Recently, I received two calls about tobacco flea beetle feeding and live beetles in tobacco that had been treated in the greenhouse with systemic insecticides (both were imidacloprid products). 

Reducing early season flea beetle feeding is one of the three main reasons we use systemic insecticides in tobacco (reducing aphidsand feeding by thrips vectorsof Tomato Spotted Wilt Virus (TSWV) are the other two).

The fact that live flea beetles were not only present, but were also actively feeding on plants that had been treated was of concern. Both locations were between four and six weeks after transplant, and the key questions I had were:

1.) Is the insecticide no longer (or never) present in the plant at insecticidal concentrations 
or
2.) are the beetles present not susceptible to imidacloprid?

One of last week’s calls was from the same grower who, in previous years, had expressed concern about difficult to control flea beetles during harvest, an issue I’ve discussed for the last several years.

In late summer 2010, we collected beetles from his field and conducted a leaf dip bioassay comparing field rates and twice the field rates of Assail (acetamiprid), Provado (imidacloprid, Admire Pro as a foliar application is now the recommended imidacloprid treatment in tobacco), Actara (thiamethoxam), and acephate.

We found that all the insecticides killed more flea beetles than died naturally in the untreated control, but that the neonicotinoid (IRACGroup 4A) insecticides (acetamiprid, imidacloprid, and thiamethoxam) acted faster than acephate. These results suggested that this population of flea beetles were susceptible to all the possible foliar applied materials in tobacco but that the Group 4A materials were faster acting. The full report from this bioassay is available here (subscription required).

Last week, I conducted a similar bioassay to narrow down the possible reasons for the flea beetle feeding activity observed in systemically treated fields. I collected live flea beetles in a field in Johnston County, which the agent had previously contacted me about regarding potentially insecticide-related plant stunting issues, and returned them to our lab.