Natures wonders: detecting predator feces protecting larvae (Introduction)

by David Turell @, Tuesday, October 08, 2019, 19:00 (234 days ago) @ David Turell

Certain insects detect predator feces when egg laying to protect the larvae when they appear:

"Scientists have demonstrated that not only plant odors determine the best oviposition site for egglaying hawkmoths, but also the frass of other larvae. They specified the repelling substance in the feces which signals the presence of competing conspecifics. Moreover, the researchers identified an odorant receptor which is involved in the detection of the typical smell of larval frass and thereby governs competition avoidance during oviposition.


"Tobacco hornworms (Manduca sexta) are extremely voracious. Their Latin name Manduca is derived from the verb manducare, which means eating or chewing. These insatiable caterpillars, which may become as big as an adult's finger, do not do anything but eat and chew all day long. Therefore, a single larva is able to defoliate a host plant, such as the wild tobacco Nicotiana attenuata or sacred datura Datura wrightii (at least small plants) completely. Hence, one plant is seldom enough to feed a second caterpillar.


"It seemed plausible that tobacco hawkmoths would try to avoid competition for their offspring. Nevertheless, that one specific class of volatile compounds in the larval feces (6-carbon aliphatic acids) would be sufficient to trigger avoidance behavior came as a surprise. Behavioral assays in the wind tunnel in which moths were exposed to individual odor components on filter paper clearly showed that moths were repelled by some of the frass-related compounds.


"...they knocked out specific receptor proteins in the antennae, the moth's nose, that were crucial for the detection of these compounds, and were thus able to show that the ionotropic receptor 8a (IR8a) governs the avoidance response to larval feces.


" It is not a new discovery that tobacco hawkmoths lay significantly fewer eggs on plants that are already being attacked by caterpillars. In addition, the smell of larval feces is known to attract predators, the enemies of the larvae. However, it was not known that the same larval frass [larval feces] ensures that conspecifics which would otherwise compete for food are kept at a distance. Thus, the chemical signal from the frass helps not only the caterpillars which are already feeding on a plant but also the hovering moths: By avoiding the smell of larval feces, they may find better food for their offspring.

Comment: this sort of avoidance activity suggest that the insect somehow knows the future consequences of its egg deposition. How do insects anticipate the future? I doubt they do and are programmed in this way to protect the larvae.

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