Insect Control: Biological and Synthetic Agents - Index of
Insect Control: Biological and Synthetic Agents - Index of
Insect Control: Biological and Synthetic Agents - Index of
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22 1: Pyrethroids<br />
used. Organophosphates <strong>and</strong> carbamates were used in<br />
Stage III <strong>and</strong> also if required for additional sprays<br />
during Stages I <strong>and</strong> II.<br />
The use <strong>of</strong> a ‘‘s<strong>of</strong>ter’’ insecticide (endosulfan) in<br />
the early season was deliberate to minimize disruption<br />
<strong>of</strong> beneficial parasitoids <strong>and</strong> predators <strong>and</strong> to<br />
avoid a potential upsurge <strong>of</strong> secondary pests such as<br />
mites, aphids, <strong>and</strong> whiteflies, which were not controlled<br />
by the pyrethroids available at the time.<br />
Examples <strong>of</strong> nonchemical countermeasures incorporated<br />
in the strategy to reduce selection pressure<br />
included the use <strong>of</strong> early-maturing crops, avoiding<br />
early-growing crops (e.g., maize) in adjacent fields,<br />
which may act as early-season nurseries for resistant<br />
H. armigera, <strong>and</strong> utilization <strong>of</strong> host plants in refugia<br />
to maintain a large pool <strong>of</strong> susceptible individuals,<br />
which would continually dilute the resistant<br />
population in the crop. Resistance was monitored<br />
on a weekly basis using a discriminating dose <strong>of</strong><br />
fenvalerate with <strong>and</strong> without the synergist PBO.<br />
Later monitoring showed a rise in metabolic resistance<br />
attributed to MFOs <strong>and</strong> this resulted in the<br />
inclusion <strong>of</strong> PBO in the last <strong>of</strong> the maximum <strong>of</strong> three<br />
pyrethroid sprays. This strategy was successful for<br />
many years but the underlying trend <strong>of</strong> upward<br />
increase in the proportion <strong>of</strong> resistant insects continued<br />
<strong>and</strong> finally led to a complete reorganization <strong>of</strong><br />
the strategy in the mid 1990s with a shift away from<br />
reliance on pyrethroids.<br />
Initial resistance to pyrethroids was thought to be<br />
due to the presence <strong>of</strong> the knockdown site-insensivity<br />
resistance (kdr) mechanism probably as a direct<br />
result <strong>of</strong> cross-resistance to DDT. Over the years <strong>of</strong><br />
the IRM strategy, metabolic resistance mechanisms<br />
appear to have taken over. However, there is still<br />
controversy over whether it is primarily due to elevated<br />
levels <strong>of</strong> esterases or mixed-function oxidases<br />
(MFOs). The main reason for the uncertainty concerns<br />
the role <strong>of</strong> synergists used in the studies. For<br />
example, PBO is now thought to inhibit both these<br />
types <strong>of</strong> enzymes. Furthermore, it has been suggested<br />
that such inhibitors may themselves have<br />
become resisted in the field over time (McCaffery,<br />
1998), thus obscuring the mechanism <strong>of</strong> resistance.<br />
The second IRM strategy involved the whitefly,<br />
B. tabaci, a representative sucking pest on a wide<br />
range <strong>of</strong> crops (Denholm et al., 1998). In 1995, overreliance<br />
on a limited range <strong>of</strong> pyrethroids in Arizona<br />
had led to a classic treadmill scenario, with farmers<br />
responding to rising levels <strong>of</strong> resistance by increasing<br />
the number <strong>of</strong> sprays (as many as 8–12 applications<br />
per season). In this pest, the haplodiploid<br />
breeding system encourages rapid selection <strong>and</strong><br />
fixation <strong>of</strong> resistance genes. Males are produced<br />
uniparentally from unfertilized, haploid eggs, <strong>and</strong><br />
females are produced biparentally from fertilized<br />
diploid eggs. In addition, for this (<strong>and</strong> other) highly<br />
polyphagous species, the interaction between pest<br />
ecology <strong>and</strong> resistance is complex <strong>and</strong> generally<br />
not well understood, making formulation <strong>of</strong> IRM<br />
strategies even more difficult. The strategy <strong>of</strong><br />
Dennehy <strong>and</strong> Williams (1997) implemented in<br />
1996 had several features in common with that for<br />
H. armigera. It too relies on the continuous availability<br />
<strong>of</strong> a pool <strong>of</strong> susceptible whiteflies in refugia<br />
(e.g., Brassica crops) throughout the year <strong>and</strong> alteration<br />
<strong>of</strong> agronomic practices (e.g., timing <strong>of</strong> planting)<br />
in crops to minimize whitefly numbers whilst<br />
still maintaining the level <strong>of</strong> natural enemies. The<br />
first <strong>of</strong> the three-stage IRM strategy involved use <strong>of</strong><br />
a single spray each <strong>of</strong> pyriproxyfen <strong>and</strong> bupr<strong>of</strong>ezin<br />
(then newly available insect-growth regulators) with<br />
a 14- or a 21-day gap. The second stage allowed<br />
nonpyrethroid conventional insecticides <strong>and</strong> the<br />
third used pyrethroids synergized with acephate as<br />
late as possible in the season. A threshold <strong>of</strong> infestation<br />
was defined to initiate insecticide applications.<br />
Mixtures were limited to no more than two compounds<br />
<strong>and</strong> any one active ingredient was restricted<br />
to no more than two applications in one season.<br />
This strategy has been extremely successful in reducing<br />
the number <strong>of</strong> sprays required <strong>and</strong> regaining<br />
susceptibility both to synergized pyrethroids <strong>and</strong><br />
key nonpyrethroid insecticides.<br />
As for H. armigera, the need for monitoring,<br />
establishing threshold levels to trigger spray applications,<br />
<strong>and</strong> cooperation <strong>of</strong> the growers was key to<br />
the strategy. Long-term success <strong>of</strong> any IRM strategy<br />
depends on many factors because, even when an<br />
IRM strategy has been successful (McCaffery,<br />
1998), effectiveness <strong>of</strong> synergized pyrethroids can<br />
be lost after just two applications in areas with a<br />
history <strong>of</strong> resistance to pyrethroids.<br />
In conclusion, the key requirement for the development<br />
<strong>and</strong> sustainability <strong>of</strong> an IRM strategy is<br />
diagnosis <strong>of</strong> the resistance mechanism(s) in field<br />
populations. This is especially important when<br />
assessing the relative importance <strong>of</strong> individual<br />
mechanisms when several are present, which influences<br />
changes to the strategy with time. As alluded<br />
to earlier, there is still much uncertainty in the diagnosis<br />
<strong>of</strong> mechanisms. The use <strong>of</strong> established inhibitors<br />
or just in vitro bioassay data has been shown<br />
to be unreliable in this regard. For example in<br />
M. persicae, elevated esterase was considered for<br />
many years to be the main resistance mechanism. It<br />
was only recently shown (Devonshire et al., 1998) that<br />
this mechanism made only a minor contribution to the