Dessiccants: High-Stress vs Low Stress

Opinions also vary on the relative value of the properties of abrasives vs. adsorbants in killing arthropods. Many claim it is the abrasion of hard particles that scratch away the critical lipid (wax-like) film which coats insects. But other researchers believe this abrasion concept is a "Myth Conception." In the Annual Review of Entomology 1971, researcher Walter Ebeling claimed that the amorphous, non-abrasive microscopic particles of silica aerogel are more effective because they absorb the thin lipid film on insects, resulting in a lethal rate of water loss. (With some insects, this film is only .25 microns think, but the film is much thicker and softer on the cockroach, according to entomologist P. Cornwell.)

LIPOPHILIC (FAT LOVING). I always assumed that silica aerogel packets absorbed traces of moisture from delicate instruments. Ebeling proved that sometimes this could be a Myth Conception. The makers of DriDie 67 said that this desiccant picks up no water at 40 percent relative humidity and only 5 percent t 80 percent relative humidity. Only at 100 percent relative humidity was 80 percent moisture absorbed. However, since silica aerogel may be preferentially lipophilous (wax absoribng), it can still absorb lipids even if saturated with moisture.

How long will pyrethrin-impregnated silica aerogel be toxic in a hidden habitat? George Hinkson of Fairfield American Corp. describes a test in which Drione was effective six months after storage, uncovered, in a room with a temperature of 80 degrees Fahrenheit, 50 percent relative humidity and eight hours per day of fluorescent light. The pyrethrin molecule is probably protected from exposure within the silica conglomerates. So much for the Myth Conception that pyrethins always degrade rapidly in lighted areas.

PORE CANALS, STATIC ELECTRICITY. The mechanics of water loss in insects is interesting. Ebeling states that an incredibly thin layer of lipids covering most insects blocks water from escaping out of myriad pore canals. He further explains in a chapter in The Insect Integument (H.R. Hepburn, editor) that there is a complex, self-regulating mechanism that controls the flow of water vapor in and out of the cuticle through the many microscopic pore canals. This permits some insects such as subterranean termites to get their water needs from humid air and lose water where humidity is low. This explains the preference of cockroaches for midnight emergence in kitchens, when there are no drafts and humidity is higher.

Another reason for the effectiveness of DriDie is the inclusion of 4.5 percent ammonium silicafluoride in its formulation. This causes the dust to develop static electricity and holds it for two to three months in most situations. I once thought that the positive charge would hold for a longer period - a Myth Conception I harbored. With the charge waning, an advantage is lost - the attraction of dust to the insect. (DriDie is the silica aerogel base which is impregnated with pyrethrins/piperonyl butoxide in Drione and TriDie).

TWO-ZONE PEST CONTROL. Which of the two desiccants is best, those with or without pyrethrin? It is my opinion that both are needed. Why? If one could treat each and every microhabitat, either pesticide would do well. But since we can reach only a fraction of the potential microhabitats, I recommend a two-zone control strategy: (1) Use a high-stress, repellent pesticide in exposed sites (baseboard, appliances, cabinets, etc.) and (2) use low-stress, non-repellent materials for hidden foci (behind walls, under floors, etc.)

To inject high-stress materials into hidden sites would kill some pests but chase others into new sites and make future problems. By injecting non-repellents into habitats which reek of attracting pheromones, death traps would be made of each site. Were it not for the impatience of some accounts for instant control, the low-stress material would do for both zones.

REPELLENCY. Any dust applied to excessive levels is unstable for insects to walk on and is thus repellent. Silica aerogel impregnated with pyrethrins is of course repellent at any level.

Conditions vary widely with regard to difficulty in reaching sites to treat. A PCO may elect to use high-stress or low-stress materials in either or in both zones.

Many PCOs regard Drione and TriDie as too expensive. I regard this as a Myth Conception. Two ounces of Drone in 100 square feet is effective for at least six months. In sewers, where dampness nullifies most toxicants, Drione can still function to kill and repel cockroaches.

One can of TriDie has 227 applications, each dispensing millions of particles. Neither need be expensive, with proper training.

MORE INFORMATION. More information on freshwater distomaceous earth is available from two sources that I am aware of: Dr. Robert Snetsinger, Pennsylvania State University, University Park, Pa., 814/863-2866 (he did research on "Shellshock," which incorporates an adhesive to secure the desiccant to the insect), and Kathey Starnes, DiaCide Distributor, Nevada, Iowa, 800/767-6222.

DiaCide available in both low- and high-stress formulations (with or without pyrethrins/piperonyl butoxide). I would appreciate hearing about any other sources.

Consultant Harry Katz is a columnist for PCT magazine and an independent pest control consultant based in Deerfield Beach, Fla.