Some Background
Roughly ninety percent of all insects spend at least some portion of their lives in the confines of the dirt kingdom. This number includes good bugs, bad bugs and those in between.
There are three types of insects, at least as it applies in this text, which can be found down-under: good bugs, which we will not concern ourselves with on this web page; bad bugs, which are bad because they cause damage to plant roots, bulbs, organic matter etc.; and bad bugs which do not cause damage to roots and the such, but are merely developing in potting soil so they can once again rise and become above-ground phytophagous or plant-eating pests.
A Variety of Pests and Soil Bugs
These pests, regardless of which category they fall into, are members of several insect orders: Coleoptera, which are beetles; Lepidoptera, which are moths and butterflies; Diptera, which are flies; and others of equal importance. We’re talking about thousands of tiny insects — wow!
This all equates to one very important fact: the soil is a good place to practice pest control, especially when dealing with pests like soil mites or tiny white bugs. Among these pests, some can include mealy bugs or scale insects, which are known for their harmful effects on plants.
Time of year will be one of the only variables you’ll be concerned with, as not all the pests you may wish to target will likely be in the dirt kingdom at the same moment.
Some pests spend time in the soil several times during every season, some are always present, and other insects visit only once a year.
Leafminers
As mentioned in the page, Leafminer Pests, those critters that are members of two orders, Lepidoptera and Diptera, can be pests that aren’t considered pests in the soil.
Again, leafminers, as larvae, when they are through tunneling through your leaves, drop to the ground to lay eggs — at least the Dipteran leafminers do. Timing isn’t too critical when it concerns these pests as there are likely to be pupae present at any given time in greenhouses, but more seasonally outdoors.
If you see the mines beginning to develop, it would be appropriate to implement leafminer parasitoids immediately, followed soon after by an appropriate soil-pest biocontrol.
Cutworms
Cutworms, one of the pests mentioned in Caterpillar Pests, are extremely vulnerable in the soil. And this is good as this is when and where they exhibit their undesirable behavior — they are soil pests. Cutworms stay in the root-zone of target plants during daylight hours and are of little concern to growers.
Cutworms stay in the root-zone of target houseplant soils during daylight hours and are of little concern to growers. At night, they are of great concern — especially to gardeners and farmers (they are not really a greenhouse pest in most cases).
They come to the soil’s surface and wrap their bodies around the stems of newly emerged plants. There, they feed on the plant’s stem, often severing it. At sunup, they return to the safety of as much soil as possible.
When the gardener heads back out to the garden, he or she will notice a plant lying over with its stem chewed and nearly cut off at or very near ground level. The plant will be doomed unless it can send up new shoots.
Targeting cutworms under most circumstances requires a preemptive action, before the plants are lost. With some crops the gardener or farmer gets a second chance.
Beetle Grubs
Not previously mentioned on this website are Japanese beetles, pests with which gardeners, orchardists and nurserymen are all too familiar. Japanese beetles, members of Coleoptera, devour the leaves of many other plants, shrubs and trees. Common hosts include bush beans, roses and flowering crab trees, for starters.
Moreover, they not only have their larval stage in the soil but they’ve been known to deaden patches of lawn if their “grub” numbers are significant. This makes Japanese beetles a unique pest, being in their glory above and below ground.
Non-chemical means of controlling the adults once they’re seen on the foliage is difficult and time-consuming at best.
The larvae or grubs are a different matter. There are generally two times to target the grubs: spring and late summer into fall — when they’re in their larval stage and residing in the potting mix.
Fungus Gnats and Flies
One important pest — or category of pests — is very common to greenhouses and other indoor plant areas, and the main focus of this chapter would be fungus gnats, or mushroom flies as they’re called in that industry.
This is especially true where artificial, sphagnum peat-based, manure-rich or compost-laden growing media are used.
These pests are generally from three genera (a taxonomical grouping) in the order Diptera: Lycoriella, Sciara, and Bradysia spp. It is now common knowledge that gnats can be quite devastating to crops.
