More Sketchy Science: Do Tomatoes Really Eat Insects?

I’ve been seeing this story more and more around the news, exemplified by this Telegraph article…apparently, there are some researchers that discovered that tomatoes have hairs on them which trap and kill small insects. The Telegraph reports:

New research shows that they capture and kill small insects with sticky hairs on their stems and then absorb nutrients through their roots when the animals decay and fall to the ground.

It is thought that the technique was developed in the wild in order to supplement the nutrients in poor quality soil – but even domestic varieties grown in your vegetable patch retain the ability.

This is kind of a cool discovery. As a biologist, it doesn’t really surprise me that plants would defend themselves through these means. As large, mobile animals we tend to think that critters are complex only if they move which totally isn’t the case. Plants produce a lot of compounds in self-defense…anyone who’s had poison ivy knows this. Anyone who’s seen a TV show where folks were poisoned with ricin knows this. A large part of plant-insect ecology revolves around plants producing toxins which kill insects. Most of what you taste in food are chemicals which defend the plant against insect invaders.

However, I’m skeptical of what was going on here as presented. In this article, as well as the multitudes of others, they made it sound like the insects trapped by the tomatoes had to be small. And well…small insects are tiny. Like…look at them under a microscope to identify them tiny. Thrips and whiteflies, which are common greenhouse pests are commonly about the size of the dot at the top of the letter i on this page (assuming you’re not using a friggin 50-inch LCD TV as a monitor) and I really don’t see them deriving a whole lot of nutrition from the types of insects which commonly infest them.

It’s a cool finding, and it could give us some insight into predatory plants like the sundew in the video below evolved. However, I sincerely doubt that this is a self-fertilization mechanism as the authors in the article claim. To their credit, they admit this…but that doesn’t stop the science journalists from turning something which is relatively unsurprising into a sensationalist news story about murderous plants.

I’m also finding this quote from the authors quite grating:

We suspect in the domesticated varieties they are getting plenty of food through the roots from us so don’t get much benefit from trapping insects. In the wild they could be functioning in the way that could properly be considered carnivorous.

When it comes to ecological roles, I’m a splitter and favor complex explanations of different feeding modes. I differentiate tachinid (and wasps) flies as parasitoids instead of parasites because their ecological roles are quite different from those of parasites. In a similar vein, I consider something to be a predator only if they consume the prey directly. Killing something, then allowing it to decay and give you nutrients which filter down from the soil is not predation in my eyes because the plant isn’t actually consuming the insect. It’s simply taking up nutrients which are leached off the insect’s corpse as it decays.

If this were considered predation, then any plant which secretes insect-killing chemicals could rightly be considered a predator under their definition. Furthermore, under this definition the soil-dwelling nematodes which live in the soil around the plants should be considered kleptoparasites, as should the fungi living in the soil. Instead, I think this is much closer to saprophagy, but I think I would ultimately describe this mode of feeding as self-fertilization.

No…it’s a cool but relatively unsurprising discovery. However, classifying these plants as predaceous is going a bit too far in the eyes of this not-so-humble entomology student.