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.