Insect Control: Biological and Synthetic Agents - Index of

10 1: Pyrethroids
It should be noted that commercial pyrethroids
can exhibit significant phytotoxicity which can
limit their application. For example, deltamethrin
is too phytotoxic to be used on Sitka spruce (Picea
sitchensis) transplants to control the large pine
weevil (Hylobius abietis), and black pine beetles
(Hylastes spp). Permethrin exhibits little phytotoxicity
and cypermethrin is intermediate between the
two (Straw et al., 1996).
1.4. Mode of Action of Pyrethroids
1.4.1. Classification of Pyrethroids
Pyrethroids, which comprise a diverse range of
structures, have historically been classified into
two broad groups (Type I and Type II) on the basis
of their biological responses (Table 3). Interpretation
of most mode of action studies on insects
has been predicated on this classification, though
this is now considered to be an overly simplistic
approach.
The two types of symptoms have been associated
with the absence (Type I) or presence (Type II) of an
a-cyano group. For example, permethrin exhibits
Type I behavior; introduction of an a-cyano group
gives cypermethrin which shows Type II behavior.
This modification is accompanied by an increase in
activity of an order of magnitude, and indeed all
commercial Type II compounds have an a-cyano
group in conjunction with a 3-phenoxybenzyl alcohol
moiety. However, introduction of an a-cyano
group does not always lead to an increase in activity.
For example, in bioresmethrin it leads to a loss
in activity and in NRDC 196 activity is only marginally
increased. Furthermore, the level of activity
is influenced by the choice of test species. Thus
Table 3 Classification of pyrethroids
classification as Type I or II cannot be based solely
on the presence/absence of an a-cyano group. It is
most likely that Type I and Type II represent the
extremes of a continuum, with many pyrethroids
exhibiting intermediate properties.
Despite the uncertainty over such a classification,
it has been suggested (Soderlund et al., 2002; Vais
et al., 2003) that, on the basis of electrophysiological
properties, there are two distinct binding
sites for pyrethroids on the sodium channel. However,
this would not explain why some pyrethroids
(e.g., fluvalinate and bifenthrin, respectively with
and without an a-cyano group) appear to manifest
intermediate responses in insects, which behavior
might indicate the presence of only a single binding
site. The most likely explanation for the differential
responses of Type I and Type II pyrethroids in
in vitro assays lies in the occurrence of pharmacologically
different voltage-gated sodium channels in
insects (see below).
1.4.2. Site of Action
Pyrethroids are recognized for their rapid knockdown
action on insects but are generally poor at
killing them, a fact not fully appreciated for many
years. Measurements of LD50 are usually made 24
and 48 h after treatment, at which times paralyzed
insects are regarded as dead. However, insects often
recover fully, even at higher doses (e.g., 5 LD 50)
(Bloomquist and Miller, 1985), over the following
few days. In practice, such long-term recovery does
not limit pyrethroid efficacy in the field, as death
occurs by secondary processes including desiccation
and predation.
Pyrethroids are nerve poisons that disrupt nerve
conduction in both invertebrates and vertebrates
Response/action Type I Type II
Poisoning symptoms Rapid onset of symptoms even at
sublethal levels
Slow onset of symptoms
Hyperactivity often leading to Convulsion followed by
knockdown
paralysis
Low kill with high recovery High kill with low recovery
Inversely related to changes in Little effect of temperature
temperature
change
Electrophysiological response in nerve tissue Repetitive discharges in axons Blockage of conduction at
synapses
Action on sodium-channel function Monophasic and rapid decay of tail Biphasic and very slow decay
currents
of tail currents
Bind preferentially to closed
Bind preferentially to open
channels
channels
Level of resistance due to resistant houseflies with
super-kdr mechanism
Below 100-fold Over 200-fold