Treatment for Prevention of Emerald Ash Borer

A recent inquiry from an Extension and Outreach colleague about timing of preventive treatments for the emerald ash borer (EAB) was insightful. This note is written to help direct types of treatments to the seasons they are most appropriate.

Insecticide product labels state that there are two windows for preventive treatments when applied to the soil or externally to the trunk of the tree: Spring and Fall.  Iowa State University Extension and Outreach publication PM2084 currently includes both windows, in keeping with the product labels.  Trunk injections can be done in a wider window (May through September 1) when the tree has a full crown and there is good soil moisture.

However, with the recent EAB confirmation in Boone (Boone County), ISU E&O’s news release indicated that the only treatment window still open during August was trunk injection.  It appeared that this was conflicting information…

Point #1 – Fall is not the best window for treatment.

Woody trees have completed their growth and primary food storage by late summer. Uptake of insecticides in the late summer and fall is much less than in the spring.

Soil treatments take 4-8 weeks with good soil moisture and active tree uptake to be distributed throughout the tree. From early August, this timetable would extend into September and October when leaves have senesced and begun falling from the tree.

Soil treatments in the fall stay in the soil around the tree base and/or under the dripline. Significant moisture (rain, snow) could move the insecticides off-site during the fall and winter months.

In general, fall treatments require twice the amount of insecticide and have yielded less effective control compared to spring.

Point #2 – Spring is the best window for soil or external trunk treatments. EAB researchers in Michigan and Ohio have reported that spring treatments are most effective because the ash tree is actively growing and uptake is at its peak. In Iowa, ‘spring’ means mid-April to mid-May.  This gives time for the systemic insecticides to be distributed throughout the tree prior to adult emergence in June and July. These insecticides will kill adult beetles and the larvae under the bark once they start feeding on the ash tree.

Point #3 – Some curative benefits from treatments.

Researchers have found that the products used as EAB preventive treatment have curative action as well. Thus, early EAB infestations in a tree (less than 30% of crown showing decline) can be controlled and the tree will be also protected from new attempts to colonize the host.  So, if a person finds out ‘late’ that their tree is infested by EAB and the damage is recent, treatments can give their ash tree important aid in fighting the infestation.

Further information on ash tree treatment is online at  


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