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 
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.

Four treatments compared

I then set up four treatments:

1.) Untreated control leaves from field grown tobacco plants that were never treated with systemic or foliar insecticides,
 2.) leaves from plants which were treated with 1.2 fl oz Admire Pro/1000 plants 6 weeks ago and had no flea beetle damage following transplant,
 3.) leaves from plants never treated with systemic or foliar insecticides, dipped in a pesticide solution equivalent to 0.7 fl oz Admire Pro/acre (lowest labeled foliar application rate) in 30 gpa water, and
 4.) leaves from plants never treated with systemic or foliar insecticides, dipped in a pesticide solution equivalent to 1.4 fl oz Admire Pro/acre(highest labeled foliar application rate) in 30 gpa water.

The results of this assay were interesting. The greenhouse treated tobacco leaves no longer contained sufficient insecticide to kill the flea beetles when compared to the untreated control leaves.

Given that this assay was conducted six weeks after the plants were treated, this is not entirely surprising. We have a parallel assay with green peach aphids using leaves from these same plants which will be repeated for several more weeks. However, both concentrations of Admire Pro applied as leaf dips (a simulated foliar application) killed nearly all the flea beetle adults and decreased their feeding activity.

What do these results mean in the context of field infestations of flea beetles on imiacloprid treated plants?

It does not appear that the flea beetles collected at the Johnston County site are resistant to imidacloprid as evidenced by the fact that they were quickly killed by the leaf dip treatments at labeled rates of Admire Pro.

However, it also appears that leaves collected from our research plots do not contain sufficient imidacloprid to kill flea beetles six weeks after treatment. This suggests that plants which were damaged by flea beetles despite being treated in the greenhouse also did not contain sufficient insecticide.

Non-uniform greenhouse insecticide application can lead to both too high and too low insecticide application rates, which may result in flea beetle damage concentrated on plant which do not contain sufficient insecticide to kill them. Fortunately, early season flea beetle injury may be unattractive but only impacts yield in very severe cases.

This has been a strange spring for recently transplanted tobacco, and flea beetle injury is yet another unusual observation.