Fungus Gnats and Shoreflies in Greenhouse Crops
Table of Contents
Fungus gnats and shoreflies are small black flies often observed around the growing media of greenhouse crops. Although growers often regard them as nuisance pests, they can facilitate and transmit root diseases in all greenhouse vegetable and ornamental crops and reduce the aesthetic quality of ornamental crops.
Adult fungus gnats are grey to black and about 3-4 mm long. They have long legs, thread-like antennae and large compound eyes that meet above the base of the antennae. Adults resemble small mosquitoes (see Figure 1) and are weak fliers. They are often seen resting on the media surface. Each female lives about 10 days and lays about 150 oval, white eggs in the organic matter of the media. Eggs hatch in 2-7 days, depending on the temperature, producing white larvae that are 4-6 mm long. The larvae have 12 abdominal segments and a distinctive shiny black head (see Figure 2). They feed for 5-14 days before pupating. Adults emerge 4-6 days later. As with other insects, fungus gnats become more active and reproduce more quickly at warmer temperatures. The life cycle can be completed in 21 days at 24°C, compared with 38 days at 16°C.
Figure 1. Adult fungus gnat on sticky card. Note the long legs and antennae.
Figure 2. Fungus gnat larva
Adult shoreflies are similar in size to fungus gnats but resemble small houseflies. They have a dark, chunky body with short, bristle-like antennae and short legs (see Figure 3). They are stronger fliers than fungus gnats and have five clear spots on the wings. Adult shoreflies prefer wetter conditions than fungus gnats and are often found in wet areas under benches and other water-soaked areas. Females lay eggs on algae or moist growing media, and larvae hatch in 2-3 days. Shorefly larvae are cream to muddy brown in colour (see Figure 4) and appear headless. They feed on algae and other micro-organisms found in the media for 3-6 days before pupating. The adults emerge 4-5 days later. Shoreflies take approximately 9-14 days to complete a generation at greenhouse temperatures.
Figure 3. Shorefly adult. Note the very short antennae, stout body and clear spots on wings.
Figure 4. Shorefly larva.
The immature stages of fungus gnats generally feed on decaying organic matter, soil fungi and algae. However, they can also feed on fine roots, root hairs and tender lower stems, causing direct damage. All greenhouse crops can be affected. In addition, fungus gnats can transmit disease-causing organisms. They are actually attracted to infected plants, and studies show that the adults can spread spores of disease-causing fungi such as Rhizoctonia (see Figure 5) by flying to non-infected plants and excreting the spores. Adult flies can also spread Fusarium, Verticillium and other fungi by means of spores that are caught on their legs and bodies. Organic media such as peat and cocofibre favour reproduction of fungus gnats.
Figure 5. Fungus gnats can transmit diseases such as Rhizoctonia shown on this poinsettia.
In contrast, immature shoreflies are semi-aquatic. They feed mainly on algae. Although they do not normally feed on plant parts, they will feed on roots infected with fungi. As a result, they can spread disease if the spores they ingest remain viable in their gut until the shoreflies become adults. Shoreflies also cause cosmetic damage to ornamental crops by depositing black drops of excrement on leaves and flowers.
Generally, controlling these flies can be difficult because of their above-ground stage, continuous overlapping generations and short life cycles. To avoid problems associated with these flies, implement control measures early. The strategies for managing these flies include monitoring and cultural controls.
You can place yellow sticky cards at the normal position at the top of the canopy to monitor for these flies. However, to detect them earlier and trap them more effectively, it is better to place the cards horizontally at the base of the plants. To monitor levels of fungus gnat larvae, place slices of raw potato on the media and examine them after 24 hr with a magnifying lens (see Figure 6).
Figure 6. Use of potato slices can be very effective in monitoring for fungus gnat larvae and the predatory rove beetle.
The best management strategy is to prevent these flies from establishing themselves in the greenhouse. Implement good hygiene practices, and have good drainage to eliminate puddles and formation of algae (see Figure 7). Minimizing exposure of media surfaces will block light, helping to prevent the growth of algae.
Figure 7. Controlling algae on rockwool (top) and under benches (bottom) is important in managing fungus gnats and shoreflies.
Several commercially available biological control agents (BCAs) can be used to manage fungus gnats and/or shoreflies. These include:
There are also several naturally occurring BCAs that are not available commercially but are often found in greenhouses where pesticide use has been reduced:
Keep the following points in mind when using BCAs.
Bacillus thuringiensis subsp. israelensis (Bti): Bti consists of two kinds of spores: one is active, while the other is a storage spore that includes a toxic protein crystal. When a larva ingests these spores, the alkaline pH in its gut releases the toxic crystal. This crystal destroys the gut wall of the insect, allowing the active spores to pass into the bloodstream. The insect then dies from blood poisoning. Within 24 hr of ingesting Bti, fungus gnat larvae stop feeding and become limp. Death follows 1-7 days after ingestion. This bacterium does not kill on contact, and only the larval stage is killed by ingesting Bti. Because studies show that the younger larval stages of fungus gnats are more susceptible, multiple applications are required to achieve control. Adults do not feed on these spores and are therefore unaffected. When using this bacterium, make sure that the pH of the water used for mixing is neutral or slightly acidic. If the pH is higher than 7.0, the organism will not be effective.
