A critical evaluation of the effects of plant essential oil formulations against silverleaf whitefly, Bemisia tabaci, B biotype (Gennadius) and its natural enemies

Abstract

The toxicity of 30 botanical products and mixtures were evaluated using preliminary bioassay tests to find out their overall effectiveness against the developmental stages of silverleaf whitefly (SLW) Bemisia tabaci, B biotype, (Gennadius) (Hemiptera: Aleyrodidae). Effective formulations were prepared to be tested in replicated experiments against the egg, nymphal and adult stages of SLW.From the results, mustard oil showed an ovicidal effect, lauryl glucoside, a surfactant, had produced high mortalities against the nymphal stages whereas monoethanolamine, diethylene glycol monomethyl ether (cellosolve acetate) and laureth -7ethylene oxide-carboxylate (laureth carboxylate) had produced high to moderate level of adulticidal effects. Three formulations were prepared from those effective products and then used in replicated experiments testing their lethal (toxicity) and sublethal (repellency and oviposition deterrent) effects against SLW. Additionally, trials were conducted to investigate their impacts on one of the main SLW parasitoids, Eretmocerus hayati (Zolnerowich and Rose) (Hymenoptera: Aphelinidae).Leaf dipping and spraying methods were used to assess the toxicity effects of the products, and choice and no choice repellence tests were used to determine the repellence index (RI) and oviposition deterrent index (ODI). A glass -slide bioassay was used to determine the lethal effects of the formulations against the SLW parasitoid. Different ranges of the tested rates of the formulations were prepared starting from 0.001% v/v to 10% v/v to investigate the proper effective rates that provide sufficient mortality rates of the developmental stages of SLW.After promising results of the mixture containing mustard oil and liquid soap, replicated experiments were conducted against each developmental stage of SLW. When tested on eggs, at a concentration of 0.25%, mortality was 95.8%, whereas the mortality percentages were 43% and 50% at tested rates of the mixture at 0.1% and 0.05%, respectively. Mustard oil evaluated against nymphal stages was effective at a rate of 0.25% and above against young and old nymphs (86.4% and 47.4% mortality, respectively). Tests against the adult stage at 0.25% and 0.5% resulted in low mortality; 34.0% and 37.0%, respectively. However, at 1% and above mortality was high (94.17% at 1%).On the basis of these results, three formulations were prepared: formulation one (F1) containing mustard oil, MW-100 emulsifier, lauryl glucoside and cellosolve acetate, formulation three (F3) containing mustard oil, MW-100 emulsifier, laureth carboxylate and monoethanolamine and formulation four (F4) containing mustard oil, MW-100 emulsifier, lauryl glucoside and monoethanolamine. These formulations were evaluated at different concentrations (0.25%, 0.44%.ii 0.69%, 1% and 1.23%). The formulations had an effective impact on the eggs of SLW, disrupting the embryogenesis process of the eggs. These formulations also affected all nymphal instars.However, the tested rates did not show sufficient effects...