I was asked to make some observations concerning mushroom flies.
Growers have told me they believe flies often come with compost. How likely is this? Several tests have been completed that indicate no survival of fly eggs, larvae, pupae or adults through phase II composting. For more information, contact Steven Jess at DARD, Newforge Lane, Belfast 9. He is conducting an in depth study.
It is possible that flies could contaminate compost while it is on the bagging conveyors, or if it is overnight stored (in bags or as blocks) or possibly while in transit to the grower. However, this is a relatively minor source of contamination simply because there are few mushroom flies in a bagging shed and because compost is usually on the move, thus limiting opportunities. I have observed contamination to occur when compost is allowed to sit on pallets outside the bagging area, for example for half a day, awaiting loading. Ask your compost maker if this is regular practice. With some it is, with some it isn't. Again, although such practice should be avoided, it is still only a fairly low level of risk because the time available is relatively short and bags are less easy for flies to penetrate when they are both wrapped up with film and on pallets.
A similar argument could be applied to contamination of casing. Even if it is in plastic bags, these are usually perforated or of woven material that is not fly-proof. Thus casing should not be allowed to await dispatch for long periods, or for there to be a long delay after arrival with the grower.
It is with the grower that the greatest risks of contamination occur, simply because of the relatively much longer period of time involved. Furthermore, most growers have some ongoing level of fly problem. Consequently, there are several types of mushroom fly around growing houses and these have adapted to feed on phase II mushroom compost. The grower may argue that the compost is inside an enclosed house, out of harms way. However bear in mind that mushroom flies have had innumerable generations to adapt their behaviour to make best use of prevailing circumstances. They don't just fly aimlessly, they actively seek fresh compost.The ones that do this successfully are most likely to produce offspring, so cycle after cycle successful behaviour becomes genetically 'built in'.
Flies not only reduce the number of pins, and thus the yield, they also annoy pickers, reduce quality and spread several important mushroom diseases. I have observed them carry Trichoderma and Cobweb spores. Furthermore, a proportion of such flies placed individually in bags of compost, or on the surface of casing, lead to colonies of such fungi, the cause of serious loss.
There is some debate as to the effectiveness of fly-traps in mushroom houses. Electrostatic UV traps appear effective, however they are expensive to buy and to maintain, especially if constantly on. A cheaper alternative is a tungsten bulb with a 'lampshade' made of sticky strips. This may be seen to catch many flies. However, its light can have a detrimental effect by aiding their breeding behaviour (particularly that of phorids) consequently, it could lead to a worse infestation rather than to less flies. My own observations are;- low wattage, (9-11 watts) long life, fluorescent bulbs have greater attracting power than tungsten-filament bulbs and also have the advantage - of using only a fraction of the electricity. This saving soon pays for the much greater cost of florescent bulbs.
I have found a very effective trap is made as follows. A black 'bucket'about nine inches (23cm) high and of similar width is fitted with 3 metalstrip legs so that it can stand over a similar bucket with a gap between the two rims of about one inch (2.5cm). Using small bolts a bulb holder is attached to the inside bottom of the upper bucket, the lead coming out of a central, clearance-size hole. In use, the lower bucket is filled with water (plus a few drops of washing-up liquid). The trap is placed in the middle of the house. Light shining on the water makes it glint brightly but without noticeably illuminating the surrounding compost bags. It is nevertheless very attractive to flies (provided the house lights are off) and flies are usually found drowned in their thousands at the bottom of the bucket. Each week the contents are inspected, then poured down a drain and the bucket refilled.