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Are these Lesser Mealworm Beetles with my yellow mealworm larvae?

edited June 2016 in Mealworm Kit Support

Hi, Thanks in advance for assistance.

I've been trying to figure out what the little black beetles in my nursery bins are. I think they are lesser mealworm beetles, but I am not sure. If they are, is this a positive symbiotic relationship that can be left alone, or should I worry? I read that they more or less keep the environment tidy, and I don't see them showing up in my breeding bins - at least, not yet. I'm very new at this.

I hope the link to the video works. I've not been able to get a still shot with them in it. They move pretty fast. with tiny Beetles 6.15.16.mp4?dl=0

I apologize if I've put this in the wrong category, or if this has been answered elsewhere and I just haven't found it (I did look). I'm not working with a mealworm kit primarily because I never heard of it before a few weeks ago. I've been mealworm ranching (grin) since March 24 and started out with 2,000 mealworms that I think have at least quadrupled (or more!) in number, but I have no idea how to count all those lovely, squirming tiny bodies (yet). Very exciting!!! I think they are doing well, overall, though I need to better regulate humidity and temperature, and come up with a good system for removing dead beetles. I think most are just dieing of old age, no disease. They don't look deformed, though there are some that die before they turn black.

I appreciate all advice and information!


  • There are over 10,000 kinds of Tenebriod beetles; nicknamed "darkling" beetles since prefer moving around in the dark ( rather than for dark color). If you got your yellow mealworm stock from another colony & supplier's set-up had lesser mealworms also in with them then that would explain some emerging.

    Get one to measure & see if it is close to 6 mm long (~1/4 inch), give or take 0.2 mm, which is average lesser mealworm beetle length. They are known to move pretty fast. If have a magnifying glass or microscope & see yellow colored "hairs" profusely covering the antennae they are probably lesser mealworms.

    When associated with poultry they can be pathogen carriers, but since likely from your supplier's colony are clean of pathogens. If seller offers various insects for reptile owners to breed their own reptile feed, or sold stock was breeding to feed his own reptiles, the lesser mealworms could have got into the yellow mealworms then.

    The 2 varieties can live together competing for the same food & the lesser beetle is a dead insect eater. I would try to get rid of most of the adults & by dropping the temperature to 70 Fahrenheit makes their eggs less likely to hatch. Cull any larvae that have their more distinct segment blaze (search for pic of them as larvae) which eventually could show up to shut down bit by bit their capability to reproduce. You can raise back up temperature keep yellow mealworms in time after put a dent in the lesser mealworms, but if you currently have bins at the theoretical "ideal" temperature for yellow mealworms of 78 Fahrenheit be aware at 80 Fahrenheit the lesser mealworm bugs go through their life cycles pretty fast.

  • Thank you, very much, gringojay, for this information (how to I tag you?). I don't know how I'm going to get them down to 70 F. Will have to think on it. My budget is much too limited to lower the air conditioning beyond 78-80 F. I may have to pick out adults as much as possible, find larvae (tricky with my eyesight, I think), and wait until winter. I bought them from Bassett's Cricket Ranch via Amazon. There were adults in the package. :(

  • In pondering this for a couple of hours, I'm thinking that I could just keep removing beetles to breeding bins with new sterilized substrate and not worry too much about the other beetles. This is what I've been doing, so that's why I'm not seeing them in the breeder bins, and shouldn't in subsequent nurseries, correct? I thought to allow 9 months - 1 year to learn and build my population while creating a good system with proper climate control (which I don't have now), and feeding some for my chickens.

    The beetles fit the size parameters, but I don't have a glass strong enough to see the color of the hairs on the antenna, so will have to acquire one.

    Thank you again for your response(s). I appreciate this resource greatly.

  • @4paws - tag by putting "@" in front of the user name (i believe case sensitive)

  • edited July 2016

    You stated you got your mealworms from Bassett's, they have an infestation of lesser mealworms. I bought 1000 mealworms from them to replenish my breeding stock. I dumped the bag in with about 60,000 mealworms. As soon as I noticed the little beetles I knew I had just infested my colony. It killed me...but I destroyed the whole bin with hot water. I told Bassett's they had an infestation and they called me a "liar" and did not "trust me". I sent them back a sample of what they sent me. Please, please, email Bassett's about your plight...I want them to know other people have had the same problem.

  • @Phyrnosoma, I will do that and post it in my review on Amazon. Why did you destroy your colony? How will they hurt? I've been picking out adult beetles and putting them in their own bin, but does this mean that the quality of my worms is reduced? It's been several months now and was hoping to start selling some soon. Thanks for the reply.

  • @Phyrnosoma, turns out, I'd already posted my review about the other beetles, with a video.

