IGRs -- A Growing, But Misunderstood Group

November 4, 2003 - 13:04

To take full advantage of these safer chemicals, you will want to understand a bit of insect physiology along with similarities and differences of the registered active ingredients in IGRs.

Over the past decade, the agrochemical industry has
developed a range of safer products. These new crop protection tools are much
more selective against target pests, exhibit low human toxicity and have
minimal impact on the environment. Insect growth regulator (IGR) products, old
and new alike, have always been characterized as highly targeted and
environmentally safe. In fact, all IGRs registered since 1993, the year the
"Reduced-Risk" initiative began, have been classified as such by the
U.S. EPA.

Insect Physiology 101

The safety of IGRs stems from the fact that these compounds
generally affect only animals that molt; so they generally do not affect
non-target species such as mammals, birds, fish or other invertebrates. IGRs
interfere with immature insect growth and development. Although they may affect
other life stages, IGRs kill insects primarily in the larval/nymphal and, in
some cases, the quiescent pupal stages.

The molting process in insects is under the direct control
of hormones. The two major hormones involved in insect molting are ecdysone and
juvenile hormone (JH). Levels of these hormones fluctuate and are
counter-cyclical during natural growth and development (See Figure 1, right).
Large peaks of ecdysone, the "molting hormone," serve to trigger the
molt. JH determines whether or not an insect will molt into a juvenile form. If
JH is present during one of the small ecdysone peaks, called commitment peaks,
then the insect molts into a larger larval form. If JH is absent, the insect
molts into a pupa or adult.

Because they control pests by interfering with the insect's
molting process, as a rule, IGRs take longer to kill than traditional
insecticides. Death is typically observed in 3-10 days, depending on the
product, the insect, its life stage at application and its developmental rate.
But some IGRs, such as azadirachtin products, cause insects to cease feeding
long before death occurs.

Modes of Action

IGRs erroneously have been bunched together and termed the
"IGR chemical class" for purposes of contrast to other product groups
in IPM discussions. Until recently, crop protection companies had not done a
great job of making it clear to growers and extension specialists exactly how
to classify these products. Products in this group are, in fact, quite diverse
in their chemistries. Regardless, it is their modes of action that are much
more relevant to determining proper use than are their chemical classes. IGRs
can be classified easily into three modes of action.

Juvenile hormone (JH). One of the earliest IGRs brought to market was
kinoprene (Enstar), developed to mimic the chemical structure of JH. This work
led to the development of a class of insect growth regulators called JH analogs
or JH mimics. By mimicking JH, members of this class cause premature molting of
young immature stages, throwing larval development into disarray. Today, the JH
mimics are represented by three products: the improved s-kinoprene (Enstar II),
pyriproxyfen (Distance) and fenoxycarb (Precision, Preclude). Of these, only Enstar
II and Distance are readily available to growers. Federal registrations for
fenoxycarb have been maintained, but at present the product is not marketed by
either Syngenta or its former licensee, Whitmire-Microgen.

Chitin synthesis inhibitors style='font-weight:normal;font-style:normal'>. The second IGR class introduced
into the U.S. greenhouse market, these products disrupt molting by blocking the
formation of chitin, the building block of an insect's exoskeleton. The
exoskeleton, the insect's skin and bones, must be absorbed and then re-formed during
each successive molt. Without the ability to synthesize chitin, molting is
incomplete, resulting in malformed pests that soon die. The first of these
products were cyromazine (Citation) and diflubenzuron (Adept), though it took a
few years to bring the latter to the greenhouse from its original uses in field
crops and mosquito control. Two new greenhouse products joined the ranks of
chitin synthesis inhibitors in 2002 with the introduction of novaluron
(Pedestal) and buprofezin (Talus).

While Pedestal shares both its mode of action and its
chemical class (benzoylurea) with Adept, it provides the added benefit of
controlling immature thrips. Pedestal enters the insect mainly by ingestion,
though it does have some contact activity. While it does not have direct
ovicidal activity, a high percentage of first-instar larvae hatched from eggs
laid on treated foliage may die. Research on whiteflies has indicated that
Pedestal also reduces adult female fertility. Pedestal is labeled for control
of thrips, whiteflies, armyworms and for suppression of leafminers.

