Soil Pests: Grubs, Gnats

Background

Roughly ninety percent of all insects spend at least some portion of their lives in the soil — good bugs, bad bugs, and everything in between. There are three categories of soil-dwelling insects that matter to growers: good bugs (which we’ll set aside here); bad bugs that damage plant roots, bulbs, organic matter and tubers; and bad bugs that don’t damage roots themselves but simply use the soil as a development chamber before emerging as above-ground pests.

A Variety of Soil Pests

Regardless of category, soil pests are members of several insect orders: Coleoptera (beetles), Lepidoptera (moths and butterflies), Diptera (flies), and others. We’re talking about thousands of species — which all adds up to one important fact: the soil is a great place to practice pest control, especially when dealing with pests like soil mites, fungus gnats and grubs.

Time of year is one of the most important factors when planning soil-pest controls. Outdoor pests are seasonal, while protected-culture pests can be present year-round. Match your scouting and your interventions to the lifecycle stage that’s actually in the soil right now.

Leafminer Pupae

As covered on the leafminers page, these pests — from the orders Lepidoptera and Diptera — can be controlled in the soil as well as on the leaf. When Dipteran leafminer larvae finish tunneling through your leaves, they drop to the soil to pupate. Timing isn’t critical in greenhouses, where pupae are present year-round, but is more seasonal outdoors. If you see new mines forming, deploy leafminer parasitoids immediately, then follow with beneficial nematodes against the soil-stage pupae.

Cutworms

Cutworms (covered also on the caterpillars page) are extremely vulnerable in the soil — which is good, because that’s where they spend their dangerous hours. They stay in the root zone during daylight and emerge at night to wrap around plant stems and chew them off at soil level. They’re mostly a garden and field pest rather than a greenhouse one.

Beneficial nematodes work well against cutworm larvae in moist soil. Diatomaceous earth rings around the stem of valuable seedlings — or simple physical collars — are also effective.

Beetle Grubs

Japanese beetles (Coleoptera) defoliate many ornamentals — bush beans, roses and flowering crab trees among them — and their larvae are equally damaging, killing patches of lawn from below. Other key grub groups include chafers, June beetles and root weevils. They’re all uniquely vulnerable to parasitic (entomopathogenic) nematodes applied when the soil is moist and temperatures are in the active range. For especially severe Japanese beetle pressure, also consider milky spore (Paenibacillus popilliae) as a long-term soil treatment.

Fungus Gnats and Flies

The most important soil pest in protected culture is the fungus gnat — or “mushroom fly” in that industry. They’re especially problematic where peat-based, manure-rich or compost-laden media are in use. The pestiferous genera are Lycoriella, Sciara and Bradysia spp. — all Diptera.

Fungus gnat larvae feed on root hairs, slowing plant development and providing a wound channel for Pythium, Fusarium and other root pathogens. Damping-off losses on young plants are often blamed on disease alone, when fungus gnat larvae are the actual cause. Adults emerge to mate and lay more eggs — a complete metamorphosis runs in days under warm conditions.

And More

Other pests that are easier to control underground than above ground include: black vine, carrot and strawberry-root weevils; sod webworms, wireworms and potato-tuber worms; apple, carrot-rust-fly, onion and cabbage maggots; and flea, June/May and cucumber-beetle larvae. There are far more species than we can list here. Even fleas can be dealt with this way, along with filth-breeding flies, and many others. Soil-applied beneficial nematodes are the single highest-leverage intervention for most of them.

Serious Pests

Fungus gnats are easy to detect with yellow sticky traps and potato disks, and their control is straightforward — but they become a serious problem if left unchecked. Fungus gnats can affect any plant, large or small. The smaller or younger the plant, the more quickly it falls. Fine-milled sphagnum mixes used for sowing tend to be cleaner of viable eggs, so seedlings are usually safe until re-potting into coarser mix.

Eggs hatch, larvae feed and pupate in the media, and the adults emerge to mate and repeat the cycle. Rotting debris and algae from over-fertilization create perfect conditions for them — if you fix those, you reduce your fungus gnat pressure significantly. Algae blooms from nitrogen-rich liquid fertilizer aren’t easy to avoid unless you change your fertilization method.

Some Advice

Shore flies are larger and fuller-bodied than fungus gnats. They’re attracted to algae and are often confused with fungus gnats, but their larvae are strictly algae-eaters — so they’re effectively harmless. In retail environments, however, customer perception is a real issue, so the right move is to cure the algae problem and any standing water (and hang plenty of yellow traps for the visual reassurance). Humpbacked flies, moth flies and drain flies are similarly nuisance-grade but not directly damaging.

For the Scout

Fungus gnats are four to five millimeters long, delicate and mosquito-like, with a distinctive Y-shaped wing vein (the open end of the Y points to the wing tip). They readily stick to yellow sticky traps. Traps reduce adult numbers and are useful scouting aids. For best results, lay the traps sticky-side-up on the media surface — just peel back one side at a time. Expect some debris from watering to accumulate.

For larvae, use potato disk monitoring: slice raw potato disks, press them into the media for 24-48 hours, then lift and inspect for tiny white fungus-gnat larvae. Larvae per disk gives you a population trend over time.

Some Solutions for Soil Pest Control

There are relatively few biological control agents available for the multitude of soil pests — but the ones that exist are cost-effective, readily available and very effective. Entomopathogenic nematodes (parasitic nematodes) control a huge number of soil pests, including all the ones discussed on this page. They can even reach some above-ground borers and similar pests if applied appropriately.

Predatory soil mites like Stratiolaelaps scimitus hunt fungus gnat larvae, thrips pupae, springtails and other small soil-stage pests. Use them preventively in seedling and propagation areas. BTI (Bacillus thuringiensis israelensis) granular formulations are also extremely effective against fungus gnat larvae and are safe for beneficials.

Combine beneficial nematodes for the larger soil-dwelling larvae with Stratiolaelaps for the tiny ones — one program covers ninety percent of the soil-pest spectrum.