Hi'iaka's Healing Herb Garden

by Barbara Fahs

New Development in Coqui Frog Control

A non-corrosive product made from acidic calcium sulfate (ACS) is effective as a spray when used for coqui frog control, according to David P. Davis, PhD, of Ono Loa Orchards in Mt. View. ACS is a bio-security product used to control harmful bacteria such as salmonella that sometimes contaminate food products.

The ingredients of ACS are “generally regarded as safe” (GRAS) and it has been approved for use in food by the Food and Drug Administration and the US Department of Agriculture. ACS is a strong acid, yet is also non-toxic and low in corrosiveness. You can hold a strong solution of it in your hand without harm, Davis said. When mixed in different concentrations and with different additives it has many uses, some of which include meat, fish, and poultry processing, and as a fruit and vegetable wash. When it comes to coqui frogs, however, Davis’ formula, which he calls ACS-P, is quite lethal because it is quickly absorbed through their skin. One person who has tested it on palm trees calls ACS-P the “silver bullet” for coqui control.

Davis moved to Hawai‘i four years ago and quickly learned about the coqui frog problem. He tried the usual remedies of citric acid and hydrated lime and found them to be too expensive, too corrosive, or too difficult to apply. As a former employee of the company that makes ACS, he realized he could use this product to make a more cost effective, non-toxic, less hazardous and more easily applied product for controlling frogs. Davis has been working on developing this product for the past two and a half years and has periodically had it evaluated by the USDA and University of Hawai‘i, receiving mixed reviews as to its effectiveness and commercial viability. After each evaluation he has made adjustments and believes he now has a formula that is superior to either citric acid or hydrated lime—and it’s more cost-effective than citric acid. The emergency approval given by the Environmental Protection Agency (EPA) for the use of hydrated lime to kill frogs expires this month, so Davis believes the chances of winning approval of ACS-P as a “minimal risk pesticide for use against coqui frogs” will be improved.

Davis has many ideas about how ACS-P could be used. Besides application as a spray for use on plants, it can be applied as slurry and used to paint benches, platforms, or other barriers in plant nurseries. Once the product is applied it does not break down until it is further diluted with water. When the slurry dries on a surface it creates a white coating that is revitalized when it gets wet. He believes that benches in nurseries could be painted with it to prevent coqui frogs from getting into the plants. “If not killed after coming into contact with the treated surface, frogs will vigorously seek to avoid further contact,” according to Davis’ information. Davis’ experiments have also shown that ACS-P destroys frog eggs. By applying ACS-P slurry on the ground under his hapu‘u ferns, juvenile frogs hiding in the leaf litter were killed.

Due to the strength of the solution needed to kill coquis, damage to some of your plants might occur after spraying them with ACS-P. Davis suggests you test your plants first or hose them off with water within 30 minutes after applying it. I sprayed some on a liliko‘i vine in which I heard a coqui, and a day or two later, some leaves began to turn yellow, and then fell to the ground. But I have not heard the frog since.

ACS-P is not an organic substance, so if you’re HOFA certified, it would not be approved. However, Davis states, “ACS-P is biodegradable and there is negligible risk of ground water contamination from the small amount of runoff that would result from spraying.” As with any other chemical, protect yourself with a facemask or goggles when applying this material.

The price is right, too: Davis plans to market his product so he can make it available at less than half the present cost of citric acid. And you can make it go a long way—for smaller areas it can be applied as a mist instead of hosing down the area, as is done with citric acid.

Davis concluded his study by stating, “The disadvantages of ACS-P are few, and it represents a potentially valuable addition to the present array of products available for combating the spread of coqui frogs.”

For more information about ACS-P, contact Dr. Davis at coquicontrol@gmail.com.

Update on experiment

In February’s column I announced that I would conduct an experiment with young tomato plants to determine whether water heated in a microwave oven has an effect on the plants’ growth and general health. The plants are about four inches tall now and I have started watering two with cooled microwaved water and two with regular catchment water. After three applications of water to date, I have noticed no differences in the two sets of plants. More next month!