Flea Control Using Boric Acid: How Safe Is It?

During the past few years, manufacturers have made significant improvements to the products they offer for flea

control. For instance, we now have pyrethroid products available as spot-ons; the insect growth regulator methoprene comes in a flea collar or can be sprayed directly onto animals and carpets; boric acid is available for broadcast application to carpets; and most recently, "the pill" containing the chitin synthesis inhibitor (CSI) lufenuron was introduced.

Despite all these new technologies, we still find ourselves in the flea control business and using conventional

strategies. Regardless of these technological advances, we still have not found a single product that can deal with all aspects of a flea's biology in a single shot or in a timely fashion. There always seems to be a loose end. We have good tools for controlling the eggs and/or the larvae, or products that work well on adults; however the pupal stage remains an elusive problem. Treating outdoor areas poses another set of unique problems, which often require different products.

While many of the IGRs and CSIs are excellent tools and I endorse their use, they have two major drawbacks: They are slow to reduce the population, and the pupae and adults are unaffected. Thus most companies still rely on residual broadcast applications to carpets for the control of flea larvae and emerging adults. Because of this, the industry has been pressured to find less toxic products for flea control. This article provides one perspective on how to approach selecting least-toxic products.

Ask any pest control technician or customer, "What pesticide, in your opinion, is the safest?" The overwhelming response will most likely be boric acid. This response is based on a number of factors, not the least of which is the product's relatively low toxicity. In addition, many of the proponents of reduced-risk pest management have publicly promoted the perception that natural products such as boric acid are safer than their chemically synthesized count- erparts. Dr. Nancy C. Hinkle of the University of California-Riverside recently reviewed some of the "natural" flea control products and their side effects (1995). You might be surprised to learn the variety of adverse effects associated with some of the "safer" products.

When we consider the issue of "least toxic," we must consider more than a product's derivation — i.e. natural or organic — and its acute toxicity. For instance, rotenone, nicotine sulfate, and strychnine are derived from plants and have very low toxicity. Many synthetic products can be viewed in a similar fashion. For instance, some of the organophosphates that have been used in the industry are extremely to moderately toxic, while the insect growth regulators have extremely low toxicity. Thus some of the synthetic products, such as IGRs and CSIs, are often many times less toxic than their natural counterparts.

One of the fundamental things you learn in this industry when working with pesticide products is the definition of Hazard (= Dose ´ Exposure). There are many factors that must be considered in determining what the hazard is, and in turn which products are least toxic. The following is a list of factors I use to compare products and to determine which are least toxic. It is important to consider several factors and not necessarily focus on a single issue.

• Acute Toxicity — usually measured in terms of LD₅₀ or LC₅₀ , LD₉₅ or LC₉₅.

• Formulation — dust, emulsifiable concentrate, microencapsulate.

• Concentration — amount of active ingredient in applied solution; usually measured as a percentage.

• Application Rate — ounces or pounds per 100 square feet.

• Application Site — carpet, crack and crevice, spot, broadcast, outdoors.

• Exposure — skin, inhaled, ingested.

The tendency in determining what is least toxic usually centers around acute toxicity, which is measured in terms of lethal dose (LD), lethal concentration (LC) or lethal time (LT). This is the dose, concentration or time required to kill a percentage of the test population (usually rats), and is usually reported for the 50% or 95% levels. But this tells us nothing about the exposure. Exposure is related to where the product was applied, how much, and in what concentration. The formulation may also be a factor in how available the product is for exposure. In terms of dislodgeable residues, usually dusts > emulsifiable concentrates > microencapsulates.

Chronic toxicity is one area of concern that we cannot easily use in our decision- making process. This information is not easily quantified and is not well documented for most products. However, the other information on most products is sufficient to make informed decisions.

For the purpose of this discussion, I would like to compare the most widely used product in indoor flea control, chlorpyrifos, to what is considered by many to be one of the safest (perceived) products, boric acid. Furthermore, the highest labeled concentration and rate of application are used for the chlorpyrifos product, and the lowest labeled concentration and rate of application are used for boric acid. Using factors listed in the table below and the following formula, the toxicant load for each product, in terms of milligrams of active ingredient per 100 square feet, can be calculated.

In this case, there is 532 times more boric acid toxicant per 100 square feet than chlorpyrifos and a significant amount of dislodgeable residue in the form of dust. It is important to note that Hinkle et al. (1995) found that applying boric acid at a rate of 6.6 ounces per 1,000 square feet resulted in 90% suppression of flea larvae. Thus it becomes readily apparent that the rate of application for the boric acid product used in this comparison — 10 pounds per 1,000 square feet — far exceeds what is necessary to effect control. Hopefully, the manufacturers using these rates of application will reconsider the concentration and/or rate of application for their products.

The purpose of this exercise is not to extol the virtues of any product or to provide a recommendation for residual flea control. However, it serves to demonstrate that we cannot blindly accept the perception that a product is safe without examining the facts. Decisions on what products are least toxic should not be driven by marketing strategies, environmentalist hype or perception, but should be based on logical evaluation of the factors previously listed. After considering these factors, we can select products that pose the least risk to our customers and the environment.

REFERENCES

Hinkle, N. C. 1995. Natural born killers. Pest Control Technology, 23(7): 54, 56, 116.

Hinkle, N. C., P.G. Koehler and R. S. Patterson. 1995. Larvicidal effects of boric acid and disodium octaborate tetrahydrate to cat fleas (Siphonaptera: Pulicidae). J. Med. Entomol. 32(4): 424-427