IGRs and Flea Control

Insect growth regulators (IGRs) are playing an increasingly prominent role in controlling insect pests in diverse situations. Initially introduced in the 1970s for control of mosquitoes, a variety of IGRs are now used in agriculture and horticulture, as well as against stored product pests, ants, cockroaches and fleas. Insect growth regulators are among those materials which have been classified "biorational" to distinguish them from conventional pesticides. For many people the term "biorational" implies that IGRs are natural, biodegradable and specific to the target pest. Conventional pesticides, on the other hand, are synthetic and often perceived as being persistent and having a broad spectrum of activity. Such comparisons between IGRs and so-called conventional pesticides are overly simplistic and may be misunderstood by PCOs.

As pest management professionals we should be aware that IGR products are synthetic, not natural. And although more limited in pest spectrum than conventional insecticides, two of the most commonly used IGRs, methoprene and fenoxycarb, are active against homoptera (e.g. scale insects), diptera (e.g. mosquitoes), lepidoptera (e.g. moths), coleoptera (beetles), as well as against siphonaptera (fleas) and many other pest and non-pest insects. Of course, the manner in which these IGRs are formulated and used helps achieve target specificity, but this also can be said of conventional pesticides.

Regarding degradability, it is perfectly true that IGRs are biodegradable, but it is also true that all the organophospates, carbamates, pyrethroids and other synthetic pesticides used by PCOs are biodegradable. In fact, IGRs are marketed and labeled as residual pesticides and persist longer indoors than some conventional pesticides.

In summary, although there are actually several similarities between IGRs and conventional pesticides, IGRs are quite different in terms of their mode of action, and this gives them some particular benefits in flea control.

HOW THEY WORK. The IGRs currently available for flea control are based on methoprene (Precor) and fenoxycarb (Torus). Both are "juvenoids," a term used for synthetic chemicals which mimic the naturally occurring juvenile hormones which help control development and molting of immature insects. A second group of IGRs known as chitin inhibitors (e.g. diflubenzuron) is under development and this type interferes with cuticle development and prevents molting. The primary mode of action of juvenoids is to inhibit insect transformation from one stage to another (e.g. larva to pupa, nymph to adult). This mode of action is particularly suitable for pests such as fleas or mosquitoes where the larvae do not cause a nuisance. However, for many insects the larval or nymphal stages are harmful, and prolonging these stages is undesirable. For instance, in the case of moths and cockroaches the use of IGRs alone is not ideal because the larvae and nymphs are pests. (Editor’s note: Where use of an adulticide is recommended, IGRs have been used successfully as the primary control agent for cockroaches in sensitive areas such as hospital and research facilities.)

It should also be remembered that IGRs do not kill adult insects, so any adult fleas present at the time of treatment will continue to be a nuisance until they die from other causes, unless an adulticide is also used. In addition, like conventional pesticides, IGRs will not affect existing flea pupae, which are well protected within their pupal skin and within the silk cocoon spun by the final larval stage. However, both methoprene and fenoxycarb have ovicidal action which prevents treated flea eggs from hatching. So while existing pupae and adults will survive and IGR treatment, eggs and larvae will not develop to produce a new generation. Before considering how to further exploit this mode of action for flea control, it is worth looking at some of the important benefits of IGRs.

COLLATERAL BENEFITS OF IGRs. Because the metamorphosis and molting of insects has no counterpart in higher animals such as people and domestic animals, IGRs can not exert their mode of action in such nontarget species. (However, beneficial insects can be affected.) This contrasts with most conventional insecticides which affect enzyme systems found in insects, people and pets. As a consequence IGRs are characterized by extremely low toxicity to people and pets.

Toxicity levels of IGRs are generally much lower than conventional pesticides. However, as a result of formulating IGRs with various solvents, the actual products used by PCOs may be more toxic than the technical material and may cause skin or eye irritation. These hazards, not limited to IGRs, are clearly stated on the label.

