An investigation was made of the action of the juvenile hormone analogues (JHAs) methoprene (ZR-515) and 3-[5-(4-ethylphenoxy)-3-methylpent-3-enyl]-2, 2-dimethyloxirane (R-20458), and the insecticides chlorpyrifos and methomyl, when applied, separately or in combination, to the late insect stages of susceptible (S) and aminocarb-resistant (R
m) strains of
Spodoptera littoralis Boisd. Sixth-instar larvae of the R
m strain showed detectable levels of cross-resistance to chlorpyrifos, methomyl and R-20458. However, cross-resistance to methoprene was less pronounced in the R
m strain. Treatment of the same instar with the ED
50 of methoprene produced an appreciable level of sterility in the S strain, but this level decreased in the R
m strain. Similar treatment with R-20458 caused a lower level of sterility in the S strain and the R
m strain was less affected. In the S strain, the ED
25 of either chlorpyrifos or methomyl when applied simultaneously with the ED
25 of R-20458, produced an antagonistic effect and the R
m strain was more capable of resisting the joint action of these compounds. The treatment, ED
25 chlorpyrifos + ED
25 methoprene produced an additive effect on the S strain, while the R
m strain tolerated their combined action. Nevertheless, the treatment, ED
25 methomyl + ED
25 methoprene produced additive effects on both the S and R
m strains. The prepupae of the R
m strain tolerated the action of the insecticides methomyl and chlorpyrifos. A similar pattern of cross-resistance was also detected against the action of the two hormones at the ED
25 level, while at the ED
25 level, both the S and R
m strains were almost equally sensitive to the action of the two juvenoids. Chlorpyrifos-JHA combinations produced additive effects on prepupae of the S strain while the R
m strain completely resisted their joint action. Methomyl-JHA combinations produced high potentiation in the S strain, but the R
m strain remained insensitive to their joint action. Detectable levels of tolerance to the action of chlorpyrifos and methomyl at the ED
25 and ED
50 levels were indicated in 2-day-old pupae of the R
m strain. This was less evident in the case of JHAs, particularly methoprene, which was to some extent equally effective on both strains; the reproductive ability of the S and R
m strains was highly affected by this compound. The pupae of the R
m strains were equally as affected as those of the S strain by the combination ED
25 chlorpyrifos+ ED
25 R-20458. Nevertheless, the two strains showed antagonism to the action of chlorpyrifos with methoprene. A high level of potentiation was produced in the S strain to the combination of methomyl and R-20458 but the R
m strain was able to withstand their combined action. Of interest in this respect was the action of the combination of methomyl and methoprene, for which high levels of potentiation were detected in pupae of the S and R
m strains. This combination also Produced a high percentage of sterility in mated females of the R
m strain.
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