8.3 Chemical Applications

Pesticides derived from biological or microbial sources are also effective and widely available. Biopesticides are derived from natural materials such as animals, plants, bacterial, and certain minerals. Common biopesticides include biofungicides (Trichoderma), bioherbicides (Phytopthora), and bioinsecticides (Bacillus thuringiensis, B. sphaericus). B. thuringiensis (Bt) has become an increasingly common mechanism to target specific vegetable pests. Bt consists of a spore that contains a toxic protein crystal.

Certain insects that consume the bacteria release toxic crystals into their gut, blocking the system, which protects the pest’s stomach from its own digestive juices. The stomach is penetrated, causing insect death by poisoning from stomach content and spores themselves. This same mechanism is what makes Bt harmless to birds, fish and mammals, whose acidic gut conditions negate the bacteria’s effect.

Microbial pesticides come from naturally occurring or genetically altered bacteria, fungi, algae, viruses or protozoans. These compounds can take different modes of action, including release of toxic compounds, disruption of cellular function, and physical effect. Beauvaria bassiana, for example, is a fungus that gets under the chitin (shell) of hard-bodied insects, resulting in dehydration and death.

Chemical pest controls used for aquaponic farms include neem oil and extracts, soaps, pyrethrum-based products, and anything that is OMRI approved. These chemicals should be used in moderation and label instructions should be followed to avoid any plant or fish damage. Before any chemical is applied to the aquaponic system, the impact on the fish and biofilter must be considered. Limiting contact between the chemical and water is critical and may be more difficult in deep-water culture and media-based systems. The following is an example on how to calculate if a pesticide is safe to apply to the aquaponic system (Storey 2016).

Note: Refer to the Safety Data Sheet (SDS) and find the LC50 value or the lethal concentration of a pesticide at which 50% of the tested population dies. Rainbow trout or tilapia are often reported. The lowest concentration over the shortest time should be used.

Example 1: Pyrethrum – the active ingredient in Pyganic 1.4

Step 1: Determine the LC50 value from the chemical’s SDS sheet – 0.0014 mg/L

Step 2: Determine the LC50 value for your system. Take the volume of your system in liters and multiply it by the LC50 (96 hr) value. Let’s use a 2,000-gallon (7,580 L) system as an example.

$7,580\ \text{L/sys. X }0.0014\text{ mg/L }= 10.61\text{ mg/system}$

Step 3: Take the pyrethrin concentration and determine how much pyrethrin is being mixed.

The label recommends mixing 1–2 fluid ounces of Pyganic 1.4 with every gallon of water in compressed sprayers, which is between 2–4 Tbsp/gallon. In a 2,000 gallon system, the entire crop can be sprayed with 0.75 gallons of mix, which at the highest application rate is around 3 Tbsp (or 1.5 fluid ounces).

The label tells us that 0.05 lbs of active ingredient (pyrethrin) is the equivalent of 59 fluid ounces.

0.05 lbs pyrethrin/59 fluid ounces = 0.0008475 lbs pyrethrin/fluid ounce

0.0008475 lbs pyrethrin/fluid ounce X 453,592 mg/lb = 384 mg pyrethrin/fluid ounce

Step 4: Determine how much pyrethrin is being applied to the system.

1.5 fluid ounces/system X 384 mg pyrethrin/fluid ounce = **576 mg pyrethrin/system **

Step 5: Compare application concentration to LC50 of your system. 576 mg pyrethrin/system is much larger than the LC50 value for a 2,000-gallon system (10.61 mg/ system from step 2). This means that this product is NOT a good choice for application.

Example 2: Azadirachtin – active ingredient in AzaMax Biological Insecticide, Miticide, and Nematicide

Step 1: Determine the LC50 value from the chemical’s SDS sheet – 4 mg/L (96 hours) for rainbow trout.

Step 2: Determine the LC50 value for your system. Take the volume of your system in liters and multiply it by the LC50 (96 hr) value. Let’s use a 2,000-gallon (7,580 L) system as an example.

$7,580\text{ L/sys. X }4\text{ mg/L mg/L} = 30,320\text{ mg/system}$

Step 3: Take the pyrethrin concentration and determine how much pyrethrin is being mixed. The label recommends mixing 1–2 fluid ounces of AzaMax with every gallon of water in compressed sprayers, which is between 2–4 Tbsp/gallon. In a 2,000 gallon system, the entire crop can be sprayed with 0.75 gallons of mix, which at the highest application rate is around 3 Tbsp (or 1.5 fluid ounces).

The label tells us that the product contains 0.35 g of azadirachtin per fluid oz. Convert g to lb:

0.35 g azadirachtin/ounce ÷ 454 g/lb = 0.0007716 lbs pyrethrin/fluid ounce 0.0007716 lbs pyrethrin/fluid ounce X 453,592 mg/lb = 350 mg pyrethrin/fluid ounce

Step 4: Determine how much pyrethrin is being applied to the system.

1.5 fluid ounces/system X 350 mg pyrethrin/fluid ounce = 525 mg pyrethrin/system

Step 5: Compare application concentration to LC50 of your system.

525 mg pyrethrin/system is much smaller than the LC50 value for a 2,000-gallon system 30,320 mg/ system from step 2). This means that this product is SAFE to use in your aquaponic system. Even if a product is generally safe, limiting exposure to the water and organisms is still critical.

Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.