Despite advances in modern agriculture, insect pests continue to be one of the most significant and long-standing threats faced by farmers. Now, researchers in the lab of Michigan State University entomologist William Wetzel are trying to find new ways to combat them by taking advantage of one of nature’s built-in defenses – diversity.
Long before farmers were protecting crops from pests, the ancestors of those crops were protecting themselves with a suite of natural defenses, including leaf hairs that entangle insects, toxins like caffeine or sticky substances that make feeding difficult. Just like no two humans are identical, neither are plants. Different varieties of the same crop exhibit different types of natural defenses. When planted in conventional monocultures on farms, however, only one combination of plant defenses is found throughout the field. This makes it easier for pests not challenged by those particular defenses to infest the field.
A long-standing, but largely unexplored, observation among entomologists contends that fields with a greater diversity of natural defenses will see reduced pest pressure, increasing yields and decreasing both the cost of insect control and the likelihood of resistance to conventional insecticides developing among pest species.
Through a pair of projects, Wetzel and MSU postdoctoral researcher Moria Robinson are attempting to explore the mechanism behind this phenomenon.
“Increasing plant diversity leading to reduced pest density is something people have seen for a long time,” Wetzel, assistant professor in the MSU Department of Entomology, said. “We know there is a link between diversity and pest damage, but if we want to take advantage of that as a management strategy we need to know which plant defenses have an impact on pests.”
Through a nearly $500,000 grant from the U.S. Department of Agriculture National Institute of Food and Agriculture, or NIFA, Wetzel’s lab will conduct a four-year study of natural defenses in tomato plants. Plant breeders have developed numerous tomato cultivars that each emphasize a different plant defense, leading to the organism’s designation as a model system for studying plant defenses. Wetzel’s team will study the crop in both traditional monocultures and in experimental polycultures at MSU’s W.K. Kellogg Biological Station, or KBS, in Hickory Corners, Michigan, to determine which mixtures of defenses produce the greatest impact on pest damage.
If successful, Wetzel’s work will give farmers a powerful new tool against insect pests without increasing the complexity of their farm operations.
“This won’t make farming more difficult,” Wetzel said. “From a growing perspective, every tomato plant in the field will be identical, requiring the same resources and agronomic practices. The only difference will be the diversity of plant defenses.”
Plant defense diversity can also occur at even smaller scales than among plants in a field. Among leaves on a single plant, significant variability can occur in levels of toxins or numbers of ensnaring hairs, lending the same benefits diversity provides fields of plants to the individual.
Through an approximately $155,000 grant from NIFA, Robinson will compare 30 cultivars of the crop alfalfa, which has one of the longest domestication histories in agriculture, with its nearest wild relatives from the Middle East to determine the impact crop breeding may have on the variation of natural defenses within a plant.
“Through crop domestication, humans have been conducting this big evolutionary experiment for centuries,” Robinson explained. “It’s important for humans to have a uniform diet, and we’ve selected traits that have homogenized plant nutrition for that purpose, but those traits are often also beneficial to insects, like higher nutrient levels and lower chemical concentrations. It’s possible we’ve changed not only the average levels of those things, but also their variation across the individual plant.”
In the course of her two-year project, Robinson will examine two defensive traits present in alfalfa – trichomes, small hairs that impede insect pests and saponin, a natural insecticide. She’ll also examine a pair of nutritive traits – leaf water and nitrogen levels – to see if millennia of plant breeding has reduced the diversity of defensive traits in favor of increased nutritional value.
Beginning this summer, Robinson will grow both the wild and domesticated plants together in a common garden at KBS and measure the level of damage both sustain at the hands of pest insects, as well as the types of insects they attract.
Robinson hopes this study will improve our understanding of how pest insects behave, what they need to thrive and how we can better manage them in agriculture. Since insects often target areas of crops that are not harvested for human consumption, Robinson sees increasing defense diversity as a way of improving a plant’s ability to fend off invaders without altering its nutritional value.
“There are certainly herbivores that eat tomatoes, for example, but there are many more that eat the leaves,” Robinson said. “If we can change the variability of the leaves without changing the tomato, we’d be able to give farmers a powerful new pest management tool without altering the end product they rely on.”