How do I use walnut leaf extract to control leaf miners?

Answer: Walnut leaf extract, principally Juglone, was found to reduce leaf miner damage to less than 3% but requires weekly application to maintain efficacy. In one study, researchers produced a tincture by extracting the phytochemicals from powdered leaves with ethyl alcohol. They used 50 g of material and 250 ml of pure ethanol in a Soxhlet apparatus (Rizvi et al., 2015). This can be done at home with 50% or higher clear alcohol such as vodka. Fill a quart-sized mason jar with dried walnut or black walnut leaves, cover with alcohol, and let sit for eight weeks. After this time, the liquid should be poured through cheesecloth into a dark-colored, glass storage container. The leaves also should be squeezed through cheesecloth into the container. This extract should be mixed with water in a spray bottle and applied weekly to discourage leaf miners from depositing eggs on plants.

As with all botanicals, strength of the solution will depend on many factors so when testing a newly made product start off with a weaker solution, less than 5%, and work incrementally up until the desired results are achieved without causing harm to the treated plant. This will be relative and very case specific due to potency at an individual plant level. This is due to differences in soil, nutrition status of the plant, the weather and UV intensity. Phytotoxity usually appears in the form of curling leaves, drying leaves, or dead leaf edges. This process will also work with almond leaves.

You can find additional solutions in ATTRA’s Tipsheet: Leaf Miners — Botanical Control Formulations.

In addition, check out ATTRA’s Biorationals: Ecological Pest Management Database. This online pest management tool for farmers is a database that highlights reduced risk materials that can be integrated with ecological pest management strategies. The database covers pests including diseases, insects, mollusks, vetebrates, and weeds. It is searchable by pest category, pest name, pesticide trade name, and active ingredient.

Reference:
Rizvi, S.A.H., M.N. Ikhlaq, S. Jaffar, S. Hussain. 2015. Population Dynamics, Efficacy of Botanical Extracts and Synthetic Insecticides for The Control of Pea Leaf Miner (Phytomyza horticola Goureau) (Diptera: Agromyzidae) Under the Climatic Conditions of Baltistan, Pakistan. International Journal of Applied Research. 1(12): 758-761.