We received an email about what appears to be an outbreak of Eurasian mealworms in Elverta, in Sacramento County, California.

Cindy, the homeowner, sent photos, posted below, and said “For the last two summers now we have them falling from any ceiling opening into our house! They drop down from the attic from any light fixture and sprinkler heads. They are also all around the exterior of our house.”  Cindy also observed: “They do seem to be worse on the back patio where we have a porch light so I do think they are attracted to light. When they land on our floors they find something to hide under. So they will gather under our rugs and floor mats.”  At this Elverta location, the home is a manufactured house, and there is no accessible attic.

Her observations  match what we have heard from Sheri and Lori, in the Sonora area, that the larvae appear to be attracted to lights, and that the larvae hide under things like rugs and mats. At Sheri’s house, in Tuolumne County, you could find the larvae under artificial turf, outside, also.

The mealworms were dropping from the ceiling, through the holes for light fixtures. The homeowners placed baggies under the lights and collected a great many mealworms.

Mealworms collected in a baggie, placed below a light fixture in the house. Many mealworns are falling through the hole.

Eurasian mealworms hiding under a carpet, in the house.

Adults beetles. These were just outside the back door.

Eurasian mealworms, larvae and adults, under a door mat by back door.

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.

Lori Dunlap recorded these videos showing large numbers of Eurasian mealworms near her home. 29 and 30 August 2018, Sonora area.  We see the larvae far from the house, near the house, taking refuge in joints in the concrete, in the garage, in the house, and in the tanbark.

And thanks to Warren Steiner… he had a better name for the video.

Lori Dunlap brought in samples of pupae, and possibly eggs, of the Eurasian mealworm. Here are images of the pupae. Specimens collected 17 August 2018, Sonora area.

One of the pupae managed to make it to adult. Images 15-17, below.

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In our first call about Eurasian mealworms, in 2015, the customer said these insects were falling from the ceiling. That was unusual for mealworms, and then, before we knew what species they were, and more about their biology (which is still not properly known), we assumed they had originated in the attic, breeding in a food source there. We could find no such source, and now know that these mealworms readily climb.

In this video, at 6:57, we document the Eurasian mealworms climbing fence posts. And here we provide some video showing how they have no problem climbing rocks.

It is clear now that the attic invasion calls (we have had more, since) are the result of the Eurasian mealworms climbing walls, all the way up into attics, and then down into the living areas of homes. This is indeed an unusual mealworm.

We tested, earlier, two insect baits, Maxforce Quantum Ant Bait, and Maxforce Complete Granular Insect Bait to see if the Eurasian mealworms would eat them. See the food preference videos located here.  The mealworms showed no interest in Quantum Ant Bait, but readily accepted the Maxforce Complete Granular Insect Bait.

We wanted to test more granular insect baits, and contacted John Woodward, of Geotech Supply, in Sacramento, who graciously sent us samples of two more baits, Advance 375A Granular Ant Bait, and Advion Insect Granules. In addition, we had Niban Granular Bait in stock, and I took all three baits to the Springfield Road jobsite for testing, on 13 August 2018, at night.

As a control, we used rolled oats, a food we had previously found the Eurasian mealworms readily accepted.  We set up six side-by-side choice tests, with the three baits and rolled oats, and videoed the responses over about an hour.

Overall, it seemed that the Eurasian mealworms accepted Advance 375 best, followed closely by Niban, with Advion the least popular bait.  We’d previously shown that Maxforce Compete was well accepted.

We could state something like this: (Maxforce Complete=Advance 375=Niban) > Advion. That is, it appears any of the first three would give good results, and then, if needed, one of the other baits could be used later, if needed.

These tests were only to examine how well the baits were accepted by the Eurasian mealworms, not the overall degree of control. Hopefully, we will run those tests soon.

Thank you to Dr. Doug and Sheri Fleming for allowing us to run these tests at their home, and to John Woodward of Geotech Supply for the donation of the insect baits.