IGRs — A Growing, But Misunderstood Group

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

Insect Physiology 101

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

The molting process in insects is under the direct controlof hormones. The two major hormones involved in insect molting are ecdysone andjuvenile hormone (JH). Levels of these hormones fluctuate and arecounter-cyclical during natural growth and development (See Figure 1, right).Large peaks of ecdysone, the “molting hormone,” serve to trigger themolt. JH determines whether or not an insect will molt into a juvenile form. IfJH 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 insectmolts into a pupa or adult.

Because they control pests by interfering with the insect’smolting process, as a rule, IGRs take longer to kill than traditionalinsecticides. Death is typically observed in 3-10 days, depending on theproduct, the insect, its life stage at application and its developmental rate.But some IGRs, such as azadirachtin products, cause insects to cease feedinglong 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 groupsin IPM discussions. Until recently, crop protection companies had not done agreat job of making it clear to growers and extension specialists exactly howto classify these products. Products in this group are, in fact, quite diversein their chemistries. Regardless, it is their modes of action that are muchmore relevant to determining proper use than are their chemical classes. IGRscan be classified easily into three modes of action.

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

Chitin synthesis inhibitors. The second IGR class introducedinto the U.S. greenhouse market, these products disrupt molting by blocking theformation of chitin, the building block of an insect’s exoskeleton. Theexoskeleton, the insect’s skin and bones, must be absorbed and then re-formed duringeach successive molt. Without the ability to synthesize chitin, molting isincomplete, resulting in malformed pests that soon die. The first of theseproducts were cyromazine (Citation) and diflubenzuron (Adept), though it took afew years to bring the latter to the greenhouse from its original uses in fieldcrops and mosquito control. Two new greenhouse products joined the ranks ofchitin synthesis inhibitors in 2002 with the introduction of novaluron(Pedestal) and buprofezin (Talus).

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

Talus delivers toxicity to pests via contact, ingestion andvapor activity. There is little or no systemic movement of Talus in plants. Butits vapor activity allows Talus to reach leaf undersides where many pestspecies dwell, and it helps protect new growth. In addition to its primary modeof action, Talus also suppresses egg-laying and causes egg sterility in treatedadults through secondary hormonal activity. Talus is available in water-solublebags for use in greenhouse tomato production for whitefly control. A broadornamental label and separate package will be available in January 2004 thatincludes the full pest spectrum of whiteflies, mealybugs, scales and plant- andleafhoppers.

Ecdysone inhibitors. This class interferes with metabolism of themolting hormone, thereby breaking the life cycle at all larval stages.Additionally, the reprogramming of larval tissues into adult tissues during thepupal stage is interrupted by the action of ecdysone antagonists, causing pupalmortality. Ecdysone inhibitors may offer more opportunities to control pestssince they directly affect both the larvae and the pupae. One activeingredient, azadirachtin, represents this class in the form of severalproducts, including Aza-Direct, Azatin and Ornazin.

Azadirachtin is a naturally occurring substance derived fromthe 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’sproduction and release of the vital hormone. Azadirachtin also serves as afeeding deterrent for some insects. Upon ingestion of even minute quantities,insects become quiescent and stop feeding. It may also act as an insectrepellent for some pest species. Though percent azadirachtin is maintained, thelevel of other neem-derived active ingredients called liminoids and the overallquality control differ among azadirachtin products.

Rotation, Rotation, Rotation

IGRs have always gained favor as rotation partners withtraditional insecticides. But because IGRs fall into three distinct modes ofaction, they themselves can be rotated effectively without undue fear ofdeveloping 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, butgenerally, each product has its forte. See Figure 2, below, for moreinformation.

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 dipteranpests such as fungus gnats and shore flies, in that members of all three modesof action elicit control. For purposes of this example, let us assume thatcultural controls (proper sanitation, moisture control, etc.) were insufficientin effectively managing the pests in a propagation scenario and that apre-existing, heavy population of fungus gnats exists. An effective chemicalrotation might include a pyrethroid (Astro, Decathlon, Talstar/Attain) forknock-down of adults, followed by successive drenches of Azatin for control oflarvae and pupae and then Enstar II for additional larval control.

IGRs and IPM

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

Since IGRs are not quick-kill products, it is generallyadvisable to apply them for two to three applications (see label restrictions)to keep pests under control. But in cases where adult pest populations explodedue to migration or introduction into the greenhouse, use adulticides with ashort residual (pyrethroids work well) before releasing beneficials. An IGRregimen could be implemented one or two weeks following an adulticideapplication. In this way, IGRs provide the flexibility of using biocontrols tostretch out intervals between applications. Similar to chemical rotations andtank-mixing, this approach combines the strengths of two separate types ofinsecticides along with a biological control program to achieve effectiveinsecticide resistance management and maximum control.

Heed the Labels

In addition to resistance management restrictions that limitthe number of applications or total amount applied, be sure to check the labelfor indications of plant safety before applying to your crop, as many labelslist specific crops to avoid, along with optimum pH levels to achieve. Withproper use, IGRs are highly-effective and exceptionally-flexible tools in yourpest management arsenal.

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.