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

The easiest way to get rid of gnats and to prevent further infestation is simple detergent - plain, old-fashioned dish detergent or liquid hand soap. Mix one large drop of detergent into each gallon of fertilizer water each time you fertilize, and water your plants with it normally. Detergent works similarly to insecticidal soap and is 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 plants if done with every watering.

If you grow hibiscus indoors and use our HVH Houseplant Formula for your fertilizer, your hibiscus will stay free of gnats, because detergent is one of the disease-prevention ingredients in the houseplant formula.

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. 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.

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. Allowing pots to dry down so the surface appears dry between waterings is helpful in preventing fungus gnats from colonizing pots. Keeping the growing area clean and weed free and any stored potting mix dry and covered will also help. But by far the best method of gnat control is 1 drop of plain detergent to each gallon of water before watering.