Insecticidal Soap - A Promising Option for Growers?

Mushroom growers across the United States and Australia are all too familiar with the tiny but troublesome sciarid and phorid flies that plague Agaricus bisporus crops. These pests are more than just a nuisance; they can spread destructive fungal diseases such as Trichoderma green mold and dry bubble, wreaking havoc on production. With few chemical options and the ever-present risk of insecticide resistance developing in flies, the hunt is on for softer, sustainable alternatives to use in an integrated pest management strategy.

Key Points

• Mushroom flies (sciarids and phorids) are major pests spreading fungal diseases in mushroom crops.
• Pennsylvania State University has tested a range of insecticidal products available in the USA against phorid flies.
• Agriculture Victoria also tested insecticidal soap made from potassium salts of fatty acids (PSFA) mixed with an oil adjuvant for sciarid fly control.
• The Agriculture Victoria team found that PSFA and oil adjuvant promoted mushroom mycelial growth in compost tests.
• Misting sciarid flies with PSFA and oil adjuvant caused 63% mortality over 3 days.
• Misting phorid flies achieved variable results depending on the product – but a few products achieved over 80% phorid mortality.
• Results highlight potential for misting flies as a low-impact option in an integrated pest management (IPM) program.
• While these products are not yet registered for use as an insecticide on mushroom farms and therefore cannot be misted, fogged, or sprayed to kill insects, they can be used to clean surfaces on the farm.

What is insecticidal soap?

Potassium salts of fatty acids (PSFA) are soaps made by adding potassium hydroxide to fatty acids derived from plants or animals. When the resulting soap is sprayed onto insects and mites it rapidly disrupts their protective cuticle and penetrates underlying cell membranes. The cell contents leak out, causing the insect to dehydrate and die.

PSFAs are not new; the first pesticide product using them was registered back in 1947. The USFDA classifies them as "Generally Recognized as Safe" due to their low toxicity. They are even used in processed food, acting as emulsifiers, stabilizers and thickeners. PSFAs are rapidly broken down by microbes in the environment, so their half-life is less than a day.

However, it is essential to note for mushroom production that there are generally two kinds of PSFAs, which are distinguished by their fatty acid chain structure: One group is insecticidal with longer carbon chain lengths ranging from C14 - C20. The other group is fungicidal, with shorter carbon chain lengths between C6 – C12. Some products may contain both long and shorter carbon chain lengths, providing broad spectrum activity. This is why testing products for both insecticidal activity and mushroom growth compatibility is a highly important step in the research.

Research in Australia and the USA

In a wonderful example of international synchronicity, both the AgVic team, lead by Dr Aimee McKinnon and the Penn State Mushroom Fly Research Team, lead by Dr Michael Wolfin, have been investigating whether insecticidal PSFA products intended for insect control can suppress mushroom fly populations.

Compatibility with mushroom growth

To check that the PSFA product available in Australia was not likely to have negative impacts on mushroom growth, initial trials were run by the AgVic team to test its effects on compost colonized with A. bisporus. The PSFA solution was applied to compost at a 3% concentration and mixed with a 2% emulsifiable canola oil, based on the label recommendations of the product. Mycelial growth was then monitored over four weeks and compared with untreated controls (Figure 1).

The results were surprising. Rather than hindering the mushroom mycelium, the PSFA treatment actually promoted mycelial growth (Figure 2). "We didn't expect to see such a clear growth enhancement," said Dr Aimee McKinnon. "It suggests that insecticidal fatty acid soaps – in combination with an oil adjuvant - may have a secondary benefit for mushroom cultivation that warrants further testing."

Misting the soaps– AgVic and Penn State

Both teams tested the insecticidal effects of misting PSFAs against their key pest fly species; sciarids in Victoria, and phorids in Penn State.

The Agriculture Victoria researchers collected live sciarid flies from commercial mushroom farms to rear them. The flies were then exposed to a fine mist of the PSFA formulation. Using hand-held atomizers, the team sprayed adult flies at a low rate of 3% PSFA with the 2% canola oil adjuvant, while control groups received only water or water mixed with the oil adjuvant.

At this point the researchers realized that simply designing the experiment was a key challenge. The flies needed to be treated independently and realistically. "Our initial trial resulted in an average of 90% of sciarid flies dying immediately, regardless of whether the PSFA soap was included" explained Dr McKinnon, "The few remaining flies succumbed within five days. This indicated that the misting process itself can be lethal to adult flies, perhaps bywetting their wings and blocking their breathing pores."

To understand and compare spray insecticides, the mist application had to be adjusted to avoid drenching the flies excessively. It also needed to limit spray drift between containers, to prevent cross contamination. The research teams at Agriculture Victoria and Penn State University independently came up with innovative approaches to applying sprays that were surprisingly similar in design (Figure 3).

"Once we understood the best way to deliver the mist, we found it easy to test for insecticidal effects. We could apply the mist and release the surviving flies into cages for daily monitoring" – explained Aimee. "We found that the 3% potassium salts of fatty acids (PSFA) spray killed flies more rapidly than water or the adjuvant only, but both treatments killed up to 63% of sciarid flies within three days after the exposure event" (Figure 4).

The Penn State Mushroom Fly Research Team tested five different PSFA products against mushroom phorid flies using a similar experimental method (Figure 3). One or two concentrations were tested for each product. A single assessment of mortality was conducted 24 hours post exposure (Figure 5). Note that the effects of these products on Agaricus mycelium was not measured.

PSFA 1 was not effective at killing mushroom phorids, even at relatively high concentrations (6 % and 12 %), and PSFA 2 was only marginally effective at killing mushroom phorids. However, PSFAs 3, 4 and 5 were very effective, achieving over 80% average mortality across all treatments.

A win-win for sustainable pest control

The dual effects observed in the AgVic trials – growth promotion and fly mortality – are encouraging for an industry seeking sustainable pest management solutions. While 63% might not seem very effective for sciarid flies, multiple applications or a higher concentration could improve efficacy. It would certainly be worth testing.

The good news is, there are PSFA and oil adjuvant products that are readily available. Growers could one day soon have access to simple, low-impact treatment options that support mushroom growth while knocking down fly numbers. These products are not yet listed as insecticides and therefore cannot be applied on farms to kill flies.These soaps can only be used on farms to clean surfaces.

However, researchers caution that more work is needed before on-farm applications can be recommended. Both teams have similar ideas; "Our next step would be to test whether the action of misting is comparable to a fogging machine. It would also be good to explore when applications can be used for best effect."

These products were effective at killing both sciarid flies and mushroom phorid flies. Because mushroom farms usually have one dominant species of fly, these products could be useful to farms regardless of the species of fly infestation.

Looking ahead…

While still preliminary, these findings point toward a practical addition of a fogging treatment for integrated pest management (IPM) programs (Figure 6). If further studies confirm the efficacy of PSFA or the oil adjuvant in the mushroom grow room environment, insecticidal soap could become a valuable tool in the grower’s pest control kit, one that's complementary for mushroom production yet still effective for flies.