Thrips? Whiteflies? Fungus Gnats? How to tell?

Discussion in 'Sick Plants and Problems' started by NtothePtotheK, Mar 17, 2004.

  1. I have an infestation of bugs which I think are either thrips or fungus gnats. Someone else suggested they might also be whiteflies.

    Most of the time they hide in the soil. For every one I squish it seems 10 more pop up. They are very tiny and multiplying very quickly. They are in all my pots now. I need control ASAP...

    [edit]: I deleted the rest of this post because what's below is more interesting. Synopsis: I didn't know how to identify them, or get rid of any of the respective pests I mentioned above. A pocket microscope (mine is 60-100x) and this link were helpful in identifying the fiend.
  2. OK kids. You know what they say, sometimes you just have to take matters into your own hands.

    I was reading a book in front of my plants when one of my little enemies landed on the page I was reading. I quickly snapped it shut and back open again, and I knew right away even with my naked eye that I had done far less damage to this insect than I usually do when I squish them with my fingers.

    About 80x of magnification later, I got a disturbingly good look at this ugly, mosquito-like fella, and a little google research made it VERY clear that my pests are definitely fungus gnats.

    I also found an incredibly useful (looking) database of pests here!

    This particular page allows you to search out a known pest by name, and brings up a list of products specific to controlling that pest.
  3. There are a few generally accepted solutions to fungus gnats (sciarid fly) in the legit gardening world which should work well without interfering with our consumable crop:


    One is a beneficial nematode, Steinernema feltiae or S. feltiae. According to my research, other species of beneficial nematodes will not work as well (or at all). They are active between 60-85 degrees F. The top of the soil should be kept moist to help them move around, but not to the point of overwatering. Both predator and prey live in the first inch or so of soil. This only kills the larvae, so put up sticky traps to take care of the adults. Some commercial products which employ this species of nematode are Nemasys and Entonem.


    Another way to effectively target them is through a bacteria called Bacillis thuringiensis or "Bt". There are three strains of this insect pathogen, the one effective against fungus gnats is Bacillis thuringiensis israelensis a.k.a. "BTI". BTI is ok to use in conjunction with other predators. BTI is also safe to use right up until harvest. Some commercial BTI products are Vectobac, Skeetal, and Mosquito Attack.

    Predator Mites

    If mites are your thing, Hypoaspis miles is the predator you're looking for. Additionally, these mites destroy thrips. The mites can be stored dormant, but above 54 degrees F they feed and reproduce. They live in the top 1/2 inch of soil. "Feeding trials in Denmark indicate that Hypoaspis eats in order of preference: fungus gnat larvae, thrips pupae, spring-tails, nematodes and leafminer pupae." (source) They can also survive on plant matter when prey is lacking.

    Hydroponics notes:

    The mites and bacteria can be applied to rockwool blocks at the base of the plant, and you can use them simultaneously. I have seen no mention of applying the nematodes in a hydroponic environment.

    Soil/Soilless Mix Notes:

    It seems if the predator mites run out of larvae, they will attack your nematodes (good or bad), so recommended combinations are either:

    Mites + Bacteria
    Nematodes + Bacteria


    1\ Most likely you will not have fungus gnats without fungus (especially in Hydro)! Fungus invites them to dinner, moist soil invites them to stick around and breed. They can't breed in standing water and I doubt adults will even lay their eggs in hydroponic medium unless it's already afflicted by fungus.

    2\ In soil (or soilless mix), use a medium with excellent drainage. Make sure the surface of the soil dries out quickly! If you are concerned that your plants will need water too often, water crystals might help counterbalance things. Instead of starting seeds in excessively large pots, transplant a couple times. A 5 gallon pot with a seedling is just asking for trouble, there aren't enough roots to dry out the soil quickly. Allowing your pots to dry as much as possible without harming the plant is a good way to kill existing larvae - but if you water your plants at different times, adults will lay eggs in whichever pot has been most recently watered. I have observed this myself and it is FRUSTRATING.

    3\ "...make your houseplants unattractive to the adults who are searching for a place to lay their eggs. Put about 1/4" of sand on top the soil in each of your houseplants. The fungus gnat will land on the sand and say "Hey, this is a beach, there's no soil here!" and the little guy will keep on looking" (source)

    If you already have a serious infestation, vacuum up the top inch or so of soil (to get rid of most of the larvae and eggs) before adding sand.


    I am not a fan of chemical insecticides, but not all of them are dangerous to consumable crops (when used as directed) so I did some very quick research for those of you who don't mind, or are desperate. This is what I found:

    "Spraying with a pyrethum-based insecticide is a safe way of dealing with the adults, but the larvae in the compost are a more difficult proposition. Insecticides containing permethrin or diazinon, which can be mixed with or watered on the compost, are the most suitable line of attack." (source)

    Good luck and good riddance!

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