The theoretical basis for interpreting interactions between agents is rooted in the history of testing combinations of chemical poisons, such as pesticides, but this theoretical framework is broadly applicable to many biotic and abiotic factors that may interact in bees (section 3 of this manuscript). Bliss (1939) recognized three basic types of interactions between agents that can be observed: Independent Joint Action, Additive Joint Action and Synergistic Action (Robertson et al., 2007).
The simplest interaction between agents, and the implicit null hypothesis in experiments testing for interactions, is termed “Independent Joint Action”. In independent joint action, the different agents act on bees through different modes of action and no combinatorial effects are observed. The more highly toxic agent in a combination is understood to cause the observed mortality (or other toxicological endpoint) and the observed mortality is indistinguishable from mortality when the more toxic agent is administered alone.
An agonistic interaction occurs when the toxicity of two agents applied together is higher than that of either agent when applied alone. If an agonistic interaction is observed and agents are known to work through similar modes of action, then the term additive toxicity is used. For example, if bees are exposed to different pyrethroid pesticides which share the same mode of action, then the observed toxicity is a sum of the doses of the different pyrethroid pesticides (e.g. tau-fluvalinate and bifenthrin, Ellis and Baxendale, 1997). Differential potencies between different agents with similar modes of action may need to be taken into account (Robertson et al., 2007).
Agonistic interactions may also be synergistic in nature when the toxicity of a combination of agents cannot be predicted from knowledge of the toxicity of each agent alone. Synergistic interactions do not generally occur at the active site (but see Liu and Plapp, 1992), but instead occur when one agent affects the absorption, distribution, metabolism or excretion of the other agent, rendering it more toxic to bees. For example, piperonyl butoxide acts synergistically with both thiacloprid (Iwasa et al., 2004) and tau-fluvalinate (Johnson et al., 2006) by inhibiting the metabolism of these pesticides and greatly increasing their toxicity to bees.
Antagonistic interactions, where a combination of agents is less toxic than each agent alone, may also be observed.
The potency of an interaction can be substantially affected by the ratio of the different agents, for example the level of exposure to coumaphos affects bees’ susceptibility to tau-fluvalinate (Johnson et al., 2009). A range of ratios between agents can be explored using the methods described.