This is the third and last report on the conference held in Hillgrove in October. My aim is to summarise the information on offer and/or to paraphrase what I felt was written in too technical a way for growers to access.
The work of Steven Jess and Jack Bingham was reported by Steven in a very interesting talk with slides given at the conference. This is written up in the booklet given out but in a style more suitable for a scientific journal. The technical language of entomology is used. I have taken the liberty of loosely translating what I believe to be its content for the benefit of growers who could not make it to the conference.
Less available chemicals result in a need for alternative strategies
Over the last ten years, Northern Ireland growers have used much smaller quantities of chemicals to kill mushroom flies. Nevertheless chemicals are still the principle means of control. The industry now only uses two units of chemicals where it previously used ten. However, most of this large reduction is because Diazanon, which used to be added to compost to control Phorids, has been banned since 2000. There is thus no chemical method to control Phorids. A particular species of eelworm, named Steinernema feltiae (S.f. for short) has been increasingly used to kill the larvae (maggots) of Sciarid flies in casing. This is achieved by applying a culture of these eelworms to the casing, a type of control known as biological.
Safety in variety
Reliance should not be restricted to a single method of fly control in case this starts to fail or is suddenly terminated. It is advocated that several methods are used simultaneously. In particular, research is now being aimed at using S.f. eelworms in compost as well as in casing; two types of predatory mite that belong to the genus known as Hypoaspis are being tested as well. These can also be introduced into compost as well as into casing. The two species of mite being tested are called Hypoapsis aculeifer (H.a) and H. miles (H.m).
Biological methods and questions posed
Although biological control methods are theoretically preferable for control of flies there are potentially more problems with them than with chemicals. A chemical can be applied and, in compost at least it stays put. A biological agent can move about and/or multiply, or it can die out if it is put on too early and there is no food base such as fly maggots available for it to feed on. Nevertheless it is very important that fly infestations are ‘nipped in the bud’, before they have time to cause damage or produce flies that then increase maggot numbers so that these cause damage. Comparisons have been made between the eelworms and the predatory mites. S.f. eelworms can move easily by ‘swimming’ in water films through casing (this often being very wet) but can they move in compost that is quite dry in comparison? This is being tested. The predatory mites, which look like tiny very active spiders, are able to run wherever they choose. However, will they choose to go deep down into the compost in search of prey or will they tend to stay near the surface or are they only ‘happy’ in the casing? Are there any basic differences between the two species of mite in how they control fly larvae? These are some of the questions that the research program aims to tackle.
Research results to date
So far it has been found that in general H.a mites go deeper into the compost than H.m mites do, both species can penetrate compost or casing in experimental containers to a depth of between 2-12cm (1-5 inches). By contrast the S.f. eelworms, that kill fly larvae by penetrating natural openings such as breathing pores, mostly did not go deeper than 2-4cm into compost or casing. H.a mites were found to be best at killing both Sciarid and Phorid larvae and that this was for two reasons. Firstly they penetrated deeper into the substrate and more often and of course Phorid larvae are usually found relative deep as well as near the surface of compost. Furthermore compost is obviously eventually underneath a layer of casing. Secondly mites attack fly larvae in much the same way that a cat would attack a rabbit or a mouse. By contrast eelworms (Sf) cannot kill unless they penetrate through the maggot’s pores but in small immature larvae these pores are too small for penetration.
Two posters were prepared in order to report on this biological control work. One I singled out in a previous article as being particularly good. It showed a very informative picture of a mite ‘grabbing’ a fly maggot. In another picture a number of S.f. eelworms could be seen penetrating a maggot via its breathing pores. These latter are spread out along the sides of the maggot’s body. It is said a picture is worth a thousand words i.e. roughly the size of this article!
Mites and logistics
A second poster, principally the work of H. Schweizer along with Steven’s team and collaborators, dealt with the question, ‘How many H.m mites have to be inoculated into a unit of casing in the first instance to give an equivalent control to that given by a standard treatment using the popular insecticide Dimilin? The answer is that 800 adult mites applied per square metre resulted in Sciarid emergence from the test material as low or lower than was achieved by the active ingredient in Dimilin, (diflubezuron) used at one gram per square metre.
This type of research work is extremely difficult. Predatory mites are very active and I can only imagine working with them is very akin to herding cats! I assume the mites have to be bred, collected, inactivated by some means that does not harm them, counted out into replicated containers that have known levels of fly infestation; etc. etc. I suspect these experiments are time consuming and not easy to carry out.
Diflubenzuron is itself an interesting chemical. I believe it prevents the fly larvae producing chitin. This material makes up the armour plating of insects. It is the building material for the maggot’s jaws for example. For a maggot no jaws, no joy! Humans do not make chitin so that from this point of view at least, Dimilin will be harmless to us.
I know that growers are often desperate for new and better ways of fly control. Flies particularly the various species of Sciarids reduce yields. They infuriate pickers and spread mushroom diseases such as Verticillium (Dry Bubble) not only around a house also between houses. I do not know if Steven will read this article or will approve of my interpretational overview of his teams work. If he does read this I would love some answers to the following questions.
‘A fool can ask questions that a wise man cannot answer’ (old quotation)
I assume mites H.m. were for comparison with Dimilin because it was known that H.a. mites would be even better?
I know that red pepper mites can spread Trichoderma Th2 (green mould) spores. They are said to have built-in ‘pouches’ for this purpose. Could active predatory mites spread disease spores accidentally, and has there been any work on this?
I have been in mushroom growing houses where the concrete floor was very cracked. In such houses predatory mites of whatever species appear to carry over from crop to crop. Is this a good thing that should be encouraged in some deliberate way or is there a possibility that the mites could cycle diseases? Of course cracks in the floor may help cycle mushroom diseases anyway.
What sort of lifespan and life cycle do these predatory mites have? Do immature (baby) mites attack immature fly maggots? If so, cannot immature S.f eelworms penetrate the pores of immature fly maggots?
In compost in which mushroom mycelium is growing, there is usually a progressive build up of carbon dioxide (CO2). Are predatory mites or eelworms going to be able to tolerate this to the same degree that fly larvae appear to do? Could it be dislike of high CO2 levels that encourage these predators to remain relatively near the surface?
Peppermint flavoured compost or "A rose by any other name would smell as sweet."
To Sciarids fresh un-spawned compost smells most attractive. To Phorids it is spawn run compost that is most attractive. The fact that insects find their host substrate by smell is the basis for yet another angle and different approach to fly control.
This work is the subject of a third poster by Steven’s team including this time Michael Harriott, a high powered biochemist. The idea of making compost smell differently is good and it is simple. BUT how do you find out what is attractive and unattractive to Sciarids and Phorids. One cannot just ask.
The answer is to use Electroantennography. Translated this simply means that a living feeler or antenna of a fly is placed between two electrodes in a ‘machine’ that produces waves on a piece of graph paper, these waves being large if the compound is picked up by the antenna and small if it is not. Many essential oils are known to repel insects. The team hope to find out which one is best in the case of compost and mushroom flies?