  • edited July 2016

    @4paws thanks for letting Bassetts know about lesser mealworm issue. I also bought mine off of amazon and posted a negative review. Main reason I killed off my infested mealworm bin was because it would have been nearly impossible to get rid of the infestation. I sell a bunch of my mealworms and did not want to perpetuate the problem. They reproduce much quicker than regular mealworms and can out compete them. Lesser mealworms can also turn into a home pest. However, I do have a lesser mealworm colony in with my orange head and dubia roach bins. I put them in before I bought Bassett's infested mealworms. They work good at breaking down the roach waste and dead roaches. I have to thin down the lesser mealworms frequently though. Its fairly easy to do with a food trap. I put food in a small container so they can crawl into it, they canot climb back out. I appriciate the function they serve. Just not in my mealworm bins. I have some white head roaches coming soon and am unsure if I am going to use lesser mealworms with them yet.

  • @4paws, I wonder if putting the lesser mealworm infested mealworms in the fridge for a few days would kill them. But I have put my eggflats in the freezer for several weeks to kill lesser mealworm eggs...and it did not kill them.

  • Is this a problem for all sellers? I got 10K from someone else last Wednesday and today spotted a lesser beetle in them. I didn't combine them.

    It seems if I had pick pupae and beetles, I should eliminate the lesser mealworms in future breeding bins, yes? It's my intention to sell as well.

    The lesser worms seem to be able to crawl better than the yellow. Should I be worried about them escaping their bin with no lid?

    @gringojay had said to chill them to 70 slow their reproduction/eliminate them, but currently in FL it's hard to keep the room at 84 degrees F. I pick out every adult I can find, often catching them still brown (no way I can cull the larvae among thousands). Pupae are put in separate hatching bins, then beetles transferred to breeding bins. Over time, that should be then end of them in the yellow mealworm habitats, eh?

    Thanks for the information.

  • @Phyrnosoma, why not in your meal worm bin? What do you sell them for?

  • @4paws I sell them for reptile food, but many people also want to breed them. So I refuse to sell mealworms that have a lesser mealworm infestation. I haven't ever received infested mealworms from any other company. Bassett's is the only one that I have had a problem with. But I rarely ever buy them to replenish breeding stock. You could spend the time to remove all the pupae since they are easy to identify. But I wasn't going to spend the time doing it. I have too many other things to do in my day. I keep and breed a lot of reptiles which takes up much of my time. If your diligent at removing the pupae you may be able to weed out the infestation. Lesser mealworms are more mobile than regular mealworms, and its unlikely they will climb up walls of a smooth bin. I have heard the beetles fly, but I have never seen it. What I was more worried about was sifting the worms out. The eggs are nearly microscopic and I figure they float really easily. Last thing I wanted to do is have a chance that I could infest my garage with them. I haven't tried refrigerating the lesser mealworms to see if they will die. But I did freeze eggflats that were in with lesser mealworms for 3-4 weeks. Lesser mealworms eventually hatched into a cricket bin that I placed the flats in.

  • :( I do not have the time to pick out all, either. I've set up a new breeding bin and hope that will solve the problem with all new substrate and hand placed beetles. If I sell any of the worms, I'll be sure to tell customers that they are NOT for breeding because of the potential, unless they are good with that. Otherwise, I'll be feeding all to my chickens.

    I bought another 10K from a different source and didn't combine them, so those should be fine. I hope.

  • The chickens will like both types of mealworms. So at least they won't be going to total waste. I haven't contaminated my mealworm colony from the roach bin lesser mealworms. But I only have to clean out the roach bin about every year, and I do it in my backyard over 100 yards from my house. I also remove my clothing inside the garage, and shower up as soon as I finish. I am pretty sure stirring up the bins while cleaning them probably gets lesser mealworm eggs stuck all over me. I also wear good eye protection and a respirator. I'm one of the many that have a roach allergy. Most do not bother me too bad...but hissing roaches are another story. I had to quit breeding them because it was causing asthma attacks and abdominal pain and bloating. Took me a year of being ill to finally figure out what was causing it. If your careful you can reduce your chances of cross contamination of both your mealworm bins. But lesser mealworm eggs can hitch rides on your beetles from your contaminated bins. So don't mix any dirty beetles with your clean bins.

  • @Phyrnosoma, thanks very much for the heads up. I wouldn't have thought of cross contaminating with the beetles. Is this true even if the pupae had been removed and hatched elsewhere? Suppose so...

  • I suppose you could rinse the mealworm pupae off and put them in a clean bin. Can't say for sure if it would work, but it seems reasonable. Sorry your going through this disaster...

  • It's discouraging, but better sooner than later. Only have four months into it. :(

  • @Pyrnosoma, is it ok to refrigerate pupae? That would kill any Lesser Mealworm eggs stuck to them, wouldn't it? Would they still become beetles when warmed?

  • @4paws... Not sure, I haven't tried it. Sounds like a good experiment :)

  • Just thought I'd update. I had segregated all the Lesser beetles I could find, thinking I'd raise them in the summer heat as an alternative to the Yellow Mealworms, but after reading how potentially destructive they could be and that they're inclined to fly, decided to more or less start over and bought new stock from two other vendors.

    I did rinse some worms and kept them separated from the rest of my set up for nearly a year. No further sightings of Lesser Mealworms since.

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