Talus delivers toxicity to pests via contact, ingestion and
vapor activity. There is little or no systemic movement of Talus in plants. But
its vapor activity allows Talus to reach leaf undersides where many pest
species dwell, and it helps protect new growth. In addition to its primary mode
of action, Talus also suppresses egg-laying and causes egg sterility in treated
adults through secondary hormonal activity. Talus is available in water-soluble
bags for use in greenhouse tomato production for whitefly control. A broad
ornamental label and separate package will be available in January 2004 that
includes the full pest spectrum of whiteflies, mealybugs, scales and plant- and
leafhoppers.

Ecdysone inhibitors. This class interferes with metabolism of the
molting hormone, thereby breaking the life cycle at all larval stages.
Additionally, the reprogramming of larval tissues into adult tissues during the
pupal stage is interrupted by the action of ecdysone antagonists, causing pupal
mortality. Ecdysone inhibitors may offer more opportunities to control pests
since they directly affect both the larvae and the pupae. One active
ingredient, azadirachtin, represents this class in the form of several
products, including Aza-Direct, Azatin and Ornazin.

Azadirachtin is a naturally occurring substance derived from
the neem tree and is the key insecticidal ingredient in these products.
Azadirachtin-containing products are thus considered "botanicals,"
and some are Organic Materials Review Institute-listed for organic use.
Azadirachtin is structurally similar to ecdysone, blocking the insect's
production and release of the vital hormone. Azadirachtin also serves as a
feeding deterrent for some insects. Upon ingestion of even minute quantities,
insects become quiescent and stop feeding. It may also act as an insect
repellent for some pest species. Though percent azadirachtin is maintained, the
level of other neem-derived active ingredients called liminoids and the overall
quality control differ among azadirachtin products.

Rotation, Rotation, Rotation

IGRs have always gained favor as rotation partners with
traditional insecticides. But because IGRs fall into three distinct modes of
action, they themselves can be rotated effectively without undue fear of
developing cross-resistance. The key is to rotate among, rather than within,
the three modes of action.

IGRs tend to work well within a limited pest spectrum.
Labels list additional pest species often at higher labeled rates, but
generally, each product has its forte. See Figure 2, below, for more
information.

As an example of rotating among the three modes of action,
let us look at fungus gnat control. As a group, IGRs are strong on dipteran
pests such as fungus gnats and shore flies, in that members of all three modes
of action elicit control. For purposes of this example, let us assume that
cultural controls (proper sanitation, moisture control, etc.) were insufficient
in effectively managing the pests in a propagation scenario and that a
pre-existing, heavy population of fungus gnats exists. An effective chemical
rotation might include a pyrethroid (Astro, Decathlon, Talstar/Attain) for
knock-down of adults, followed by successive drenches of Azatin for control of
larvae and pupae and then Enstar II for additional larval control.

IGRs and IPM

The selective nature of IGRs makes them a natural fit in
greenhouse IPM. Greenhouse IGR products, differentially targeted to pests, are
not broad enough in their activity to include insect orders containing
biocontrol agents. Also key here is that adult predators and parasites are not
killed by IGRs because they are not molting.

Since IGRs are not quick-kill products, it is generally
advisable to apply them for two to three applications (see label restrictions)
to keep pests under control. But in cases where adult pest populations explode
due to migration or introduction into the greenhouse, use adulticides with a
short residual (pyrethroids work well) before releasing beneficials. An IGR
regimen could be implemented one or two weeks following an adulticide
application. In this way, IGRs provide the flexibility of using biocontrols to
stretch out intervals between applications. Similar to chemical rotations and
tank-mixing, this approach combines the strengths of two separate types of
insecticides along with a biological control program to achieve effective
insecticide resistance management and maximum control.

Heed the Labels

In addition to resistance management restrictions that limit
the number of applications or total amount applied, be sure to check the label
for indications of plant safety before applying to your crop, as many labels
list specific crops to avoid, along with optimum pH levels to achieve. With
proper use, IGRs are highly-effective and exceptionally-flexible tools in your
pest management arsenal.

About The Author

Insect growth regulator (IGR) products, old and new alike, have always been characterized as highly targeted and environmentally safe. IGRs have always gained favor as rotation partners with traditional insecticides. But because IGRs fall into three distinct modes of action, they themselves can be rotated effectively without undue fear of developing cross-resistance.

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