Insect Control: Biological and Synthetic Agents - Index of

polymorphisms in the protein. Of the 20 amino-acid
polymorphisms each uniquely associated with pyrethroid
resistance, those occurring at four sites have
so far been found as single mutations in resistant
populations. These are valine 410 (V410M in H. virescens;
Park et al., 1997), methionine 918 (M918V
in B. tabaci; Morin et al., 2002), leucine 1014
(L1014F in several species; L1014H in H. virescens;
Park and Taylor, 1997; L1014S in C. pipiens;
Martinez-Torres et al., 1999a), An. gambiae (Ranson
et al., 2000), and phenylalanine 1538 (F538I, in
B. microplus; Heet al., 1999). All these sites have
been located on trans-membrane regions of the
channel and it is inferred that they are close to
the binding site(s) of pyrethroids and DDT.
Additionally, other polymorphisms have been
found that only occur in the presence of L1014F
and appear to act in a similar way to M918T in
houseflies, causing enhanced resistance.
1.5.2. Resistance to Pyrethroids in the Field
The main cause of development of resistance in
insects in the field is a persistent and high selection
pressure as a consequence of repeated applications
of a single class of insecticide (or another with the
same mode of action). Therefore, despite effective
control in the initial stages, a small number of survivors
with innate resistance then rapidly multiply
until control fails. In this regard, pyrethroids are
no different from other insecticide classes. It is noteworthy
that it was the development of resistance to
pyrethroids that first prompted companies to take
resistance seriously and to take joint action. Pyrethroids
suffered an inherent disadvantage at the
outset in that kdr also confers resistance to DDT,
and prior use of DDT had already selected kdr
alleles to significant levels in the same pests. Presently
over 80 species have developed resistance to
pyrethroids (Whalon et al., 2003).
Until the late 1970s, the major agrochemical companies
had seen the development of resistance to
established classes as commercially beneficial and
motivation for the introduction of new classes.
However, the increasing cost of discovery, together
with the realization that pyrethroids could be rendered
ineffective in the field within a much shorter
period of time than other classes of insecticides,
forced them to take collective action. In 1979 they
set up the Pyrethroid Efficacy Group (PEG), which
in 1984 become a sub-group of a larger international
organization called the Insecticide Resistance
Action Committee (IRAC). This group communicates
its actions through a website (http://www.
irac-online.org) and also sponsors the biannual
Resistant Pest Management newsletter. Before the
1: Pyrethroids 21
establishment of PEG, the investigation of resistance
mechanisms and their consequences had remained
the domain of academic research, and the development
of resistance to pyrethroids was largely overlooked
and even denied by some companies. The
primary aim of PEG was to prolong the effectiveness
of pyrethroids in the field. It encouraged and
assisted in the investigation of all aspects of resistance
to pyrethroids and particularly the development
and implementation of insecticide-resistance
management (IRM) strategies.
Persistent selection with a single class of insecticide
will invariably lead to resistance, even if
synergists are used. Therefore the overall aim is to
minimize use of any single insecticide class so as
to limit selection pressure and thereby conserve susceptibility
in pest insects. This requires an in-depth
knowledge of factors ranging from resistance
mechanisms to genetic and ecological attributes of
both pest and beneficial insects. Any strategy has
also to integrate the judicious use of different insecticide
types (namely those with different modes of
action and synergy where possible) with other pestmanagement
options (e.g., agronomic practices),
together with regular monitoring of both the levels
of resistance and the nature of the resistance mechanisms.
Finally, the key requirement for the success
of any strategy is the cooperation of the growers. By
way of example, two IRM strategies are considered
below which encompass these factors.
The first successful and the most publicized IRM
strategy came from Australia. Synthetic pyrethroids
were introduced in Australia in 1977 when there
was already widespread resistance to virtually all established
classes. However, within six years of introduction,
resistance to these pyrethroids had also
developed in commercially important insect species
(Forrester et al., 1993). A resistance-management
strategy for H. armigera was implemented in the
1983/4 season. A different approach was used for
each class of insecticide, depending on the severity
of the resistance risk and predicted selection
pressure. It integrated the use of chemical and
nonchemical control methods, especially biological
and cultural. For chemical control, unrelated chemistries
were used with a strong emphasis on pyrethroid/ovicide
mixtures. In essence, a three-stage
strategy was implemented. In Stage I (September to
February), only endosulfan (an organochlorine) was
permitted. Stage II (January to February) allowed
a maximum of three pyrethroid sprays within a
42-day window (later reduced to 38 days), enough
to control only one of the five H. armigera generations
present within a single growing season. In Stages I and
II, ovicidal compounds (e.g., methomyl) could also be