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Title: Monogalactosyldiacylglycerols as host recognition cues for western corn rootworm larvae (Coleoptera: Chrysomelidae)

Author
item BERNKLAU, ELISA - Colorado State University
item Hibbard, Bruce
item DICK, DONALD - Colorado State University
item RITHNER, C.D. - Colorado State University
item BJOSTAD, LOUIS - Colorado State University

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2015
Publication Date: 4/2/2015
Publication URL: http://handle.nal.usda.gov/10113/60874
Citation: Bernklau, E.J., Hibbard, B.E., Dick, D.L., Rithner, C., Bjostad, L.J. 2015. Monogalactosyldiacylglycerols as host recognition cues for western corn rootworm larvae (Coleoptera: Chrysomelidae). Journal of Economic Entomology. 108(2):539-548.

Interpretive Summary: Insects typically utilize various chemical and/or physical factors to recognize their hosts. Host recognition cues may also be of use in the management of insect pests. We isolated MGDG (Monogalactosyldiacylglycerol) from a corn root extract as a host recognition cue for larvae of the western corn rootworm using insect behavior to exposure to the chemical as the key component in the chemical isolation process. The bioactive fraction of the corn extract contained four different MGDG compounds differing in the length and makeup of the waxy portion of the molecule. Biological activity also required a separate fraction which contained a blend of sugars (glucose:fructose:sucrose:myoinositol). Activity of MGDG increased after exposure to and subsequent re-exposure to western corn rootworm larvae, suggesting that larvae may be responding to compounds that they produced after breakdown of MGDG when exposed to insect saliva. In tests with synthetic blends composed of theoretical MGDD-breakdown products, larval responses to four synthetic blends were not significantly different than their response to isolated MGDG from corn extracts. Chemical analysis of MGDG after exposure to rootworm larvae, were consistent with enzymatic breakdown and further supports the possibility that it is the breakdown products of MGDG which are primarily behaviorally active on this major pest. MGDG and its breakdown components could have potential uses in managing this major pest.

Technical Abstract: Monogalactosyldiacylglycerol (MGDG) was identified as a host recognition cue for larvae of the western corn rootworm Diabrotica virgifera virgifera LeConte. An active glycolipid fraction obtained from an extract of germinating maize roots was isolated with thin layer chromatography using a bioassay-driven approach. When analyzed with LC-MS (positive ion scanning), the assay-active spot was found to contain four different MGDG species: 18:3-18:3 (1,2-dilinolenoyl), 18:2-16:0 (1-linoleoyl, 2-palmitoyl), 18:2-18:2 (1,2-dilinoleoyl) and 18:2-18:3 (1-linoleoyl, 2-linolenoyl). A polar fraction was also needed for activity. When combined with a polar fraction containing a blend of sugars (glucose:fructose:sucrose:myoinositol), the isolated MGDG elicited a unique tight-turning behavior by neonate western corn rootworm larvae that is indicative of host recognition. In behavioral bioassays where disks treated with the active blend were exposed to successive sets of rootworm larvae, the activity of MGDG increased over four exposures, suggesting that larvae may be responding to compounds produced after enzymatic breakdown of MGDG. In subsequent tests with synthetic blends comprised of theoretical MGDD-breakdown products, larval responses to four synthetic blends were not significantly different (P < 0.5) than the response to isolated MGDG. GC-MS analysis showed modest increases in the amounts of the 16:0, 18:0 and 18:3 free fatty acids released from MGDG after a 30 min exposure to rootworm larvae, which is consistent with enzymatic breakdown.