The unique mode of action of IGRs has particular benefits regarding pet safety. Where fleas are a problem, pets are often subjected to a battery of treatments with conventional pesticides administered by owners, veterinarians or pet groomers in the form of dips, shampoos, sprays, dusts, feed-throughs or collars. These on-animal treatments often involve the same family of active ingredients (organophosphate or carbamate) as found in conventional pesticides used by PCOs for treating the premises. This can result in additive effects, and sometimes noticeable symptoms of cholinesterase inhibition, when treated pets re-enter treated premises.

In the case of IGRs there are no such additive effects. Indeed, with the recent availability of the first on-animal IGR, a methoprene-based product from Zoecon (Petcor), it is now possible for PCOs to treat the home with Precor and leave behind Petcor for homeowners to treat the pet, using the same IGR ingredient without ill effect.

Among other benefits of IGRs particularly relevant to flea control are their low odor and low potential for staining carpets. In these respects IGRs are as good as the best conventional pesticides. In other respects IGRs have advantages over conventional pesticides. For instance, on theoretical grounds and on the basis of more than 10 years of commercial use of methoprene by PCOs, it seems less likely that insects will develop resistance to IGRs than to conventional synthetic organic pesticides. Also, IGRs are surprisingly persistent indoors, and some formulations of IGRs are resistant to ultraviolet light.

Regarding the length of residual action of IGRs, a thorough spray treatment of the premises with Torus 2E or Precor EC can last up to six months or more.

HOW TO APPLY IGRS. The methods of application of IGRs against fleas in structures are the same as for conventional residual insecticides. This is not surprising when you consider that conventional residual insecticides are aimed against flea larvae as well as adult fleas. Sprays of IGRs are targeted at all indoor areas that may harbor fleas, including carpets, furniture, pet sleeping areas and throw rugs. The underside of cushions of upholstered furniture also should be treated.

In the case of Torus E2 and Precor EC, the spray should be applied at a rate of a gallon per 1,500 square feet of surface area. As with conventional residual insecticides, pets and people should not contact surfaces sprayed with IGRs until they are dry.

STRATEGIES FOR USING IGRS. While the application methods are the same, the strategies for using IGRs against fleas are quite different than with conventional pesticides. The key difference is in the timing of treatments. Because IGRs don’t kill existing adult fleas, it is best to use them early in the season before adult infestations build up. In the warmer parts of the country this means using IGRs in April or even earlier. If applied soon enough, one or two applications of IGRs can provide season-long control. In other words, where initial numbers of adult fleas are low, the use of IGRs alone can be sufficient to achieve acceptable levels of control. This is because most existing adult fleas will die naturally within two weeks and the IGRs will prevent a new crop of adults from developing.

It is recommended that PCOs use an adulticide in conjunction with an IGR when one or more of the following three conditions exist:

• The population of adult fleas in the account is already high.

• New adult fleas are constantly being brought indoors by pets which roam outside.

• There is a large reservoir of pupae in low temperature conditions, which may result in an extended period of adult emergence, perhaps for six months or more.

Under these three circumstances it may be necessary to use appropriately labeled conventional insecticides to kill adult fleas. Fortunately, the labels for both methoprene and fenoxycarb allow for tank mixtures with conventional insecticides, so a separate spray application is not necessary. When selecting which conventional insecticide to mix with IGRs to kill adult fleas it is important to choose one which is labeled for this use, safe to use in a particular circumstance, and chemically compatible with the IGRs. For best results it is also preferable to use a conventional insecticide which has a good residual action against adult fleas. Among the conventional products which have proved particularly useful in mixtures with IGRs are those based on benidiocarb, chlorpyrifos and propetamphos. When tank mixing a wettable powder product with an IGR, it is best to first mix the wettable powder in the full volume of water before adding the IGR.

CONCLUSION. In summary, IGRs are an extremely useful tool for achieving long-term control of fleas. However, whether used alone or in a combination with conventional insecticides which kill adult fleas, IGRs should form part of an integrated program involving cleaning the premises and treating those pets which are vulnerable to fleas.

 

The author, a frequent contributor to PCT, is president of Winchester Consultants, Winchester, England.