Hi Ninnah,
This is a great question but it has not been thoroughly researched. Considering the length of time the pesticide must pass into the tree, into the leaves, then survive the decomposing process of the leaf litter and finally make its way into the vegetable garden and the fact that the contaminated leaves have been diluted (by other tree species) and that the pesticide breaks down rapidly by microbial activity and sunlight I would not be concerned about using the leaves as a garden mulch.

Emamectin benzoate is broken down rapidly by microbial activity and sunlight. Because of its short residual activity on the surface of leaves, it is considered a biorational insecticide compatible with integrated pest management programs, including biological control. These characteristics suggest that environmental impacts will be negligible as emamectin benzoate is released from decomposing leaves. Regulatory agencies concluded that foliar applications of emamectin benzoate to vegetable crops will have no adverse effects on ground or surface water, birds, mammals, fish, or aquatic invertebrates when used as directed. Here is a link to this study: http://www.emeraldashborer.info/documents/Potential_Side_Effects_of_EAB_Insecticides_FAQ.pdf

I consulted with the National Pesticide Information Center, the Colorado Department of Agriculture web site, horticulture agents and certified arborists who use the EAB treatment products regarding the safety of using those composted leaves in your vegetable garden and they said it should be ok depending on how long the composting process takes. They did say to not eat fruit that grows directly off a treated tree, but since these leaves are turned into compost it should be safe to use.

This is a link to the brand name label which is Mectinite. The product label says: Do not apply to trees that may yield food consumed by humans or used in animal feed. https://rainbowtree.widen.net/s/ctcfn9rzpc/mectinite_specimen_label

The product is registered with the Environmental Protection Agency: https://www3.epa.gov/pesticides/chem_search/ppls/<personal data hidden>7-20190624.pdf

The product is OMRI Listed. It may be used in certified organic production or food processing and handling according to the USDA National Organic Program regulations. https://www.omri.org/mfg/aos/certificate/13906

I’m assuming that your compost takes several months till ready to use in which case water, sunlight and microbial activity can significantly break down any residual pesticide. For an extra precaution you can mix the ash tree leaves with other leaves.

If you have any additional concerns or questions, please contact Ask Extension again. It’s great to hear that you are using your leaves because they are such a valuable resource that many throw away.

Jeffco Master Gardener Clinician Nancy