Hidden Valley Hibiscus
Growers & Hybridizers of Exotic, Tropical Hibiscus
Plant Care
Fungus Gnats & Shoreflies
There are little bugs flying around my indoor hibiscus!
If you have ever grown plants indoors, or in a cooler climate, you have almost certainly seen tiny flying insects buzzing around your potted plants. You may have noticed tiny black insects landing on the leaves or on the top of the potting mix. Even though they don't seem to be doing anything terrible, most of us think of them as a nuisance we would rather not have on our potted plants. What are they, really? Are they dangerous to the plants?
What Are They?
These insects are almost always either fungus gnats or shoreflies. Fungus gnats can be quite harmful to plants; shoreflies are usually not destructive. Fungus gnats look like small mosquitoes. They have long legs and antennae, and the way they fly is very similar to mosquitoes. Shoreflies look like small house flies, are stockier than fungus gnats, and have shorter legs and wings. They fly more like a housefly than a mosquito. We have both species here along the California coastal region.
What Harm Do They Do to Plants?
Fungus gnats lay their eggs in wet potting mix. When the eggs hatch, the larvae seek food nearby. They consume any fungus they find, then continue to feed on the roots of the plant growing in the potting mix. This makes them doubly dangerous, because as they feed on the roots they also can transfer fungus to the damaged roots where it can gain easy entrance into the plant. As we all know, fungus diseases are the biggest threat to hibiscus outside of freezing temperatures.
Shoreflies do not feed directly on plants. They prefer algae, which is often found on potted plants or the surface of the mix the plants are growing in. The presence of shoreflies can be considered a nuisance, but they are far less likely to damage your plants than fungus gnats.
How to Control a Fungus Gnat Infestation
Once fungus gnats are identified, treatment should start as soon as possible. There are several possible treatments you can use:
The standard treatment in commercial greenhouses is to target the larvae rather than the adults in order to control an infestation of fungus gnats. The treatment involves drenching the pots with a very specialized bacteria, Bacillus thuringiensis var. israelensis that is usually used to control mosquitos. This product, Mosquito Bits kills the larvae of both mosquitos and fungus gnats very effectively. The fungus gnat larvae in the pot will eat the bacteria, which in turn make a toxic substance in the larvae's stomach that cause it to quit feeding and soon die. These bacteria are deadly only to fungus gnat larvae and only last for two days in the pot. For complete control of the gnats plan on making 3 treatments in a week, skipping one day between each treatment.
It is important not only to control the fungus gnats in the potted plant mix, but also to correct conditions in the area that made it attractive to fungus gnats in the first place. After the final treatment let the pots dry out more. Constantly wet potting mix is what attracts fungus gnats because wet soil is necessary for the larva to survive. Pots that are thoroughly watered, and then allowed to dry down until the surface is dry rarely have any problems with fungus gnats.
Take a look around the growing area. It needs to be weed free and any spilled or stored potting mix should be kept dry and covered.
If there are a lot of adult fungus gnats present, they can be gotten rid of with standard treatments of pesticides. Bayer Advanced 3-in-1 is an excellent choice as a spray. Clean Leaf spray or horticultural oil can also help control adults. With fungus gnats it is not sufficient to treat only for adults as the larva will soon turn into adult gnats and lay new eggs. By treating for both larva and adults the infestation can be quickly halted.
How to Control a Shorefly Infestation
Shoreflies are treated like adult fungus gnats. Bayer Advanced 3-in-1 will kill them. By removing any algae that is growing on the surface of the potting mix their food source is removed and a new infestation is less likely to occur.
Hibiscus develop fungus diseases without warning sometimes, and we always wonder why it was that plant and not another one that was attacked. It is hard to know, but I suspect that fungus gnat larvae are often involved. Be aware of these little pests and have some Mosquito Bits available to treat the pots.
How To Prevent an Infestation from Starting
It is always better to prevent any problem with hibiscus from occurring than to treat an established problem. As noted above, allowing pots to dry down so the surface appears dry between waterings is an excellent way to prevent fungus gnats from colonizing pots. Keeping the growing area clean and weed free and any stored potting mix dry and covered will also help. Cindy swears by a home remedy both to prevent and eliminate fungus gnats in our houseplants. She adds 1 drop of plain detergent to each gallon of water before watering. Detergent works similarly to insecticidal soap and appears to be quite deadly to fungus gnat larvae. The "wetting action" of detergent also helps to maintain the ability of the potting soil to absorb water and nutrients. We have seen no toxic effects from using detergent this way over many years as long as the small dose is maintained. Just because 1 drop is good does not mean that 10 drops are better. Please, only use the one drop if you try this technique. It is most effective at maintaining gnat free pots if done with every watering. We include detergent in our HVH Houseplant Formula.
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