Desiccants: Dry As Dust Means Insect Deaths

It’s not only world politics that are on a fast track. Pest control strategies and customers are changing more rapidly then most PCOs realize.

Public perception of the hazards of pesticides has created a bonanza for PCOs who promote pest control with zero or minimal use of traditional toxicants. PCOs who have been essentially "contract spray vendors," to quote PCO Bill Hawks of Wichita, Kan., will lose more and more business to a new breed of PCOs who promote environmentally compatible strategies. These include heat, cold, Electroguns, live trapping and other methods.

Our old IPM strategies: sanitation, blocking, etc., are still the mainstays of a first-rate pest control program. However, new environmentally inspired buzzwords are good bait for doing business with people who have been conditioned by anti-pesticide propagandists. They have created a political climate in which a pesticide is considered dangerous if more than one person in a million can develop a cancer after 70 years of exposure.

Among the choices available to PCOs who aim for this market are two oldies in our bag of remedies: diatomaceous earth and silica aerogels - desiccant dusts which kill by drying up the insect. I investigated these old timers and found that I had been harboring some Myth Conceptions about these desiccants.

For starters, I had thought diatoms were primitive, tiny sea animals. But from NPCA’s Harvey Gold, who earned a master’s degree in marine biology, I learned that they are microscopic plants that come in many shapes. He said their tiny, skeletal cellular walls are heavily perforated. Hugh accumulations of these cells, together with considerable detritus from ancient sea beds, are now mined for many industrial uses.

I asked Whitmire’s Dave Naffziger to examine a diatom under his 400x microscope. He obliged and reported that it was rectangular and relatively smooth with sharp edges. When I mentioned this to Bob Fee of the CR Chemicals Corp. in Golden, Colo., he said that a 1000x magnification would have shown the perforations - an indication of the incredibly small pore size! Fee continued, "Only marine diatoms have that rectangular shape. Freshwater species have a wide variety of shapes: triangular, round, cylindrical, snowflake, spicular, etc.

MARINE VS. FRESHWATER DIATOMS. From Penn State’s Bob Snetsinger, I learned that my idea that all diatomaceous earths were essentially the same was a Myth Conception. He said diatomaceous earth from fresh water seas does not clump the way salt water diatoms do. For pest control use, this is critical. Early efficacy tests on the more common marine variety may have had different results with the fresh water diatoms. This distinction blurs the various claims about generic diatomaceous earths.

The distinction of marine vs. freshwater diatoms has helped clear up another Myth Conception. I’ve been told that free silica particles in diatomaceous earth could cause silicosis, a condition of massive fibrosis of the lungs marked by shortness of breath and caused by prolonged inhalation of silica dusts. According to Fee, marine diatomaceous earth could contain as much as 60 percent crystalline silica, whereas the freshwater material has less than 1 percent crystalline silica.

PARTICLE SIZE AND HARDNESS. Most of the texts that I have state that diatomaceous earth passes through a 325-mesh screen, the finest commonly used for pesticide dusts. This particle size measures 44 microns. But Fee states that a 325-mesh screen retains on ½ percent to 4 percent of the dust. Actually 90 percent of the particles have a mean micron size of 9, with 10 percent of them below 1. So the 325-mesh label is misleading - another Myth Conception!

One of my early beliefs when I tested a sample of SG67 (DriDie) from Davison many yeas ago was that the DriDie particle was sharp, that it abraded the integument and removed the critical lipoid layer on the target insect. I was wrong about the abrasion. The particle may look like a miniature porous rock with jagged points, but it’s soft as a sponge. However, it does a superb job of absorbing the thin lipoid film that keeps the body fluids from escaping. Diatamaceous earth, on the other hand, is truly abrasive. It is the whitish residue in car polish rubs. Bottom line: Both systems work, abrasion or absorption.

 

PORE CAPACITY AND REPELLENCY. Both products are pore thirsty. The silica aerogel has greater pore capacity and can hold oil up to 300 percent of its weight, while the fresh water diatomaceous earth can hold 114 percent of its weight, according to Fee.

I’ve been told that DriDie is so sharp it cuts the body surfaces of insects, making it repellent. A Myth Conception! Entomologist Walter Ebeling, who is probably the foremost authority on desiccants, believes that insects avoid DriDie because they "sense what’s bad for hem" (losing body fluid). Fairfield American’s George Hickson, on the other hand, cites field experiences in which repellency was not demonstrated. Perhaps the different viewpoints relate to dosage. I once rolled DriDie on one side of a boiler room and later found that air currents had redeposited it in excess on the opposite side. Diatomaceous earth, however, is not repellent if applied lightly, according to Bob Snetsinger. He believes that anything over 3 ounces per 100 square feet may be repellent. Excessive layers of any dust are repellent because of an unstable, shifting base. Of course, when pyrethrins are added to either product, it becomes very repellent.

In next month’s Myth Conceptions column, I’ll discuss how the desiccants work on arthropods, how best to use them and the pros and cons of silica aerogels and diatomaceous earth desiccants.

 

Industry consultant Harry Katz is a columnist for PCT.