Impact of Neem Seed Extract on Mortality, Esterase and Glutathione-S-Transferase Activities in Thai Polyvoltine Hybrid Silkworm, Bombyx mori L.

Neem, a biopesticide, offers a safer alternative to the synthetic insecticides commonly used in mulberry cultivation, which can harm silkworms. This study aimed to investigate the effects of Thai neem seed extract on all instar larvae of the Thai polyvoltine hybrid silkworm, Bombyx mori L., Dok Bua strains, focusing on the mortality rate and the activities of esterase (EST) and glutathione S-transferases (GST) enzymes. Acute toxicity was assessed using the leaf-dipping method. Results showed that the mortality rate tended to be higher in younger instars than in older ones. The first instar larvae exhibited the highest mortality rate at 94%, whereas the LC50 was highest in the third instar at 5.23 mg L-1 at 72 h. This trend aligns with the activities of EST and GST, which were evaluated in the whole bodies of the first instar larvae and the midgut tissue of fifth instar larvae. As the extract concentration increased, EST activity decreased while GST activity increased in both the first and fifth instar larvae. These findings highlight that neem extract is toxic to all instar larvae, with GST playing a crucial role in detoxification, particularly in the whole body of the Thai polyvoltine hybrid silkworm.

Effects of rotenone from derris crude extract on esterase enzyme mechanism in the beet armyworm, Spodoptera exiqua (Hubner).

This research was to elucidate the toxicity and esterase enzyme mechanism of derris crude acssst (Derris elliptica Benth) in the beet armyworm (Spodoptera exigua Hubner) by larvae dipping method. Derris crude extract was extracted by Soxhlet's apparatus with ethanoL solvent. The ethanolic derris crude extract was diluted into 5 concentrations to test the toxicity value. The result showed that the toxicity dramatically increased when the concentration of substance and the time exposure were increased. Toxicity in terms of LC50 value against 2nd instar larvae indicated that ca. 69.15 (r2 = 0.98), 51.32 (r2 = 0.99) and 46.60 (r2 = 0.98) ppm at 24, 48 and 72 hours, respectively. The in vivo study with Yang et al. (2004); Visetson and Milne (2001) modified method, revealed that the derris crude extract inhibited esterase enzyme activities ca. 0.75-1.07 fold without a change in the protein concentration. This result indicated that, derris crude extract had efficacy as an insecticide, therefore it had the potential to be another insecticide alternative in S. exiqua control. Moreover, some separate experiments indicated that this extract was safe to human and the environment.

Temperature effect on novel entomopathogenic nematode Steinernema siamkayai Stock, Somsook and Reid (n. sp.) and its efficacy against Spodoptera litura Fabricius (Lepidoptera: Noctuidae).

The efficacy of Steinernema siamkayai is a new species that isolate in Thailand and the well known previous species are Steinernema carpocapsae and Steinernema riobrave against 3rd instar larvae of common cutworm, Spodoptera litura, and last instar larvae of greater wax moth, Galleria mellonella was evaluated under laboratory condition. The optimal temperature for new entomopathogenic nematode, S. siamkayai at 50 and 100 IJs per larvae to penetrate into S. litura and G. mellonella larvae were highly suitable during 25 to 30 degrees C. The toxicity value in term of LC50 of S. siamkayai to S. litura larvae at age of 10 and 12 days were indicated that 37.75 (r2 = 0.92) and 61.16 (r2 = 0.95) IJs per larva after 72 hours of post infection, respectively. Moreover, S. siamkayai was better adapted to high temperatures and caused higher mortality at 35 degrees C than S. carpocapsae. According to the results of this research, the investigation should be further conducted in order to develop increasing efficacy of S. siamkayai and symbiotic bacteria. In addition, especially, the specificity of S. siamkayai to the insects should also be investigates.