It seems like a logical problem: Sarracenia (pitcher plant) and Dionaea muscipula (Venus flytrap) are entirely dependent on insects visiting their flowers and transferring pollen for reproduction. These same plants also capture insects to obtain nutrients. What if a plant accidentally eats its own pollinator? How is it then that the plant manages to survive evolutionarily? This article is an accessible version of that scientific quest: does this "pollinator-prey conflict" (PPC) truly exist, and if not, how do carnivorous plants prevent it?

What is the pollinator-prey conflict, actually?

The term comes from research by Jürgens and colleagues, among others, who conducted a large literature review in 2012 on this phenomenon in carnivorous plants in general. They state that a true conflict only arises if three conditions are met simultaneously. If one is missing, there is no problem.

The three conditions for a pollinator-prey conflict

1

Dependence

The plant needs insects to be pollinated; self-pollination is not (sufficiently) possible.

2

Overlap

The same insect species that pollinate are also caught as prey in the traps.

3

Scarcity

There are too few pollinators or too little pollen available to compensate for the loss.

In other words: even if a plant occasionally catches a pollinator, it only becomes a real problem if those pollinators are scarce. If there is an abundance of pollinating insects, a single casualty makes little difference to the plant's reproduction.

📜 Recent research: It was Charles Darwin who, in 1875, was the first to scientifically demonstrate that plants actually digest meat, with his book Insectivorous Plants. Whether the prey are the same as the pollinators only received serious attention much later: the first thorough literature study on the pollinator-prey conflict only appeared in 2012.

Flower versus trap: two very different structures

Sarracenia and Dionaea are both entirely dependent on insects for their reproduction, though they do so in different ways. In Sarracenia, the flower has a unique umbrella-like style that collects pollen; a pollinator must literally crawl into the flower to reach it, touching the stigmas in the process. This ensures both cross-pollination and (to a lesser extent) self-pollination. In Dionaea, it's different: the flowers are protandrous, meaning that the pollen is already mature before its own stigma becomes receptive. This largely precludes self-pollination.

Also read in our other blog post how you can pollinate the pitcher plant Sarracenia yourself to harvest seeds.

Fig. 1. A group of Dionaea muscipula plants, with a close-up on the right of the protandrous flower: the pollen has already been released before its own stigma becomes receptive

Both plants thus meet the first condition for a PPC: they are entirely dependent on insects for pollination. The real question is: which insects pollinate them, and are these the same insects they also catch?

Sarracenia: who pollinates, who gets caught?

In Sarracenia, it's mainly bumblebees of the genus Bombus that are considered the most important and efficient pollinators for most species in the genus. In species with smaller flowers, such as S. minor, S. psittacina, and the S. rubra group, smaller bees (including those from the family Megachilidae) play a larger role. This is logical: large bumblebees are simply too big to crawl into a small flower. Carrion flies (Sarcophagidae) are also mentioned as potential pollinators, although they may sometimes be too small to properly touch the stigmas of larger flowers.

Meanwhile, Sarracenia's prey is quite diverse, but ants (Formicidae) and beetles (Coleoptera) together make up the largest portion. This is true in nature; if you place the plants with us, they will mainly catch many flies. In species with tall pitchers like S. flava and S. leucophylla, more flying insects are also caught. For a complete list of insects caught by the carnivorous plant Sarracenia, please see our older blog post.

Fig. 2. The striking flower of Sarracenia: the flowers usually appear in spring before the traps. So, Sarracenia flowers first and only then grows leaves, the leaves themselves being the carnivorous traps that catch insects.

Dionaea: who pollinates, who gets caught?

In Dionaea muscipula, there's more research: Youngsteadt and colleagues (2018) studied pollinators and prey in the exact same research, in the same locations. This is a great advantage: it allows for direct measurement of overlap instead of comparing separate studies. They found about 64 different insect groups carrying Dionaea pollen, mainly bees (Hymenoptera) and beetles (Coleoptera). At the species level, the sweat bee Augochlorella gratiosa was the most important pollinator, followed by the beetle Trichodes apivorus and the longhorn beetle Typocerus sinuatus.

Prey turned out to consist of 40% spiders (especially jumping spiders and wolf spiders), and for the remainder largely of ants and beetles. In practice (in Belgium and the Netherlands), we ourselves primarily notice flies as prey in our plants in the greenhouse and outdoors.

So: does the conflict exist?

Based on the available literature, the answer for both carnivorous plants is: no, not to a significant extent. The second condition (overlap between pollinators and prey) is not sufficiently met in either case. But that raises a new question: how do these plants manage to prevent this conflict so well? Three mechanisms appear to play a role.

Conclusion: almost no conflict, but a beautiful piece of evolution

In short: both Sarracenia and Dionaea are completely dependent on insects for their reproduction, but there is hardly any overlap between the insects that pollinate them and the insects they catch. In Sarracenia, temporal separation plays a major role, while in Dionaea, spatial separation is key. Differences in scent, nectar, and color likely also play a role in both, although this has not been fully investigated.

The beautiful thing is that this is not so surprising from an evolutionary perspective: a plant that accidentally catches its own pollinators would eventually disadvantage itself. Any adaptation that prevents this provides an advantage and is therefore favored by natural selection. So there is still much to discover, but the main conclusion is clear: these plants, despite their dual relationship with insects, are surprisingly well-balanced.

This is, of course, also good news for our pollinators. Yes, a carnivorous plant will occasionally catch a bumblebee or bee. But this will be incidental, and the effect the plants have on pollinator populations will, in all likelihood, be positive. Bees and bumblebees pollinate the plants en masse, but are only caught occasionally. 

Sources

• Horner, J. D. (2014). Phenology and pollinator-prey conflict in the carnivorous plant, Sarracenia alata. The American Midland Naturalist, 171(1), 153–156.
• Jürgens, A., El‐Sayed, A. M., & Suckling, D. M. (2009). Do carnivorous plants use volatiles for attracting prey insects? Functional Ecology, 23(5), 875–887.
• Jürgens, A., Sciligo, A., Witt, T., El‐Sayed, A. M., & Suckling, D. M. (2012). Pollinator‐prey conflict in carnivorous plants. Biological Reviews, 87(3), 602–615.
• McPherson, S., & Schnell, D. E. (2011). Sarraceniaceae of North America. Redfern Natural History Productions.
• Schnell, D. E. (1983). Notes on the pollination of Sarracenia flava L. (Sarraceniaceae) in the Piedmont province of North Carolina. Rhodora, 85(844), 405–420.
• Youngsteadt, E., Irwin, R. E., Fowler, A., Bertone, M. A., Giacomini, S. J., Kunz, M., … Sorenson, C. E. (2018). Venus flytrap rarely traps its pollinators. The American Naturalist, 191(4), 539–546.