Analysis of free fatty acids in food substrates and in the dust and frass of stored-product pests: Potential for species discrimination?

The larger grain borer, Prostephanus truncatus, is a serious beetle pest that tunnels extensively to produce large quantities of dust and frass. The natural enemy Teretrius nigrescens is an important biological control beetle which is known to exploit at close-range solvent-extractable chemical cues in the dust and frass. The objective of the current study was to analyse quantitatively and qualitatively, the free fatty acid mixtures in different food-substrate materials both before and after insect attack by a range of stored-product pests in order to ascertain whether differences in these mixtures could explain the T. nigrescens selectivity to P. truncatus dust/frass over that of other species irrespective of food substrate. By TLC, GC and GC–MS we found triglyceride and five free fatty acids were the most abundant chemicals in dust/frass (palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3)). In maize flour, Sitophilus species did not significantly change free fatty acid concentrations whereas with P. truncatus, Rhyzopertha dominica and Dinoderus minutus there were 4–6-fold increases, and, for Tribolium species there were over 20-fold increases. These differences provide interesting insights to tunnelling/feeding habits and are correlated with known feeding preferences within grain. Principal component analysis (PCA) demonstrated that free fatty acid ratios in dust/frass of different species are most linked to the food substrate and confer little discriminatory information that could be used to distinguish between the different species. Although increases in free fatty acid concentrations are good indicators of pest infestation and this may contribute behaviourally in an additive or synergistic way, we conclude that other chemical(s) are present and are key to T. nigrescens recognition of P. truncatus on different substrates

Developing pheromone traps and lures for Maruca vitrata in Benin, West Africa

In previous work successful trapping of the legume podborer, Maruca vitrata Fabricius (Lepidoptera: Pyralidae), was accomplished using a synthetic pheromone blend consisting of (E,E)-10,12-hexadecadienal, (E,E)-10,12-hexadecadienol, and (E)-10-hexadecenal in a 100 : 5 : 5 ratio. In the present work, experiments were conducted in cowpea fields in Benin to compare different trap designs, and other aspects of the lures. A water-trap made from a plastic jerry-can was found to be superior to commercial funnel- and sticky-trap designs, and 120 cm was the optimum height for captures. Generally, lures consisted of polyethylene vials containing 0.1 mg of pheromone. Results showed that shielding the lures from the adverse effects of sunlight with aluminium foil did not increase trap catches of M. vitrata. The degree of isomeric purity of the (E,E)-10,12-hexadecadienal and (E,E)-10,12-hexadecadienol blend components, in the range 73–99%, had no significant effect on captures, while lures of 80% isomeric purity showed no loss of effectiveness for up to 4 weeks. Similar results were observed with lures from a commercial source containing 0.46 mg of pheromone in the blend ratio 100 : 11 : 6 and 95% isomeric purity. Residue analysis showed that vial lures exposed for 2 weeks in the field still contained 73% of the initial amount of (E,E)-10,12-hexadecadienal, compared to rubber septa dispensers, which only retained 22%. Females comprised 11–50% of total catches, confirming earlier, unexpected results for synthetic lures. The observations that effective traps can be made from locally available plastic containers, and that pheromone blend composition and purity are not critical, should reduce costs and improve the feasibility of traps as practical monitoring tools for M. vitrata

An astigmatid defence volatile against a phytoseiid mite

Numerous semiochemicals have been isolated from several species of astigmatid mites with various identified or unidentified functions. Alarm pheromonal activity is widespread with neryl formate and neral, being the most common compounds eliciting alarm response in conspecifics. The cosmopolitan astigmatid mite Suidasia medanensis (= S. pontifica) Oudemans (Acari: Suidasidae) has been reported to use neral as an alarm pheromone, but neral can also act as an allomone towards predators of oribatid mites. Suidasia medanensis can be utilised as a factitious prey for mass-rearing of the phytoseiid predatory mite Amblyseius (= Typhlodromips) swirskii (Athias-Henriot) (Acari: Phytoseiidae), which is used for biological control of insect and mite pests in protected crops. This study investigated the potential defence properties of the S. medanensis volatiles against A. swirskii, comparing the repellency to pollen-reared (naïve) vs. S. medanensis-reared (experienced) predators using a synthetic blend of the isomers neral and geranial (1:1) as a model compound. In a repellency bioassay, the synthetic blend elicited a significant repellence to A. swirskii with no difference between naïve and experienced predators. During capture success studies, S. medanensis under repeated attack could release sufficient quantities of the defence volatile to deter 1–5 attacks from A. swirskii, whereas hexane-treated S. medanensis artificially depleted of volatiles were significantly more vulnerable to an attack. This is the first report of an astigmatid defence volatile with repellent activity to a phytoseiid mite and the starting point to understanding semiochemical interactions in any current or novel factitious predator-prey mass-rearing system

Environmental impacts of the control with organophosphate pesticides and explosions of the red-billed quelea bird Quelea quelea in Africa

Both spraying with pesticides and using explosions to kill red-billed quelea birds may affect non-target organisms and leave hazardous contaminants in the environment. Results of monitoring such effects in Botswana and Tanzania showed that although few non-target mortalities were noted, indirect effects in terms of depressed cholinesterases in birds and small mammals were marked and pesticides and contaminants from products used for the explosions persisted at controlled sites at unacceptable levels and for long periods (at least 188 days for sprays and at least 11 months for petroleum products and plastics)