Indoxacarb and Insecticide Resistance

Indoxacarb is highly active against insect pest species that are already resistant to other insecticide classes. This has been demonstrated in the housefly

(Musca domestica) resistant to spinosad (Shono and Scott, 2003) and organophosphates (Sugiyama etal., 2001); tobacco budworm (Heliothis virescens) resistant to spinosad (Young et al., 2001); German cockroach resistant to pyrethroids (Appel, 2003; Dong, unpublished data); tortricid leafrollers resistant to organophosphates (Olszak and Pluciennik, 1998); cotton bollworm (H. armigera) resistant to cyclodienes, organophosphates, carbamates and pyrethroids (Ahmad et al., 2003); diamondback moth (Plutella xylostella) resistant to several conventional chemistries and spinosad (Boyd, 2001);

beet armyworm (Spodoptera exigua) resistant to pyrethroids, organophosphates, and benzoylurea insect growth regulators (Eng, 1999); and H. vires-cens resistant to multiple conventional chemistries (Holloway et al., 1999).

It is likely that the insects mentioned above are resistant to conventional insecticides due to a variety of mechanisms, including increased metabolism, decreased penetration and target site insensitivity. Because indoxacarb is bioactivated via esterase/ amidase enzymes, overproduction of esterases in insects resistant to organophosphates or pyrethroids could lead to faster liberation of the active toxin DCMP than in nonresistant insects. This suggests that resistant insects may in fact develop a negative cross-resistance to indoxacarb, as has been observed in laboratory strains of H. armigera (Gunning and Devonshire, 2003). However, it is important to note that we have no clear evidence thus far for negative cross-resistance to indoxacarb in the field, indicating the speed and ease with which susceptible Lepidoptera can bioactivate indoxacarb. The aggregate resistance data also indicate that insects with increased resistance to conventional chemistries also have highly sensitive Na+ channel DCMP binding sites, as would be expected since there has been no previous exposure to commercial SCBIs in the field. Thus, indoxacarb is an excellent rotation partner for alternating modes of action in insect resistance management programs.

As has been mentioned, certain field strains of C. rosaceana, which have developed resistance to many conventional insecticides, are also poorly sensitive to indoxacarb (Ahmad et al., 2002). This may be due to an enhanced ability to detoxify indoxa-carb; however, this insect is not on the indoxacarb US label and is an extremely unusual example of such poor susceptibility in Lepidoptera.

Insect pests representing eight species (H. vires-cens, H. armigera, S. exigua, S. eridania, S. littoralis, P. xylostella, and T. absoluta) in over 20 countries have been targeted for a sustained susceptibility monitoring program by DuPont Agricultural Products (Andaloro, unpublished data). These populations are evaluated by a feeding assay throughout each season in locations that have a history of insecticide resistance, and where a significant number of indoxacarb applications are made, so that early signs of resistance can be detected before widespread field failures occur. This proactive approach to indoxacarb resistance management is beneficial not only for preservation of this product in key markets, but also to maintain its usefulness as a rotation partner, thus promoting the longevity of other products with different modes of action, and thus ultimately benefiting the agricultural field in general.

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