On 24 August 2018, I treated two of the mealworm jobs at night. The numbers of larvae were much reduced, but the number of adults had increased significantly.
Daytime wide-area spray applications do not appear to be useful, as there appears to be little effect, on the larvae.
Baiting with Niban also did not seem to have appreciable impact on their numbers, either.
Nighttime wide-area spray applications do reduce numbers. But even then, the results are not as good as I’d have expected. In the tanbark areas, there were still plenty of mealworms, and there were still mealworms crawling around in areas that I’d night-sprayed, just two nights earlier (and had also been day-sprayed).
28 August 2018. In following up on some of the spray applications against the Eurasian mealworms, it is becoming clear that these insects have a resistance to pyrethroid insecticides. We have treated one location five times, with rates varying from 1-3 quarts per 100 gallons of Bifenthrin, and have used Tempo as well, with both daytime and nighttime applications, and the results are not impressive. The applications have greatly reduced the numbers, but it is clear that the insects have resistance. They must be under pesticide pressures in their native areas. This pyrethroid resistance was a surprise to me.