Ursula S.R. RÖSE, Ara MANUKIAN, Robert R. HEATH,
W. Joe LEWIS, James H. TUMLINSON
United States Department of Agriculture, Insect
Attractants, Behavior and Basic Biology Research Laboratory, 1700 SW 23rd
Drive, Gainesville, FL 32604, U.S.A.
Plants under herbivore attack release volatile compounds that can serve as cues to lead predators (1) and parasitic wasps (2) to the vicinity of their herbivorous prey or host. Therefore, volatiles released by the plant in response to herbivore attack, benefit both, the plant by attracting natural enemies of the herbivores that feed on its foliage, and the parasitoid and predator by indicating the presence of a potential host or prey.
Cotton plants attacked by lepidoptera larvae release volatiles that attract parasitoids (3). We found chemical evidence that volatiles are not only released by the damaged leaves, but systemically throughout the entire plant. The release of volatites from undamaged leaves of caterpillar damaged cotton plants was detected after two to three days of continuous larval feeding. Compounds systemically released were (Z)-3-hexenyl acetate, (E)-ß-ocimene, linalool, (E)4,8-dimethyl-1,3,7-nonatriene, (E)-ß-farnesene, and (E,E)4,8,12-trimethyl-1,3,7,11-tridecatetraene. These systemically released compounds are known to be released in response to caterpillar damage (4), and are not released in detectable amounts by undamaged plants. Other compounds (indole, isomeric hexenyl butyrates, 2-methylbutyrates) known to be released in response to herbivore damage, were not released systemically. However, when undamaged leaves of caterpillar damaged cotton plants were damaged with a razor blade, those leaves released isomeric hexenyl butyrates, 2-methylbutyrates and large amounts of constitutive compounds in addition to the previously detected compounds. Control plants that were damaged with a razor blade in the same way did not release isomeric hexenyl butyrates or 2- methylbutyrates. Indole was not detected, even after artificial damage.
In two choice flight tunnel experiments, female parasitoids (Microplitis croceipes) were given a choice between cotton plants that systemically released volatiles and undamaged control plants. The lower leaves where the initial caterpillar feeding had occurred were cut off the plant and the cut wrapped in Teflon tape. Leaves of the same size and position were cut off a control plant and the cut was covered with Teflon tape. Parasitoids were significantly more attracted to undamaged leaves of plants that were previously fed upon by caterpillars than to undamaged control leaves. In a second twochoice experiment the response of wasps to cotton plants with artificially damaged leaves was compared to undamaged control plants. Wasps did not distinguish between artificially damaged cotton plants and undamaged control plants. Furthermore, the overall number of wasps that responded to artificially damaged plants and control plants was clearly smaller than the number of wasps responding to cotton plants that released systemically induced voiatiles and control plants in the flight tunnel.
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