Act now to limit fly damage losses in 2002 In winter, cold, rain and wind minimise fly transfer from house to house. This is a major reason for seasonal decline in fly numbers. Cold also prevents breeding in the many potential sites around houses, such as in soil, spilled compost and rotting vegetation. Clear away any rubbish in the gaps between houses. Conditions within mushroom houses remain fairly constant and experimentally flies can be reared equally well all year round. Now that Diazanon granules are no longer available, growers can expect phorids to rival sciarids in importance, particularly next autumn. Their larvae feed on mycelium in the compost, not only reducing it, but also breaking the cords that link compost and mushrooms. Sciarid larvae also actively burrow and may do similar damage, particularly in the casing, seeking out decaying organic matter. Adult flies carry diseases and the females in particular are active rapidly moving from bag to bag egg-laying. The interval from egg to adult varies with temperature but there is usually sufficient time for 3 generations per crop. This means theoretically one female fly could spawn 64,000 ‘great-grand’ larvae. Consequently, it is the flies that get into a house in the first 2 weeks that are the ones that do most damage. When ‘filling’, the doors should be open for minimum periods of time. Fog the house immediately afterwards.
Houses should be as fly proof as possible
Many houses do not have fly proof mesh covering vents, this is crazy! Switch the house lights off and mark where light chinks show through from the outside. If possible fill all such gaps with foam or silicone. If any gaps remain, for example under doors, spray these areas with contact insecticide. The aim being to kill flies attempting to enter. Steven Jess of DARD at Newforge Lane, Belfast is working on a range of non toxic repellents that have strong odours, like peppermint or lemon oils, aiming to find suitable fly repellents. Such a compound even if too expensive to add to compost might have a useful role sprayed around doorways. Steven is also working on attractants for male flies. He reported recently that Newforge has just obtained a high-tech piece of equipment to aid this. In use the freshly severed head of a male mushroom fly is attached to one of its electrodes. The other electrode touches the ends of the fly’s antennae. Scents wafted across this gap alter the conductivity of the antennae, resulting in a graph with high peaks and troughs. The main idea is to find a cocktail of chemicals that male flies find equally attractive to the scent of a young female fly. This may eventually enable growers to effectively trap the males of each pest species.
Back on farm, when a house is being laid out, the heavy work involved usually tempts men to leave the doors wide open for cooling and for better light. Houses should always have good internal lights and ideally cooling should be via the ventilation system, thus providing ‘safe’ filtered air. If doors are left open, fog the house after they are closed.
Casing protection Doors should only be open for minimum times and certainly not for hours, waiting for a load to arrive. Directly the job is finished, thoroughly fog. Remember that if flies are given sufficient time, some will enter bags and evade the fog. The scents of casing and particularly of fresh compost appear attractive to sciarids while that of mushroom mycelium attracts phorids. It would be interesting to know, do houses that have vents at the door end have worse fly problems than those with air exit vents at the opposite end? I suspect vents near the door attract flies to this critical area. They are thus poised to nip in through open doors or under them. In general, the main message to growers is to fog early. Once flies become a nuisance it is far too late. If casing is stored on farm, protect it from flies. Casing manufacturers should also try to prevent any infestation of stored casing.
Dimilin or Apex applied as a chitin inhibitor aim to prevent fly larvae growing. However, these materials may have the downside of reducing mushroom yield (according to some experimental evidence from Steven Jess). Parasitic nematodes as contained in Nemasys M are used to kill fly larvae and are a useful alternative. Remember this material is a ‘living’ inoculum. It must be stored correctly or it will become ineffective.
For and against shelter-belts on farm Shelter-belts protect mushroom houses and reduce fuel bills, but they may harbour flies and reduce wind that would otherwise blow them away. The litter under trees may also be a source of flies as it comprises both rotting vegetation and Agaric mycelium.
A discussion about the effectiveness of light traps I have great ‘faith’ that light traps are effective in reducing fly numbers. Steven Jess on the other hand tells me that at the moment he holds the opposite view. He has some experimental evidence that light stimulates either fly mating or egg laying, resulting in a net increase in fly numbers in those experimental chambers that had light traps in them. I would argue that a house is so much larger than an experimental chamber, consequently it has less likelihood of wall reflections affecting trap effectiveness. Furthermore, experimental flies were ‘drugged’ with CO2, to quieten and examine them, prior to release. Could this treatment have made them less likely to fly to a light trap?
The main point that light traps could have a detrimental effect if they illuminate compost is important. Light traps are popular with growers and appear to be of three types:— 1) The UV electrocuting trap being the most effective, but expensive. 2) An ordinary bulb with a lampshade of sticky yellow strips is least effective and gives most compost illumination. 3) Low wattage fluorescent lights are less attractive than UV but much more so than an ‘ordinary’ bulb containing a tungsten filament. Last year I described a type of trap that appears to be a good compromise. This has a fluorescent bulb in an inverted black-plastic bucket provided with tripod legs and set on the floor of the house over a similar bucket filled with water and containing a few drops of washing-up-liquid. The latter is to reduce surface tension and allow flies to drown. The circular gap between buckets of about an inch (2 to 3 cm) hardly illuminates the compost at all, but is highly attractive to flies.
A year’s experience suggests that once a trap starts catching large numbers of flies the lower bucket must be cleaned out thoroughly once a week. Otherwise the surface becomes covered with a bacterial scum that prevents new arrivals drowning. A small sticky yellow strip illuminated by the trap is a good monitor of fly numbers.
Role of compost manufacturers Flies are numerous in compost yards. However, none of the life cycle stages experimentally survived the peak heat in phase II. During conditioning compost is held in a fly-proof chamber. Prior to bagging it is rapidly moving along conveyors. The danger period starts when it is stationary, awaiting transit. The shorter this interval and the tighter the protection strategies at this time the better.
