Feeding deterrence bioassays

One example for a chemically defended zooplankton species is the Antarctic pteropod Clione antarctica. This shell-less pelagic mollusk offers a potentially rich source of nutrients to planktivorous predators. Nonetheless fish do not prey on this organism, due to its efficient chemical defense. In a bioassay-guided structure elucidation, pteroenone 37 could be isolated and characterized as the main defensive principle of C. antarctica [82,83]. If embedded in alginate, this compound is a feeding-deterrent in nanomolar concentrations. This unusual metabolite is likely to be produced by C. antarctica itself and not accumulated from its food, since its major food sources did not contain any detectable quantities of 37. 

Interactions between antifecdant sesquiterpenes and other plant allelochemicals. Binary combinations of one of the more potent WCR feeding deterrents with another at a dose that gives weak feeding deterrence were explored with eight combinations of chemicals in the squash disk bioassay. No synergistic or antagonistic interactions for combinations of deterrents within or between the sesquiterpene (V-VII, IX), diterpene (XI, XII) and flavonoid (XVII) classes were noted. This indicates that the suite of antifeedants present in sunflower inflorescences act jointly in an additive fashion. 

The biological effects that we are searching for are of three types (Figure 1). The first is feeding stimulation. This is the ability of a substance to induce a fish to feed in the absence of any "whole" food stimulus. The second effect is that of feeding enhancement. The effect is manifested by an increase in consumption of an enhancer-modified food item over the amount of the same food item consumed without added enhancer. Finally, a feeding deterrent effect is marked by the ability of a substance to inhibit a fish from feeding upon a food that is normally found to be completely palatable. We measure these effects by a behavioral bioassay, which is described in detail below. 

Using innovative methods, as well as classical bioassays, we have studied feeding deterrency, larval growth inhibition and acute toxicity of a number of Nigerian plant extracts. 

Leafedisk Antifeedant Bioassay Ten l.O cm sweet potato leaf disks were placed in marked wells in an agar-coated petri dish. Five disks were alternatively treated with 10 pi of plant extract (100 pg/cm ) or solvent (acetone or methanol). Five third instar Spodoptera litura (Lepidoptera Noctuidae) larvae per dish and three dishes were used per treatment. The treated dishes were placed in an incubator at 27 C and 75-80% RH for 16-18 hours in darkness. The leaf surface consumed was measured with a video camera interfaced to a personal computer as described earlier (6). The feeding index was calculated as 1= %T / (%T + %C) (%T = % of treated disks consumed, %C = % of control disks consumed). An arbitrary level of I<20 was used as the criteria to determine effective feeding deterrents (7). 

Shaw, B.A., P.J. Harrison R.J. Andersen. 1994. Evaluation of the copepod Tigriopus califomicus as a bioassay organism for the detection of chemical feeding deterrents by marine phytoplankton. Mar. Biol. 121 89-95. 

Ftirodysinine (60) and euryfuran (52) have been identified as feeding deterrents in nudibranchs (78, 80, 524, 525). Deterrent activity has also been reported for pallescensin (50), idiadione (156), the isonitrile 769, and the isothiocyanate 770 (524). These compounds were also toxic to goldfish at 0.1 mg/mL. Axisonitrile (771) was ineffective as antifeedant, but showed strong ichthyotoxicity, the minimum active concentration being at 8 ppm. Furoscalarol (772) and deoxoscalarin (773) were active in the feeding inhibition bioassay (526). 

In view of the reported deterrent activities of Solenopsis and Monomorium venoms under field conditions (1, IJ., 12), it seemed worthwhile to evaluate the comparative repellencies to ants of some of these venom-derived alkaloids. In this investigation, the deterrency of these nitrogen heterocycles to a variety of ant species was determined by using a feeding bioassay in which the reactions of hungry ant workers to alkaloid-treated food were quantified Selection of a variety of aggressive ant species... 

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