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 on adult mortality. When the formulations were evaluated for their repellent and oviposition deterrent effects, numbers of adults on the leaves were counted 2, 6, 12, 24 and 48 h after the adult introduction. From the results of choice tests, F1 had the highest RI value (0.18), whereas the RI value of F4 (-0.01) was the lowest among the tested formulations. F4 showed certain repellence and oviposition deterrence effects (RI= -0.01; ODI= -44). In no-choice experiments, the mean number of adults attracted to the lower side of the leaflets treated with the formulations F1, F3 and F4 was calculated. The results indicated that F3 and F4 showed a reduction in adult mean number of 34.1% and 46.9%, respectively, and accordingly there were a reduction in the mean number of laid eggs by 77.3% and 81.2%, respectively, compared with the control. There were different responses of the formulations on the parasitoids. F1 had the lowest significant adverse effect on the parasitoid among the tested formulations: 11.11%. There were no significant differences between the tested rates on the parasitoids. F3 and F4 showed severe effects on the parasitoids. The parasitized mortality rates reached 60%. From the above, the mixture of mustard oil and liquid soap could be used effectively against all developmental stages of SLW. The formulations also showed high mortality rates against egg and nymphal stages but no significant effects against adults. F1 had the highest RI among other formulations in choice tests however there were no significant differences between them in their effectiveness when tested in no choice repellence tests. F1 was safer to the SLW parasitoids than F3 and F4. Therefore, F1 containing mustard oil, MW-100 emulsifier, lauryl glucoside and cellosolve acetate could be used in future against SLW developmental stages and could be incorporated in integrated pest management programs (IPM).