Insect Control: Biological and Synthetic Agents - Index of

232 6: The Spinosyns: Chemistry, Biochemistry, Mode of Action, and Resistance
profile. Additionally, tests conducted for EPA registration
showed spinosad to neither leach nor persist
in the environment. These factors all contributed to
the EPA’s registration of spinosad as a reduced risk
pesticide in early 1997 (Thompson et al., 2000;
Thompson and Sparks, 2002), and the bestowing
of the Presidential Green Chemistry Award in 1999.
6.6. Resistance Mechanisms and
Resistance Management
6.6.1. Cross-Resistance
Many of the primary insect pests in major markets
have a history of developing resistance to insect
control agents. Thus, efficacy against insecticide resistant
pest insects and the potential for the development
of resistance are increasingly important
considerations in the development decision for any
new insect control agent. Early in the development
process, spinosad was examined for efficacy against
a variety of insecticide resistant strains of pest
lepidopterans, such as larvae of H. virescens and
P. xylostella (Sparks et al., 1995). Heliothis virescens
larvae collected from a number of locations in
the mid-southern USA were subjected to discriminating
dose assays for a variety of insecticides. Relative
to the susceptible strains, larvae collected from
almost all of these locations were poorly controlled
by cypermethrin (pyrethroid) or profenophos (organophosphate).
In contrast, spinosad provided
very good to excellent control of all of the strains
tested (Sparks et al., 1995), demonstrating its utility
against insecticide-resistant strains possessing what
was likely a mixture of resistance mechanisms
(based on the diverse nature of the other insect
control agents). Likewise, studies with laboratory
strains of P. xylostella selected for high levels of
resistance to pyrethroid, avermectin, or acylurea
insecticides and known to possess enhanced glutathione
transferase and/or monooxygenase activity
(Chih-Ning Sun, personal communication), showed
no appreciable levels of cross-resistance to spinosad
(Sparks et al., 1995) (Table 9). One subsequent study
with field-selected strains of P. xylostella resistant to
the pyrethroid permethrin, has shown a low level of,
but broad, cross-resistance to several insect control
agents including spinosad and the avermectin emamectin
benzoate (Shelton et al., 2000) (Table 9).
However, virtually all other studies involving a variety
of insecticide resistant species and strains have
shown no cross-resistance to spinosad (Table 9). For
example, a multiresistant strain of housefly (Musca
domestica) that was highly resistant (1800-fold
and 4200-fold) to the pyrethroids permethrin, and
cypermethrin, respectively, exhibited no crossresistance
to spinosad (Liu and Yue, 2000) (Table 9).
Spinosad was highly effective against a number of
strains that were highly resistant to other insect
control agents including abamectin (decreased penetration
and altered target site), cyclodienes (rdl), pyrethroids
(monooxygenases, knockdown resistance,
and reduced penetration), and organophosphates
(altered acetylcholinesterase) (Scott, 1998). Only
with the multiresistant LPR strain of housefly was
any cross-resistance noted, and that was only at a
very low level (Scott, 1998). Taken in total, these
data are most consistent with the presence of a
unique mode of action for the spinosyns as well as
what may be an apparent limited susceptibility of the
spinosyn structure to insect metabolic systems.
6.6.2. Resistance Mechanisms
While almost all insecticide-resistant strains of pest
insects have exhibited little or no cross-resistance to
the spinosyns/spinosad, it is nearly axiomatic that if
sufficient pressure is put on an insect population,
resistance will be developed to any insect control
agent, and the spinosyns/spinosad are certainly no
exception. This has indeed been borne out in that
there are now a few examples of spinosad resistance
being developed in the laboratory and in the field.
Resistance to spinosad has been selected for in the
laboratory. Bailey et al. (1999) used a topical selection
protocol to generate a high level of spinosad
resistance (toxicity ratio, TR ¼ 1068-fold, topical
assay) in larvae of H. virescens after 14 generations
of selection using methods that favored the development
of resistance (NC spinosad-R) (Young et al.,
2001, 2003). Likewise, a spinosad resistant strain
(NYSPINR) of M. domestica (TR ¼ >150) was
selected in the laboratory starting from a composite
of field strains (Shono and Scott, 2003). From this
multiresistant NYSPINR strain, a further spinosad
resistant strain (rspin) was developed that was specifically
resistant to only spinosad and also incorporated
a number of recessive mutant markers (Shono and
Scott, 2003). For spinosad resistant strains of both
H. virescens (NC-spinosad-R) and housefly (rspin),
cross-resistance to other insect control agents such
as emamectin benzoate, indoxacarb, acetamiprid,
abamectin, etc. was negligible (Young et al., 2001;
Shono and Scott, 2003) (Table 9). Both studies also
showed spinosad resistance to be linked to a single,
non sex-linked, recessive gene (Shono and Scott,
2003; Wyss et al., 2003). Studies with the NYSPINR
housefly strain also showed no synergism of spinosad
activity with piperonyl butoxide, S,S,S-tributylphosphorothioate
(DEF), or diethyl maleate (DEM),
suggesting that metabolism was not likely to be the