From the standpoint of how they regulate their body temperature, we can very crudely divide the animal world into ectotherms and endotherms. Ectotherm body temperature is influenced primarily by the external environment, while endotherms maintain a relatively constant body temperature by conserving the heat they generate from the many chemical reactions that occur within their body. This division is crude because there are many, many organisms, including the bee species that is the focus of this tale, that use both approaches.
For now, I’m going to make a generalization that it is very difficult to be a small ectotherm in cold climates. After all, small animals can’t have much insulation, and most of the cells belonging to small ectotherms are located near the surface, and subject to the chilling effects of the cold air. Cold cells can’t do much because most chemical reactions that power our small ectotherm need warmth for maximum efficiency.
Perhaps the most common small-ectotherm-in cold-climates approach is to be active only on warm sunny days. This works great, so long as there are sufficient warm sunny days to get on with the business of life (primarily food, sex and not becoming someone else’s dinner). Rumor has it that there was a huge celebration of small-ectotherms-in cold-climates, when they heard about Donald Trump’s election, as they reasoned that global warming was likely to continue, and they might be able to enjoy a more successful life.
Jessica Forrest and Sarah Chisolm wanted to know how rising temperatures would affect the success of the solitary mason bee, Osmia iridis, in cold climates. This bee survives by consuming nectar and pollen gathered from leguminous plants (bean and peas are common legumes), primarily Lathyrus lanszwertii, and Vicia americana. This bee can elevate its body temperature somewhat above air temperature, but its body temperature, and consequent activity levels are nonetheless greatly influenced by air temperature. The researchers reasoned that warmer temperatures would allow the bees to fly more often and gather more nectar and pollen, so they could grow more quickly, reach a larger size and produce more bee babies.
But there’s a fly in many ointments, or in this case a brood parasitic wasp, Sapyga pumila, which can lay its egg in a nest cell where the bee has just laid its egg. The wasp egg usually hatches first, kills and eats the bee baby, and, adding insult to injury, also eats the pollen that the bee mother has left to provision its (now dead) baby. So if warmer temperature helps the wasp, by giving it more flight time and faster development, perhaps warmer temperatures will actually decrease the bee’s reproductive output.
So how does this play out?
Forrest and Chisolm marked and released 109 bees over the course of three years, and discovered that these bees, on average, built more nest cells on warmer days. This could indicate that bee reproductive output increases with temperature. But only if all other factors influenced by air temperature are equal.
But all factors influenced by temperature are not equal. As it turns out, Sapygia wasps, the most significant parasite on these bees, are most active at high temperatures as well. So while bees can make more nest cells at high temperatures, they also need to deal with more parasitic wasps.
These bees make more babies when most of their nest cells are not parasitized. Very few offspring are produced when the parasitism rate climbs above 0.5 (50%).
Overall it’s pretty much a wash for the bees. Bees benefit directly from increased nest cell construction at high temperatures, but suffer from the increased rate of parasitism. In contrast, the parasites are more successful at high temperatures. Several other studies of hosts and parasites have shown that the parasites usually benefit from increased temperature more than the hosts, but there are some counterexamples. The authors caution us that forecasting the impact of future warming requires understanding all the factors that can affect population regulation – a daunting challenge. In the meantime our bees may want to cancel their “We love Donald Trump” party, as their enemies appear to be benefiting more than they are from a warmer climate.