Greenhouse Grower Notes: January 2007 2

January 21, 2008
Written by Gillian Ferguson
It is the beginning of the season for many greenhouse vegetable crops, and everything looks fresh and clean. There are likely not many pests to be seen, if any. With time, however, we can expect to see little flies hovering around the base of the plants, close to the growing media. These flies are usually fungus gnats and shoreflies, and could also include moth flies.

Studies indicate that these flies can carry spores, both internally and externally. Moreover, spores can remain viable even after passing through the insects’ digestive tracts. In one particular study carried out at the University of Victoria, British Columbia, researchers were able to isolate spores of Fusarium sp. and Botrytis cinerea from the bodies of fungus gnats and shoreflies collected in greenhouse nurseries. More spores were collected from fungus gnats than from shoreflies, perhaps because gnats have more body hairs. Spores were found on their legs and leg joints, in grooves on their bodies, and on their heads and chest areas. This is particularly significant because Fusarium sp. and Botrytis cinerea produce spores above ground, and there is a high probability that flies will assist in the spread of these spores.

Eliminating flies cannot ensure that there will be no incidence of diseases caused by pathogens such as Fusarium and Botrytis, but it will certainly slow or reduce their spread. One preventive measure that can help to suppress the population increase of some of these flies is early release of predators such as Hypoaspis miles and the rove beetle (Atheta coriaria). This article will focus on H. miles.

HYPOASPIS MILES
This is a soil-dwelling predatory mite that usually stays in the growing media and can feed on a range of different organisms, including larvae of fungus gnats and shoreflies, nematodes, and pupae of thrips, leafminers and gall midges. However, studies suggest that H. miles prefers fungus gnat larvae to other species. Many growers are already familiar with this predator and this article is simply a reminder that early release of this predator is the best way to use it. Hypoaspis miles is most suited for preventive use because it has a long lifespan and is able to withstand long periods without food.

Research in the UK indicates that in the absence of food, young nymphs of H. miles can live for about 12 days, and older nymphs for about 22 days. Starved male and female adults live for about 24 days, but starved females do not lay eggs. If food is available, many males and females can survive for about 142 days.

Regarding effects of temperatures, H. miles requires warm temperatures to be active. For instance, at 10ºC, there is no development, and eggs fail to survive. The lower and upper thresholds for development are 10-12ºC and 32-35ºC, respectively. Data from studies done in Denmark and the UK indicate that the optimum temperature for development of this mite is about 25ºC. Although development time is shorter at higher temperatures, mortality is also higher. The time taken from egg to adult at 24ºC is about 11 days, although the actual length of time taken for development is also influenced by the prey eaten. One study showed that at 20ºC, development time from egg to adult was 14.5 days when the mites were fed fungus gnat larvae, and 16.6 days when fed mould mites.

All larval stages of the fungus gnat are attacked by H. miles but the numbers eaten depend on the size of the stage. Hypoaspis miles can eat about eight very young larvae, but only about one mature larva, and that is if the mite decides to attack this larval stage which is about seven times its size. Eggs and pupae are usually not eaten, possibly because the mites do not recognize them as prey. Hypoaspis spp. will attack shorefly larvae provided they are not in standing water.

OVERALL STRATEGY FOR RELEASE OF HYPOASPIS SPP.
Although one release at the start of the season could lead to sufficient population for the entire season, it may be better to make a second release about 2-3 weeks later to ensure that a population of this predator does establish. Given the lack of food at the beginning of the season as a consequence of good cleanup practices between crops, use of new blocks and slabs, etc., Hypoaspis spp. may face difficulty in reproducing. In such situations, this predator may either not establish or take a much longer time to attain populations levels needed for good suppression of fly pests.

ALTERNATIVE STRATEGIES FOR FLY PESTS
As noted earlier, the rove beetle can also be released for suppression of fungus gnats and shoreflies. Other biocontrol agents used for suppression of fungus gnats include the generalist predator the parasitic nematode, Steinernema feltiae, and the bacterial organism, Bacillus thuringiensis subsp. israelensis (Bti) sold as Vectobac for greenhouse crops. A brief note regarding Bti and shoreflies: one European study indicated that this bacterium showed no larvicidal activity against shoreflies.

In addition to early release of Hypoaspis miles, growers are also reminded that removal of standing water, plant debris, and minimizing algal growth will all go a long way towards minimizing establishment of flies. A fact sheet on fungus gnats and shoreflies (Order # 06-079) can be obtained by calling the Agricultural Information Contact Centre of the Ontario Ministry of Agriculture, Food, and Rural Affairs at 1-877-424-1300.

Gillian Ferguson is the greenhouse IPM specialist, Ontario Ministry of Agriculture, Food and Rural Affairs, at Harrow. • 519-738-2251 ext. 406, or
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