassays.

However, the degree of agreement from the very different bioassays described above is fortunately often encouraging. Thus, it has been found

that nerolic acid, (E)-citral and geraniol are the most important components for inducing clustering, stimulating pheromone release at the hive entrance and (usually) for attracting foragers. The presence of geranic acid has been found to diminish the attractiveness of a mixture to clustering bees and to foraging bees, although it did stimulate Nasonov gland exposure. Nero! and farnesol were not important in inducing clustering or in inducing Nasonov gland exposure at the hive entrance and in these circumstances they even tended to have a repellent effect. Farnesol, which has a relatively high molecular weight and is relatively abundant (Fig. 13.1), may well act as a keeper substance. It must always be borne in mind that in different behavioural situations bees may perhaps respond to different components or mixtures of components. Learn about pheromone attraction |
The results from the bioassay on cluster formation led to the development of a Nasonov pheromone lure. Each lure consists of a polyethylene vial (30 mm long, 15 mm diameter), loaded with a 121:1 mixture (10 mg of each) in hexane (100 ul) of (a) (E)-and (Z)-citrals, (b) geraniol and (c) nerolic and geranic acids. This mixture has the geraniol equivalent of about 5000 workers. The vial has a push—in cap which is closed immediately the chemicals have been inserted; they are absorbed into the walls and pass to the outside by diffusion.

It was with a lure just inside the entrance, and unbaited empty hives. In 39 out of 40 trials swarms occupied a hive with a lure, and usually chose the hive most visited by scout bees. Frequently there were more than 50 scout bees at the entrance of the preferred hive immediately before the swarm arrived (Free et al., 1981b).

Decreasing the amount of chemicals in a lure decreased its effectiveness, but increasing the amount of chemicals failed to increase its attractiveness to swarms, probably because the walls of the lure were already saturated with chemicals and increasing the amount of chemicals failed to increase the amount released. However, increasing the number of lures in an empty hive increased the likelihood that it was chosen by caged swarms. Experiments in cages also indicated that the attractiveness to swarms of empty new hives, empty hives previously occupied by colonies, and empty hives that contained old empty comb were all enhanced by the presence of lures (Free et al., 1984b).

In field trials pairs of Langstroth hives, each with two empty used combs, were placed 2 m apart in 50 locations in Southern England and a lure was fixed just above the entrance inside one hive of each pair. Of the 100 hives, 12 with lures, but none without, were occupied by swarms (Free et al., 1981b). Similar tests in New York State (Lesher and Morse, 1983) and Kenya (Kigatiira er al., 1986), gave similar results (Table 13.3). Simple and inexpensive traps containing the Nasonov lure have recently proved to be an opportunity.