Toxicity of certain igrs and conventional insecticides against cotton Leafworm and their effects on the development and Haemocyte counts

Abstract EN

Toxicity of two insect growth regulators (IGRs) (novaluron and chlorfluazuron) and two conventional insecticides (chlorpyrifos and lambda-cyhalothrin) were evaluated against 2nd and 4th instars larvae of cotton leafworm (CLW), Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae).

Joint toxic action of tested IGRs and insecticides against 4th instar larvae of CLW were also evaluated.

The sublethal effects of tested insecticides on the development and haemocyte counts were also carried out.

Results showed that, chlorfluazuron was the most toxic against both 2nd and 4th larval instars with LC50 values 0.12 and 1.4 mg/L after 96 hrs of treatment followed by chlorpyrifos which is followed by novaluron.

Lambda-cyhalothrin recorded the least toxicity against both 2nd and 4th larval instars with LC50 values 0.86 and 4.6 mg/L after 96 hrs of treatment.

Potentiating effect was obtained when chlorpyrifos was mixed with novaluron or chlorfluazuron each at LC25 with co-toxicity factors (CTFs) 42.85 and 46.66, respectively.

All other mixtures of chlorpyrifos with novaluron or chlorfluazuron resulted in additive effects.

All tested insecticides at concentrations equivalent to LC10 and LC25 significantly reduced larval body weights, percentage of pupation, pupal mean weight and percentage of adult emergence.

Total haemocyte count (THC) was significantly decreased and reached to 34.6, 45.2, 72.3 and 87.6×103 cell/mm3 after 24 hrs from treatment with novaluron, chlorfluazuron, chlorpyrifos and lambda-cyhalothrin at concentration equivalent to LC25, respectively, compared to 106.4×103 cell/mm3 in control.

All treatments with tested IGRs and insecticides decreased the percentages of Prohaemocytes (PRs) and Spherulocytes (SPs) after 24, 48, 72 and 96 hrs of treatment compared to control.

In contrast, the percentages of Granulocytes (GRs) and Oenocytoids (OEs) were slightly increased.

While in novaluron and chlorfluazuron treatments, Plasmatocytes (PLs) were significantly increased and reached to 48.3 and 49.2 %, respectively, it was decreased by chlorpyrifos and lambdacyhalothrin treatments and reached to 39.6 and 40.5 %, respectively, after 24 hrs compared to 44.8 % in control.

Finally, the present data suggests that the use of novaluron and chlorfluazuron in binary mixtures with conventional insecticides particularly chlorpyrifos can reduce the rate of used insecticides and consequently reduce environmental pollution.

Also, sublethal concentrations of tested insecticides can negatively affect the population of CLW.