Steinernema feltiae: These nematodes may provide more rapid control of fungus gnat larvae than the other BCAs. Pupae are not as susceptible as larval stages. The nematodes search out an insect host and enter it though body openings such as the mouth, anus and breathing pores called spiracles. Once inside the insect, the nematodes release a bacterium (Xenorhabdus spp.) that they carry in their gut. The bacteria develop within the insect, killing it within 48 hr. In theory, the nematodes can then develop by feeding within the larva (see Figure 8). However, the size of fungus gnat larvae may be too small to allow nematodes to reproduce. For best results, apply nematodes at the end of the day to avoid the risk of them drying out or being exposed to direct sunlight. Since plants are irrigated during the day, applying nematodes at the end of the day also reduces the risk of flushing them out of the growing media, particularly in rockwool. Studies show that these nematodes move very easily through rockwool. Within two weeks after application, the majority of nematodes are found in the lower parts of the rockwool media. Nematodes are most effective when the temperature and pH of the water used for mixing are the same as those required for optimum crop growth. Applications should be made weekly for several weeks, depending on pest populations.
Figure 8. Steinernema feltiae nematodes can provide effective control of fungus gnats.
Stratiolaelaps scimitus and Gaeolaelaps gillespiei: These predators are soil-dwelling mites (see Figure 9) that feed mainly on the young larvae of the fungus gnat. They feed very little on eggs and probably not at all on pupae. They do not diapause but will become inactive at low soil temperatures. If these predators are released in the seedling stage, make a second release after planting out in the main house when more food is likely to be available to sustain them. It is best to release these predators before fungus gnat populations are established. There are two advantages to using these predators: they provide season-long suppression, and they will also feed on other small insects in the growing media, such as springtails and thrips pupae.
Figure 9. Stratiolaelaps predatory mites feed on soil-dwelling organisms such as fungus gnat larvae and thrips pupae.
Dalotia coriaria: This beetle is a relative newcomer to the biological control arsenal for fungus gnats. Laboratory studies indicate that it has a lot of potential for controlling fungus gnats and shoreflies. The adult is a small black beetle, 3-4 mm long (see Figure 10). There are three larval stages that vary from white in the earlier stages to yellow-brown in the final stage. The adult and all larval stages are predatory. The rove beetle is very active, establishes easily and spreads quickly throughout the greenhouse. It often establishes naturally, and resident populations will maintain a permanent presence. Potato slices used to monitor fungus gnat larvae are also very effective in detecting the presence of Dalotia adults and larvae.
Figure 10. The rove beetle Dalotia coriaria is a soil-dwelling predator that feeds on the eggs and larvae of fungus gnats and shoreflies.
Coenosia attenuata: Also called the hunter fly or tiger fly, Coenosia is a greyish, predatory fly in the same family as the housefly (see Figure 11). It is larger than the shorefly. Coenosia preys on flying insects, catching them in flight. These include fungus gnats and shoreflies, as well as leafminers and, to a lesser extent, whiteflies. Coenosia larvae live in the soil, where they feed on other soil-dwelling organisms such as fungus gnat and shorefly larvae.
Figure 11. Coenosia attenuata is a predatory fly. The adult feeds on flying insects such as fungus gnats and shoreflies, while the larva feeds on soil-dwelling organisms. The figure shows the difference in size between a shorefly on the left and Coenosia on the right.
Synacra: This is a parasitic wasp that lays its eggs into fungus gnat larvae. The wasps develop and emerge from the fungus gnat pupae. The adult wasp is similar in size to the fungus gnat but has a typical wasp-like appearance with a pinched "waist" and a long, tapered abdomen (see Figure 12). Although its effectiveness in controlling fungus gnat populations has not been documented, large numbers of these BCAs are often found on sticky cards in greenhouses.
Figure 12. Synacra is a parasitic wasp that lays its eggs into fungus gnat larvae.
Hexacola neoscatellae: This is a parasitic wasp that feeds on shoreflies (see Figure 13). It is smaller than a shorefly and can be found in large numbers on yellow sticky cards in greenhouses with resident shorefly populations. It is black in colour and has an almost spherical abdomen, in contrast with the more elongated abdomen of Synacra (see Figure 14).
Figure 13. Hexacola neoscatellae is a parasitic wasp that lays its eggs into shorefly larvae.
Figure 14. Comparison between Synacra (left) and Hexacola. Notice that Hexacola is smaller than Synacra and has a rounder abdomen.
Pesticides used to control fungus gnats target the larval stages living in the growing medium. Best results are usually obtained when applied early in the crop production cycle, which is when fungus gnat populations are often largest. As the crop matures, the growing medium dries out more quickly and the developing root system is less prone to feeding damage. There are several pesticides registered for control of fungus gnats and shoreflies, some of which are compatible with biological control programs. Please refer to OMAF Publication 370, Guide to Greenhouse Floriculture Production, and Publication 835, Crop Protection Guide for Greenhouse Vegetables.
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