Semiochemicals bioassays

Field and laboratory bioassay of chemosignals from related sympatric and allopatric species (overlapping and discrete distributions) are essential to an understanding of the relatedness or otherwise of functionally active compounds. The semiochemicals involved in speciation surely utilise the main and vomeronasal senses, but their relative contributions cannot be predicted at present. 

Parasitic hymenoptera hold promise in integrated pest management schemes, because they parasitize many economically important insect pests in a species-and stage-selective manner. The pheromones and kairomones of the parasitic hymenoptera have been studied for a long time, and there are many examples where there is evidence of chemical mediation of parasitoid behavior. This review emphasizes work done since the last major reviews [11, 12, 42] and, where it is available, on the primary bioassay-guided chemical identification of the semiochemical (Fig. 2 and Tables 3 and 4). 

Reliable bioassays are indispensible for a successful isolation and identification of semiochemicals, especially pheromones. Recent improvements range from the construction of new olfactometers102 and the development of a delivery system for laboratory bioassays103 to measurements of odor plume structures in the wind tunnel104 and to a new method to improve olfactory responses to gas... 

Although the potential role of semiochemicals as contact or fumigant poisons cannot be minimized, the bioassay procedures for such compounds are sufficiently similar to those for synthetic pesticides that a detailed review may not be necessary here. The simplest contact bioassay is identical to that for pesticides. The candidate toxin is dissolved in an appropriate solvent, usually acetone, and fixed quantities are applied to the subject with a microsyringe. A range of concentrations are used, and, with appropriate sample sizes, the resulting pattern of mortality is analyzed by probit analysis (Robertson Preisler 1992) to determine LDso s and fiducial limits. 

In this bioassay, activity due to contact cannot be distinguished from activity through fumigant action because the containers were sealed. There are two ways to address this problem. The first is to ventilate the chamber to draw off all vapors as is done with a number of pesticide bioassays (e.g., Morse et al. 1986), thus restricting exposure to direct contact. The second simply is to isolate the test semiochemical from physical contact the subjects to that the only means of exposure is by fumigant action. This is illustrated by bioassays of chromenes and benzofurans from Encelia spp. developed by Isman and Proksch (1985) as follows. 

In two bioassays, semiochemicals were applied first to the bottom and sides of glass scintillation vials, while in the second, an equivalent quantity was applied to the inside of the top of the vial. In both cases, the semiochemicals were applied in solvent, then the solvent was allowed to evaporate. Surfaces or lids of control vials were treated with solvent only. After the solvent had evaporated, five neonate larvae of the variegated cutworm, Peridroma saucia, were introduced into each vial for a 4-h exposure period. After exposure to the test chemicals, larvae were transferred to artificial diet, and their survival was assessed 24 h later. Because larvae could not climb the slippery surfaces of the glass vials and contact the... 

Identification of chemicals that affect the naturally occurring interactions between organisms requires sophisticated chemical techniques, such as those documented in volume 1, in combination with effective bioassays. Without an effective bioassay, the identification becomes akin to looking for a needle in a haystack, but without any idea of what a needle looks like. To a large extent semiochemical identifications must be driven by bioassays. 

Another example of such a historical lesson is that translation from laboratory bioassay back to the natural context to reevaluate the results is a critical component of completing the identification of a semiochemical. If the need for simplicity has driven the investigator to laboratory bioassays, then returning to the organisms habitat represents the ultimate test of the conclusions of laboratory bioassay-driven identification. 

---