When stressed by both pesticides and parasites, honey bees do better than expected

In their latest research, Bird and colleagues perform a meta‐analysis to gain a clearer view of the combined effects of parasites and pesticides on honey bee health.

Honey bees pollinate about a third of all crops. Without them, the production of everything from almonds to zucchinis would grind to halt, which makes it especially alarming that honey bees have being dying at increasing rates over the last decade.

Research into what’s causing this increased decline has uncovered two primary factors: pesticides and parasites. As honey bees forage for nectar and pollen they also collect some of the pesticides that farmers use to try and kill pests. This exposure tends to be slight but frequent, and over time is sufficient to harm development and hasten death.

As honey bees are packed together in hives and trucked from one farm to another, they are prone to catching and spreading parasite infections. Microscopic fungi invade their guts, while Varroa mites eat their young and transmit viruses.

Of course, in the field a honey bee colony might need to cope with pesticides and parasites simultaneously, which brings us to this question: How do multiple stressors combine to affect honey bee health? Does one bad thing make another worse?

In formal ecology, this is a question about the additivity of stressor interactions. Basically, when two stressors – in this case, parasites and pesticides – combine, three things could happen.

The combination could be additive, in which case the harm done to bees is simply the sum of the harm caused by parasites and the harm caused by pesticides. Alternatively, the combination could be worse than additive – that is, synergistic – or not as bad as additive – that is, antagonistic.

Honey bee biologists have been particularly worried about the possibility of synergistic stressor effects, and for good reason. Synergistic interactions are extra dangerous; they reduce the odds that a population under stress will be able to recover.

Moreover, honey bee biologists know of a possible mechanism for synergy; they have found that pesticides can degrade honey bee immune function. Perhaps then it is understandable that researchers went looking for evidence of pesticide-parasite synergy, and were so focused on potential synergy that they ignored the potential of antagonism.

But here’s the thing; when ecologists have looked more broadly at how stressors combine to affect the health of animal populations, they’ve found that stressor antagonism is as least as common as stressor synergy.

And for honey bees, we know of at least one way in which one stressor could mitigate against the effects of another. In some cases, pesticides appear to reduce the abundance of honey bee gut parasites; they harm bees, but also the enemies of bees. So, dismissing the possibility of pesticide-parasite antagonism is probably not warranted.

Bird et al fig
Comparison of single and combined stressor treatments to predicted additive effects on the risk of mortality of honey bee workers. A risk ratio of one is no effect. The additive effect is the sum of the effects of single stressor treatments and the multiplicative effect is the sum of effects on a logarithmic scale. All treatment effects are significantly different from each other.

To get a clearer view of how pesticides and parasites combine to effect honey bee health, we performed a meta-analysis. What we found was surprising.

By and large, when honey bees are exposed to pesticides and parasites in concert they fare better than expected. Stressor effects are antagonistic. Researchers had been hunting doggedly for evidence of stressor synergy and had hardly considered antagonism. In fact, antagonism is what tends to happen.

What this means for honey bee management remains unclear. Antagonistic effects on honey bee health from pesticides and parasites could be due to multiple biological mechanisms. It could be that one stressor mitigates against another, as in the example noted above where pesticides hurt honey bee parasites.

Antagonism could also arise because pesticides and parasites activate the same multi-purpose stress-compensation responses. Maybe bees suffer more from the cost of having to turn on and run such a system, than from which stressors in particular they have to contend with.

A better understanding of the mechanisms of stressor antagonism may help pave the way for effective interventions – at least now we know that antagonism is what we need to explain.

Read the full article Parasites and pesticides act antagonistically on honey bee health in Journal of Applied Ecology

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