In focus: IPM using novel insecticides and other approaches

Larvicidal property and active compound profiling of Annona squamosa leaf extracts against two species of diptera, Aedes aegypti and Anopheles stephensi

Background & objectives

Vector control measures are important in lowering the spread of infections spread by mosquito. Synthetic pesticides used to suppress vector populations during the larval stage have had adverse impacts on people and the environment. The early III instar larvae of Aedes aegypti and Anopheles stephensi were the targets of the current experiment, which assessed the larvicidal ability of petroleum ether, chloroform, methanol, and aqueous extracts of Annona squamosa leaves.

Methods

Using the standard World Health Organization (WHO) larval bioassay test, leaf extracts were evaluated for their activity against Ae. aegypti and An. stephensi to determine lethal doses. Phytochemical analysis and gas chromatography-mass spectrometry (GC-MS) were carried out to identify larvicidal components in the extract. Further analysis using a scanning electron microscope (SEM) was done to check the extracts toxicity for both mosquito larvae.

Results

The larvicidal active components were identified by GC-MS as tetradecanoic acid, cis-vaccenic acid, and 2,4-di-tert-butylphenol etc. Methanol leaf extracts of A. squamosa (ASME) exhibited strong larvicidal activity against the early 3rd instar larvae of Ae. aegypti and An. stephensi with Lethal concentration (LC50) values of 51.450 ppm and 107.121 ppm. Cell damages to the larva post exposure to ASME were examined.

Interpretation & conclusion

This finding showed that the ASME has better larvicidal activity and its components that may be used to kill larvae as larvicides. The extracts toxicity towards damage of midgut of larva further suggests that this plant methanol leaf extracts could be effective in larval growth control approaches.

Evaluation of the pesticide Oberon on a model organism Drosophila melanogaster via topical toxicity test on biochemical and reproductive parameters

Spiromesifen (Oberon® 240 SC), a pesticide widely used to control pests like mites and whiteflies, was investigated using Drosophila melanogaster Meigen, 1830 (Diptera, Drosophilidae) as a model organism. The compound was applied topically at two concentrations (LC₁₀: 21.45 and LC₂₅: 39.53 μg active ingredient/pupa), on newly molted pupae and assessed on morphometric measurements of ovaries and the progeny of surviving adults. Results showed that spiromesifen inhibited the growth and development of ovaries, reducing at the highest dose (LC₂₅) the number of oocytes, the volume of basal oocytes and ovarian weight. Biochemical analysis revealed that the tested compound reduced the ovarian levels of carbohydrates and glycogen during the sexual maturation. Moreover, fecundity, fertility and number of descendants from parents that survived to the treatment of pupae were significantly reduced. The sex ratio determined indicated a significant decrease in treated series and males seemed more sensitive to spiromesifen than females. Lastly, the compound was found to affect the sexual behavior.

Effects of juvenoid Pyriproxyfen on reproduction and F1 progeny in myiasis causing flesh fly Sarcophaga ruficornis L. (Sarcophagidae: Diptera)

Freshly emerged virgin female Sarcophaga ruficornis were topically treated with different doses of pyriproxyfen to test the efficacy on reproduction and subsequent F1 progeny. The results included mortality of the treated adults, significant reduction in fecundity, more than 90% inhibition in larvae production, mortality in F1 generation during larval instars, reduction in pupariation, and adult emergence and production of deformed adults. There was a dose-dependent response showing high degree of correlation in the doses administered and deformities observed. The effects in F1 generation show that an intraovarial transfer of pyriproxyfen was responsible for the various deformations observed, showing the potency of juvenoid pyriproxyfen for longer durations across generations. This is the first study that clearly demonstrates the efficacy of pyriproxyfen against reproduction in S. ruficornis and its potential for the management of this notorious pest of medico-veterinary importance.

Toxicity of spiromesifen to the developmental stages of Bemisia tabaci biotype B

BACKGROUND

Spiromesifen is a novel insecticidal/acaricidal compound derived from spirocyclic tetronic acids that acts effectively against whiteflies and mites via inhibition of acetyl-CoA-carboxylase, a lipid metabolism enzyme. The effects of spiromesifen on the developmental stages of the whitefly Bemisia tabaci (Gennadius) were studied under laboratory conditions to generate baseline action thresholds for field evaluations of the compound.

RESULTS

Adult B. tabaci mortality rate after spiromesifen treatment (5 mg L(-1)) was 40%. Treatment with 0.5 mg L(-1) reduced fecundity per female by more than 80%, and fertility was almost nil. LC(50) for eggs was 2.6 mg L(-1), and for first instar 0.5 mg L(-1). Scanning electron microscopy revealed that eggs laid by treated adult females had an abnormally perforated chorion, and females were unable to complete oviposition. Light and fluorescent microscopy showed significantly smaller eggs following treatment, and smaller, abnormally formed and improperly localized bacteriomes in eggs and nymphs. The number of ovarioles counted in females treated with 5 mg L(-1) was significantly reduced. Spiromesifen showed no cross-resistance with other commonly used insecticides from different chemical groups, and resistance monitoring in Israel showed no development of field resistance to this insecticide after 1 year of use.

CONCLUSION

The strong effect on juvenile stages of B. tabaci with a unique mode of action and the absence of cross-resistance with major commonly used insecticides from different chemical groups suggest the use of spiromesifen in pest and resistance management programmes.