The Pentatomid bug, Scotinophara lurida (Burm.), is a serious pest of the rice crop in certain areas of Ceylon. Five or six generations develop on the two annual rice crops, and that which attains the adult stage at the time of harvest of either crop ‘aestivates’ and does not become sexually mature until later. The generations that become adult earlier in each crop season become sexually mature forthwith, and mate and oviposit.
In 1952, laboratory experiments to ascertain the susceptibility of adults of S. lurida to a range of modern organic insecticides, and field experiments on insecticidal control, were initiated. The insecticides tested were DDT, BHC, endrin, dieldrin, malathion, Chlorthion, Guthion, Dipterex, diazinon and parathion.
In the laboratory, freshly emerged, sexually mature and aestivating bugs were tested separately by topical application on the abdomen, and it was found that the insecticides could be arranged in the following descending order of toxicity, based on the LD50, against the sexually mature adult:— γ BHC, endrin, dieldrin, parathion, Dipterex, Chlorthion, diazinon, Guthion, malathion, DDT. The adults of the aestivating generations, at least prior to becoming sexually mature, were markedly more resistant to all the insecticides tested than were sexually mature bugs of uninterrupted development, the resistance ranging from two-fold to nineteen-fold that of the latter.
There is some indication that tolerance to every insecticide does not necessarily increase with increasing age and sexual maturity of the normal adult bug. Thus, whereas with the organophosphorus insecticides increasing sexual maturity is associated with an increase in tolerance, with DDT and endrin the bugs appear to become more susceptible with increasing sexual maturity, while with γ BHC and dieldrin there is little change.
The chlorinated hydrocarbon insecticides DDT, toxaphene, chlordane, aldrin, dieldrin, endrin, isodrin and γ BHC and the organophosphorus insecticides malathion, diazinon, Dipterex, Chlorthion, Guthion and parathion have been tested as emulsion sprays against S. lurida in the field. DDT and toxaphene gave the poorest results with about 85 per cent, control of the pest in 24 hours when applied at and above a concentration of 0·045 per cent, active ingredient. The other insecticides are arranged thus in descending order of efficacy against the pest considering the minimum dosage required to give 100 per cent, or near 100 per cent, control within 24 hours of application of the treatment: —parathion 0·02 per cent.→ endrin 0·03 per cent. = Chlorthion 0·03 per cent.→ Dipterex 0·03–0·04 per cent.=Guthion 0·02 per cent.→ dieldrin 0·06 per cent.= malathion 0·06 per cent.= γ BHC 0·06 per cent. → diazinon 0·07 per cent. Aldrin and isodrin were of about equal effectiveness giving about 90 per cent, control of the pest within 24 hours when applied at a concentration of 0·06 per cent, active ingredient.
Of the dusts tested, both γ BHC at 1·3 per cent, and dieldrin at 2·5 per cent, gave 100 per cent, control of the pest in 24 hours when applied under still and dewy conditions obtaining in the morning. Under dry, windy conditions however, γ BHC at 1·3 per cent, and 3 per cent, gave better control of the pest than did dieldrin or aldrin dusts.
Parathion, dieldrin, endrin, Metasystox and Ekatin were tested for residual action against S. lurida on the rice crop. Parathion was the most effective residually for a period of two days with dieldrin coming next in order of residual effectiveness.
Metasystox and Ekatin were tested for their systemic action against S. lurida on three-week-old and ten-week-old rice plants as foliar sprays and as root treatments. Systemic action of a low order of effectiveness against the pest was demonstrated in all cases but the poor results achieved and the high rates of application involved did not warrant their consideration for practical field use.
