Chemical stimulants of leaf-trenching by cabbage loopers: natural products, neurotransmitters, insecticides, and drugs.

Larvae of the cabbage looper, Trichoplusia ni (Lepidoptera: Noctuidae), often transect leaves with a narrow trench before eating the distal section. The trench reduces larval exposure to exudates, such as latex, during feeding. Plant species that do not emit exudate, such as Plantago lanceolata, are not trenched. However, if exudate is applied to a looper's mouth during feeding on P. lanceolata, the larva will often stop and cut a trench. Dissolved chemicals can be similarly applied and tested for effectiveness at triggering trenching. With this assay, I have documented that lactucin from lettuce latex (Lactuca sativa), myristicin from parsley oil (Petroselinum crispum), and lobeline from cardinal flower (Lobelia cardinalis) elicit trenching. These compounds are the first trenching stimulants reported. Several other constituents of lettuce and parsley, including some phenylpropanoids, monoterpenes, and furanocoumarins had little or no activity. Cucurbitacin E glycoside found in cucurbits, another plant family trenched by cabbage loopers, also was inactive. Lactucin, myristicin, and lobeline all affect the nervous system of mammals, with lobeline acting specifically as an antagonist of nicotinic acetylcholine receptors. To determine if cabbage loopers respond selectively to compounds active at acetylcholine synapses, I tested several neurotransmitters, insecticides, and drugs with known neurological activity, many of which triggered trenching. Active compounds included dopamine, serotonin, the insecticide imidacloprid, and various drugs such as ipratropium, apomorphine, buspirone, and metoclopramide. These results document that noxious plant chemicals trigger trenching, that loopers respond to different trenching stimulants in different plants, that diverse neuroactive chemicals elicit the behavior, and that feeding deterrents are not all trenching stimulants. The trenching assay offers a novel approach for identifying defensive plant compounds with potential uses in agriculture or medicine. Cabbage loopers in the lab and field routinely trench and feed on plants in the Asteraceae and Apiaceae. However, first and third instar larvae enclosed on Lobelia cardinalis (Campanulaceae) failed to develop, even though the third instar larvae attempted to trench. Trenching ability does not guarantee effective feeding on plants with canal-borne exudates. Cabbage loopers must not only recognize and respond to trenching stimulants, they must also tolerate exudates during the trenching procedure to disable canalicular defenses.