For millennia, plants have evolved innovative ways to attract animals to their flowers. As a part of their arsenal for appealing to animals, plants utilize sweet-tasting nectar to make sure that potential pollinators are rewarded for visiting. But how do plants entice effective pollinators without luring in animals that only drink their nectar and leave? Some plants, such as the balloon milkweed (Gomphocarpus physocarpus), produce chemicals in their nectar that are toxic to some animals that aren't very good at pollinating them – such as honeybees – but leave their wasp pollinators seemingly unharmed. Hannah Burger of Ulm University, Germany, and an international group of researchers from Germany, the United Kingdom and South Africa wanted to understand what makes the balloon milkweed toxic to honeybees but tasty to the vespid wasps that pollinate its flowers.
First, the team began the difficult task of discovering which chemical compounds were making the plant's nectar toxic to bees but not to the wasps. What they found in the nectar was a number of highly toxic, heart-stopping chemicals called cardenolides that are commonly found in the leaves of milkweeds. Interestingly, Burger and colleagues also found these same chemicals in the nectar of a closely related plant (Gomphocarpus fruticosus) that is pollinated by bees, but the amount of the toxic cardenolides was vastly different. The nectar of the wasp-pollinated milkweed contained larger amounts of the compounds that were toxic to honeybees. But sometimes, the honeybees would still feed on the nectar from these toxic plants, so are the vespid wasps just better at dealing with the toxins in the nectar?
Burger and colleagues allowed the bees and wasps to feed on nectar from the balloon milkweed that they modified to have either high, medium or low concentrations of cardenolides for 5 days. The bees didn't drink much of the nectar, but those that drank nectar with high concentrations of cardenolides died within 2 days, and those that drank nectar with medium concentrations didn't last much longer. However, the bees that fed on nectar with low concentrations of the toxic cardenolides seemed to be okay consuming this sweet snack. Surprisingly, the wasps couldn't handle high concentrations of the toxic compounds either – surviving for only 4 days. However, if the nectar contained only medium or low levels of the toxic cardenolides, the wasps were unharmed, suggesting that there is something about the wasps that protects them against the poisonous effects of the milkweed's nectar. The honeybees don't often visit the balloon milkweed, whereas the vespid wasps do. So is there something else besides the nectar that is attracting the wasps to the plant?
It turns out that the flowers also have 21 different chemicals that the antennae of bees and wasps respond to. The team found that the most abundant chemical in the flower scent was acetic acid, which smells like vinegar. The researchers then made a synthetic version of the scent of the wasp-pollinated milkweed's flowers to give to the wasps. Most of the wasps (22 of 25) were attracted by the scent of the balloon milkweed flowers. ‘Nectar that smells like vinegar is probably avoided by honeybees because they might think that the sweet sugar is already used by yeasts and the nectar is not a good source of sugar anymore’, speculates Burger. These floral scents and nectar compounds seem to be important, at least for the balloon milkweed, in attracting the right kind of pollinator. This is just another example of how plants attract the right pollinator that will get the job done, not just drink their fill and leave.