They are known to feed on root hairs, which are essential to a plant’s uptake of nutrients. They vector root rot, which causes bacteria, viruses, and other soil-borne diseases. They can actually kill plants outright if not kept in check.
Some growers use products like neem oil as part of their preventative control strategy, particularly for pests like fungus gnats or root aphids, which can also harm plant performance.
And More
Other pests which can be fairly easily controlled underground but more difficult to deal with above ground include the following: Black vine, carrot and strawberry-root weevils; sod webworms, wire- and potato-tuber worms; apple, carrot-rust fly, onion and cabbage maggots; and flea, June/May and cucumber beetle larvae.
There are many species of pests which can be dealt with more easily when in their vulnerable subterranean stages, way too many to list here.
Why, even fleas can be dealt with, and filth-breeding flies, and…? Well, you get the point.
Serious Pests
Fungus gnats are easy to detect — using yellow sticky traps and potato disks — and their control is pretty straightforward. If you let them get out of hand, though, they can be a serious problem. Fungus gnats can be a problem with nearly any plant, large or small.
The smaller or younger the plants, the more quickly they fall and the more vulnerable they are.
Fortunately, since many plants sown by seed are planted in a fine-milled sphagnum mix, which tends to be freer of viable fungus gnat eggs, the plants can usually be problem-free until they are re-potted into a coarser mix-unless their nursery is in a structure where surrounding plants may already be affected.
The eggs hatch, the larvae feed and pupate in the media, and the adults subsequently come forth — a complete metamorphosis. The adults mate, land on the soil and lay more eggs to repeat the cycle. If there is rotting debris on the medium’s surface or algae from top-watering a nitrogen-based fertilizer solution, they will prefer those plants. If those problems can be better dealt with, you’ll reduce your fungus gnat problem.
Algae is directly attributed to many chemical fertilizers being so nitrogen-rich and is not easily avoided unless you use another method of fertilization or alter the way it is applied, i.e., use a bottom-irrigated method in lieu of top-watering.
The amount of water being used can also contribute to the problem. If the plants will do well if their substrate is allowed to dry between waterings, let them. This will hamper the activity of gnats. Regarding debris on the medium’s surface, clean it up. Run a tight ship.
Some Advice
Shore flies, which are larger and fuller-bodied than fungus gnats, are attracted to algae and can be seen on pots, but their larvae are strictly algae-eaters, so if you can tolerate their presence, they will be non-threatening.
Like mom said: “Leave them alone, and they’ll leave you alone.” However, if you’re in a retail environment, you may have to deal with your customers’ objections — it can be bad business — so you’ll have to cure the algae problem and dry areas of standing water. Oh, and hang lots of bright-colored sticky traps.
Humpbacked flies and moth flies, or drain flies as they’re sometimes called, though not real pests in the plant-destructive sense of the word, can be a bad thing to have flying around.
They can be reduced by removing algae, standing water and its associated algae, rotting plant debris and employing sticky traps to reduce adult numbers.
For the Scout
Fungus gnats are four to five millimeters in size, delicate and mosquito-like, and have a distinctive Y-shaped vein on their wings (the top, open end of the “Y” is towards the wing-tip). They readily get themselves stuck in yellow sticky traps.
The traps can reduce adult numbers and are useful scouting aids. Yellow traps will be most effective if the sticky side is laid up on the medium’s surface; just peel back one side at a time. However, given their proximity to the action, do expect the traps to become littered with debris from watering and other common activities.
The debris may also make bug counting more difficult. Potato disks, which have been previously mentioned, are very useful in detecting the gnats’ larvae, isolating problem containers, and reducing larval numbers — assuming you frequently change the quarter-inch disks.
Moreover, potato disks are useful in playing a part in a control program and for population monitoring where sticky traps would be impractical, such as in many interiorscapes.
Some Solutions
One control, in particular, entomogenous or parasitic nematodes, controls a huge number of soil pests, including all of the pests discussed on this page (they can even control some above-ground, boring and similar pests if applied appropriately).