Shuit Hung Ho

Insecticidal and repellent properties of nine volatile constituents of essential oils against the American cockroach, Periplaneta americana (L.)

The toxic and repellent properties of nine major constituents of essential oils, comprising benzene derivatives and terpenes, were evaluated against Periplaneta americana (L.). Contact and fumigant toxicities to adult females and repellency to nymphs were determined. The decreasing order of knockdown activity via contact was methyl-eugenol>isosafrole=eugenol>safrole. The killing effect via contact was in the order eugenol=methyl-eugenol=isosafrole>safrole. Fumigant toxicity was only observed for safrole and isosafrole, with safrole being the more potent. Isoeugenol and the tested terpenes had neither contact nor fumigant toxic effect. The decreasing order of repellency to nymphs was safrole>isosafrole>methyl-eugenol=α-pinene> eugenol>isoeugenol. The benzene derivatives were generally more toxic and repellent to P. americana than the terpenes. The distance of the side chain double bond from the aromatic ring and the substitution of a methoxy group to these compounds appeared to be important determinants of their toxicity and repellency.

The oil of garlic, Allium sativum L. (Amaryllidaceae), as a potential grain protectant against Tribolium castaneum (Herbst) and Sitophilus zeamais Motsch

Abstract Fresh garlic cloves were steam-distilled to obtain the essential oil. The garlic oil was tested for toxicity against the eggs, larvae and adults of Tribolium castaneum and adults of Sitophilus zeamais. T. castaneum egg mortality increased with garlic oil concentration, complete kill of eggs being achieved at 4.4 mg/cm 2 , using the filter paper impregnation bioassay. The eggs were the most susceptible stage, followed by adults, 10-day old larvae and older larvae. T. castaneum adults were more susceptible to garlic oil than S. zeamais adults, with KD 50 values of 1.32 mg/cm 2 and 7.65 mg/cm 2 , respectively. When rice and wheat were treated with garlic oil, eggs that were laid in the media failed to produce F1 progeny at concentrations of > 2000 ppm in rice for T. castaneum and 5000 ppm in wheat for S. zeamais . The weights of F1 adults of T. castaneum and S. zeamais in treated media were not significantly different ( P > 0.05) from those of the controls. The potential of garlic oil, and the bioactive compounds therein, as an effective grain protectant is discussed.

Bioactivities of methyl allyl disulfide and diallyl trisulfide from essential oil of garlic to two species of stored-product pests, Sitophilus zeamais (Coleoptera: Curculionidae) and Tribolium castaneum (Coleoptera: Tenebrionidae).

Abstract Two of the major constituents of the essential oil of garlic, Allium sativum L., methyl allyl disulfide and diallyl trisulfide, were tested against Sitophilus zeamais Motschulsky and Tribolium castaneum (Herbst) for contact toxicity, fumigant toxicity, and antifeedant activity. The contact and fumigant toxicities of diallyl trisulfide were greater than that of methyl allyl disulfide to the adults of these two species of insects. These two compounds were also more toxic to T. castaneum adults than to S. zeamais adults. Older T. castaneum larvae were more susceptible to the contact toxicity of the two compounds, whereas younger larvae were more susceptible to the fumigant toxicity of these compounds. Both compounds reduced egg hatching of T. castaneum and subsequent emergence of progeny. Diallyl trisulfide totally suppressed egg hatching at 0.32 mg/cm2, and larval and adult emergence at 0.08 mg/cm2. Methyl allyl disulfide significantly decreased the growth rate, food consumption, and food utilization of adults of both insect species, with feeding deterrence indices of 44% at 6.08 mg/g food for S. zeamais and 1.52 mg/g food for T. castaneum. However, it did not affect any nutritional indices of T. castaneum larvae. Diallyl trisulfide significantly reduced all of the nutritional indices in all of the insects tested. Feeding deterrence indices of 27 and 51% were obtained in S. zeamais adults and T. castaneum larvae, respectively, at the concentration of 2.98 mg/g food, whereas feeding deterrence of 85% was achieved in T. castaneum adults at a much lower concentration of 0.75 mg/g food. Hence, diallyl trisulfide is a more potent contact toxicant, fumigant and feeding deterrent than methyl allyl disulfide.

Comparison of Two Bioassay Methods for Determining Deltamethrin Resistance in German Cockroaches (Blattodea: Blattellidae)

Abstract Susceptibility to deltamethrin of 10 strains of the German cockroach, Blattella germanica (L.), trapped in hotel kitchens in Singapore was determined. Two resistance detection bioassay methods (topical application and World Health Organization glass jar method) were compared. Resistance ratios based on 50% knockdown, obtained by comparison with the S strain, ranged from 17.7 to 4,235 for topical application and from 2.2 to 22 for the glass jar method. A field strain, with consistently low resistance ratios (topical method = 17.7, glass jar method = 2.2), was identified as a potential field strain to be used as a baseline for comparison with other field strains. Resistance ratios for the other field strains obtained by comparison with the R5 strain ranged from 24.5 to 239 for topical application and from 1.2 to 9.8 for the glass jar method. The results of our study demonstrate that deltamethrin-resistant German cockroaches are numerous in Singapore. Comparison between the two bioassay methods showed that there was significant correlation between KD50 and KT50 values. The glass jar method is similar to field situations but topical application is sensitive enough to define the magnitude of resistance. Differences between the two detection bioassays and the factors governing the choice of bioassay in monitoring resistance in German cockroaches are discussed.

Modes of action of fraxinellone against the tobacco budworm, Heliothis virescens

Fraxinellone significantly reduced the relative growth rate, food consumption rate as well as the efficiency of conversion of ingested food into biomass of Heliothis virescens when incorporated into artificial diets at concentrations of 4.31 × 10−5 mol/L and above. After being fed with fraxinellone‐treated diets for 24 h, the larval midguts of H. virescens possess significantly lower activities of α‐amylase and non‐specific proteases and higher activities of cytochrome P450s. In vitro, the compound did not inhibit the activities of α‐amylase and non‐specific proteases extracted from the larval midguts. Clear evidence of post‐ingestive toxicity of fraxinellone to midgut cells was observed under an electron microscope. The modes of action of the compound against insects were discussed.