I’m always amazed at how brilliant plants can be. For example Billy Krimmel and Ian Pearse showed in 2013 that many plant species exude sticky substances that entrap small arthropods, thereby attracting predators, which then rid these plants of many herbivores that might otherwise consume their leaves or reproductive structures. Jennifer Van Wyk joined this research group (which included Laure Crova) in graduate school. They were hunting for predatory hemipterans (true bugs) for a different experiment, which involved looking for them on turkey mullein (Croton setiger). They found plenty of predators, but almost no prey. This was puzzling; what were these predators eating? Intrigued, the researchers swabbed the turkey mullein leaves for pollen and found relatively vast quantities of pollen trapped in the trichomes (hairlike protuberances) of the leaves. Much of the pollen was from other species, and the researchers suspected that the trichomes were removing pollen from pollinators (primarily bees) that came to visit. Presumably the predators, which included spiders, hemipterans and ants, were attracted to this highly nutritious pollen.
Van Wyk and her colleagues wondered whether pollen capture benefitted turkey mullein. If turkey mullein used pollen to attract predators, and predators ate herbivores, pollen extraction by trichomes would be an adaptation that formed part of turkey mullein’s defense strategy. If this is true, supplementing turkey mullein with additional pollen should increase visitation by predators, and decrease herbivore abundance. With fewer herbivores, the researchers predicted less leaf damage.
Supplementing turkey mullein with additional pollen presents its own set of problems – most importantly coming up with enough pollen – in particular pollen that predators want to eat (Van Wyk and her colleagues collected a pound of oak pollen, only to find that predacious bugs were not interested in it). The researchers grew sunflowers in greenhouses, secured squash pollen from friends’ gardens and used tuning forks to vibrate pollen from tarweed flowers.
The researchers then set up experiments using 60 turkey mullein plants from one population in 2013 and 80 plants from another population in 2014. Nearby plants were paired up, with one member of the pair receiving 150 mg of supplemental pollen each week from mid-August to mid-September. They surveyed all arthropods visible to the naked eye, and categorized each species as predator or herbivore based on its primary diet (many of the arthropods were actually omnivorous).
In accordance with expectations, predator abundance was substantially greater in the supplemented populations in both years of the study. Spiders showed the most consistent increase, while Orius (the minute pirate bug) increased significantly in the 2014 population. The 2014 population had fewer arthropods of all species, possibly because it was immediately adjacent to an agricultural field.
The results are less clear-cut with herbivore abundance. Fleahoppers were 18% less abundant on supplemented plants in 2014, and slightly (not significantly) less abundant in supplemented plants in 2013. Plants with a greater number of spiders had fewer fleahoppers, suggesting that spiders were eating them (or scaring them away). The researchers were unable to measure the abundance of an important herbivore, the grey hairstreak caterpillar, which forages primarily at night, and retreats into the soil during the heat of the day.
Lastly, supplemented plants suffered much less leaf damage than did unsupplemented plants.
Taken together, these experiments indicate that turkey mullein uses its trichomes to capture pollen and attract a diverse army of predators, which reduce herbivore abundance and reduce damage to the plant. It is possible that pollen supplementation could be used on a larger scale to reduce herbivore loads on agricultural crops. More generally, it will be interesting to see whether other plants with sticky trichomes, such as the marijuana plant Cannabis sativa, also use their trichomes to attract predators and reduce herbivore abundance.
note: the paper that describes this research is from the journal Ecology. The reference is Van Wyk, J. I., Krimmel, B. A., Crova, L., and Pearse, I. S.. 2019. Plants trap pollen to feed predatory arthropods as an indirect resistance against herbivory. Ecology 100( 11):e02867. 10.1002/ecy.2867. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.