Insect Control: Biological and Synthetic Agents - Index of

6: The Spinosyns: Chemistry, Biochemistry, Mode of Action, and Resistance 229
(the 4 00 -N-demethyl and 4 00 -N,N-didemethyl derivatives
of spinosyn A, respectively) (Table 2), are nearly
as active as spinosyn A against H. virescens, asisthe
second most abundant factor, the 6-methyl analog,
spinosyn D. Loss of a methyl group at the 2 0 -, 3 0 -, or
4 0 -position of the rhamnose moiety reduces lepidopteran
efficacy by about an order of magnitude, as
measured by activity against H. virescens larvae
(spinosyns H, J, K, respectively) (Table 2). A comparable
loss in activity is also associated with loss of
a methyl group at C16 or a reduction in alkyl size
(ethyl to methyl) at C21 (spinosyns F and E, respectively)
(Table 2). Interestingly, an increase in alkyl
size at C21 (ethyl to n-propyl) results in a slight
improvement in activity towards H. virescens larvae
(Table 3), while a further alkyl extension at C21
(2-n-butenyl) is about as active as spinosyn A
(C21 ¼ ethyl). Among the spinosyns arising from
mutant strains (2 0 ,3 0 ,or4 0 -demethyl rhamnose derivatives)
(Table 2), virtually all were far less active
than spinosyn A. The one notable exception to this
is spinosyn Q (LC 50 0.5 ppm) which is close in
activity to spinosyn A.
6.5.1.1.2. Structure–activity relationships of
spinosyns – Diptera In general, the pattern of
activity for the different spinosyns towards adult
stable flies is similar to that observed for the Lepidoptera
(Kirst et al., 2002a). For example, spinosyns
A–D and Q are nearly equitoxic to the adult stable
fly (Stomoxys calcitrans), while the C16 demethyl
analog (spinosyn F) is less active, as is spinosyn
K(Table 2). However, unlike with H. virescens larvae,
spinosyns E, H, and J are as active as spinosyn
A(Table 2). The difference for spinosyn J is especially
interesting since it is at least 100-fold less active
than spinosyn A against larvae of H. virescens. Thus,
there may be some significant differences in the
spinosyn target site in adult stable flies, in the metabolism/conjugation
of spinosyn J, or both, compared
to larvae of H. virescens. Larval blowfly
(Calliphora vicina) activity of the tested spinosyns
was even more like lepidopteran activity than
S. calcitrans activity (Kirst et al., 2002a). As in
H. virescens larvae, spinosyns A–D are as toxic as
spinosyn A, while spinosyns E and F, and the 2 0 ,3 0 ,
or 4 0 -demethyl analogs (spinosyn H, J, and K,
respectively) are less active than spinosyn A. However,
unlike in larvae of H. virescens, spinosyn J still
displayed a reasonable level of activity towards
larvae of C. vicina (Table 2).
6.5.1.1.3. Structure–activity relationships of
spinosyns – Homoptera Unlike members of the
Lepidoptera and Diptera, homopterans such as
aphids are not very susceptible to the spinosyns
(DeAmicis et al., 1997; Sparks et al., 1999; Crouse
et al., 2001). With the exception of spinosyns B, K,
and O, the spinosyns of S. spinosa for which data are
currently available are not very active against cotton
aphid (Aphis gossypii) (Table 2). Spinosyn activity
against the aster leafhopper (Macrosteles quadrilineatus)
reflects a similar pattern in that spinosyn K
is again the most active of the spinosyns tested,
although comparatively, spinosyns A and B are also
active. Interestingly, other 4 0 -O-demethyl spinosyns
such as spinosyn O and the 2 0 ,4 0 -di-O-demethyl spinosyns
U and V are also quite active against the
leafhoppers, relative to spinosyn A (Table 2).
6.5.1.1.4. Structure–activity relationships of
spinosyns – Acarina The spinosyns are also active
against tetranychid mite species such as the twospotted
spider mite (Tetranychus urticae) (Sparks
et al., 1999; Crouse et al., 2001). As observed with
some of the homopteran species, spinosyns K and
O are among the most active of the natural spinosyns
from S. spinosa, while spinosyns H and Q are
comparatively weaker than spinosyn A (Table 3).
Likewise, spinosyn B is also relatively active. As
noted previously, there is only a weak correlation
between activity towards neonate H. virescens larvae
versus spinosyn activity towards T. urticae
(Sparks et al., 1999). While improvements in mite
activity are possible, the physical properties affecting
residuality and translaminar characteristics are
such that the spinosyns are typically not well suited
for mite control compared to available commercial
standards (Crouse et al., 2001).
6.5.1.2. Nontarget organisms
6.5.1.2.1. Beneficial insects The spinosyns,
exemplified by spinosad, are highly efficacious
against a variety of pest insect species, yet relatively
weak against towards a variety of beneficial insect
species, especially predatory insects. A detailed
summary of studies involving the effects of spinosad
on a wide variety of beneficial insects has recently
been published (Williams et al., 2003). Spinosyns
such as spinosad typically have little effect on predatory
insects and mites (Williams et al., 2003). Where
particular laboratory bioassays have shown some
spinosad toxicity to predators (e.g., Orius insidiosus),
corresponding greenhouse and field tests have generally
shown spinosad to have little effect (Studebaker
and Kring, 2003), principally due to the rapid
environmental degradation of spinosad (Saunders
and Bret, 1997; Crouse et al., 2001; Williams et al.,
2003). Based on data in Williams et al. (2003),
in more than 88% of the greenhouse/field assays