What happens to carnivorous plants when the insects bite back?

Usually, when people look at the interaction between carnivorous plants and insects, it’s the bug that’s the loser. But that’s not always the case. Winer and Horner have surveyed the flowers of Sarracenia alata, the pale pitcher plant, in Texas. They found that around half of the pitcher plant’s flowers were attacked by herbivores. 

Florivory, eating flowers, can significantly impact a plant’s fitness. At the direct level, getting eaten is bad, but it’s particularly bad when it’s a flower. Leaves aren’t that specialised, but the costs of building a flower with specialised structures are a lot greater than a typical leaf. Plants sink lots of resources into a flower, and these resources are lost when a flower is eaten.

Another problem is that when parts of a flower get eaten, they’re not doing their job. If a flower loses its petals, then it’s not putting on a display for pollinators. No pollinators mean no seeds, so even a small hit on the flower can have serious consequences.

Winer and Horner surveyed a bog in Texas in 2017 and 2018. They were looking for Sarracenia alata, and caterpillars that seek out pitcher plants for food and security, Exyra semicrocea, known as the pitcher plant moth. Matt Candeias has a post about them at In Defense of Plants, where he talks about their unusual lifestyle. The pitcher plant moth lays its eggs in the pitcher. When the caterpillar hatches, it starts eating the pitcher. It doesn’t slip and fall into the pitcher’s trap, thanks to its adapted feet. When they’ve eaten enough, they find a pitcher to pupate in, and when they emerge as a moth, they go to find a mate around the pitcher plants. Far from being instant death, the pitcher is co-opted as a bodyguard for the moth’s young.

Winer and Horner examined the plants when the pitcher plants were flowering to see if the caterpillars were attacking the flowers and how much damage they were doing. It seems that the damage varies from year to year. In their paper, Winer and Horner write: “There was a significantly greater density of flowers in 2017 than 2018, yet a greater proportion of flowers was damaged in 2017. Differences between years in the proportion of flowers attacked may have been due to variation in the population density of E. semicrocea. Indeed, assuming there was no difference between years in the movement of larvae among flowers, the population density of feeding larvae (estimated from the number of flowers damaged/m2) was greater in 2017 than in 2018.”

When the caterpillars attacked the flowers, they weren’t indiscriminate. The authors found that the pitcher plant moth has a taste for anthers, the pollen-laden male organs. In contrast, they note that the ovaries were attacked last, if they were attacked at all. They argue the preference might be due to the anthers being nutrient-rich and lightly defended compared to other flower parts.

The difference in preference means that there could be implications for differences in male and female fitness for plants. Winer and Horner add that it’s not clear if the consumption of over 50% of the anthers in a year also leads to pollen limitation among the plants.

While carnivorous plants can inflict damage on insects, insects have shown they can also fight back.

RESEARCH ARTICLE

Winer, Z.M. and Horner, J.D. (2022) “Floral herbivory in the carnivorous plant, Sarracenia alata,” Arthropod-Plant Interactions. https://doi.org/10.1007/s11829-021-09880-y