Plants & Karate? Using Self-Defense in IPM
When we think about plants being aggressors toward insects, the insect-devouring Venus flytrap or pitcher plant may come to mind. Most plants, however, do defend themselves against attacking pests and have adapted ways to overcome being completely eaten. These defense traits can be useful when developing resistant varieties and new tools in pest suppression.
The most noticeable plant traits to deter feeding are physical. Thorns on stems of raspberries, for example, may deter neighbors from picking every berry. On a smaller scale, some plants have dense mats of trichomes (plant hairs) that make it difficult for insects and pathogens to reach the nutritious plant tissue. These hairs are sometimes accompanied by a sticky substance that traps insects or alters their movement. For example, hairy varieties of beans and strawberries reduce eating or egg-laying by aphids, leaf hoppers, and whiteflies. And glossy or waxy leaf varieties of cabbage reduce the ability of diamondback moth larvae from mining the leaves, leaving them exposed to predators.
Plant breeders use molecular tools to determine mechanisms of plant resistance, such as with a cabbage selection found to be resistant to whitefly.
Before feeding on milkweed leaves, monarch caterpillars overcome the plant's defenses by trimming trichomes and severing leaf veins to bleed out the toxic latex.
Inconspicuous defense traits include plant-produced chemicals that make them less suitable as a food source. Several plants, like Sacred datura (jimsonweed) and foxglove are harmful if eaten by animals because of specific plant chemicals. And it has long been known that crushed chrysanthemum flower heads and seeds have insecticidal properties (pyrethrum). Some other examples of plant chemicals that deter pests are the soap-like saponins in alfalfa, the bitter tasting glucosinolates in mustards and Brussels sprouts, nicotine in tobacco, and cucurbitacins in cucumbers.
In some cases, when the plant is attacked by an insect or pathogen, levels of these chemicals become elevated. This added protection can help slow additional feeding and prevent complete death of the plant. Plant breeding has been used to select plants with high levels of these chemicals and breed them for pest resistance. The trade off, however, is that high concentrations of these chemicals can lead to a less palatable plant. In cases where pests are specialized on a particular group of plants, like cucumber beetles, these elevated chemicals can alert the pest to the location of its food source.
When plants are damaged by insects, researchers have found that saliva of the attacking pests can cause plants to release a plume of specific blends of volatile chemicals, called herbivore induced plant volatiles. This “call for help” leads to an increase of predators and parasitoids that are attracted to these plant volatiles, and then feed on the pest. The volatiles in the air sometimes create a spiral effect by promoting neighboring plants to produce the same defense chemicals.
Several plant hormones are involved in the production of defense chemicals and volatiles. Methyl-salicylate is a plant hormone that has been synthesized and is available as a lure (e.g., PredaLure). Several studies show that this lure is effective in attracting predatory insects and parasitoids that aid in pest suppression. There is on-going work being done to evaluate the complex interactions of plants and insects when using these lures.
The plant traits described above have been exploited for the development of resistant varieties, insecticides, lures, and other pest management strategies. For example, some large scale production farms are using mustard green manure as a bio-fumigant to suppress pathogens in the soil. With advances in science and technology, we are gaining a better understanding of plant resistance mechanisms, plant defense chemicals, and their interactions with pests. With this knowledge, we can improve on their use as a tool in integrated pest management.
-Ricardo Ramirez, Entomologist
Zehnder, G. 2010. Host plant resistance and tolerance to insect pests. eOrganic article 2564.
Freeman, B.C. and G.A. Beattie. 2 008. An overview of plant defenses against pathogens and herbivores. The Plant Health Instructor.
Mortensen, B. 2013. Plant resistance against herbivory. Nature Education Knowledge 4:5.