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Video transcript: Pest management: comparing conventional and organic farming

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June 2011

Professor Jane Memmott, Research Leader, University of Bristol
Good webs or ecological networks as they are more generally known as, to do with the links between species.

Video slowly reveals a chart of abundance and diversity linking different species

So traditionally ecologists have looked at species lists, patterns of abundance and diversity but over the last decade there has been increasing interest in how these species are linked together because this has a huge effect on things like the provision of eco systems services such as pollination, pest control, our capacity to mend these communities if they are damaged and degraded, their response to stress and so on. The structure of the study we used was a big ambitious project and what we did first of all was to choose ten pairs of organic and conventional farms in the South West of the UK. So we had 20 farms. To actually choose 20 farms you've got to look at 80 so there is a lot of leg work involved in actually getting the choice right.

Video scans over a map of the area between Stroud and Shepton Mallet with comparable farms on it, then shows a complexed chart of links that network them. This is followed by still pictures of the researchers in the field collecting various insects like caterpillars and then it shows the collection of jars they saved them in, with some plant material as well.

You have got a list of criteria - the farms need to be matched in size, type and so on - otherwise you are comparing chalk and cheese. Having got our farms in place, and these go from Stroud down to Shepton Mallett basically, we then constructed ecological networks for each of the farms so this is a tears worth of work. We are sampling the farms once a month, all 20 farms and we have got a huge team of people out there gathering up, counting plants, collecting caterpillars, leaf miners and ruling the parasitoid. We use that information to construct these networks which shows who's eating who on the farms. We can then analyse the structure for those networks. So that's the descriptive stage.

The next stage is the manipulative stage because to me a food web project - you either have a perturbation where there is some environmental woe affecting it in some way or you deliberately manipulate it and predict what will happen. So our manipulation in this case was to introduce a novel pest to the farms and obviously we can't use a real live pest or else the farmers won't like that at all for all the obvious reasons! What we did, we had a surrogate pest, so we introduced a plant that does grow on farmlands at all.

Video shows the Paracantha plant, then a leaf with damage from leaf minor pest.

It's a plant called paracantha which is a very urban shrub and we planted a small plot of 50 plants at each farm, we then introduced species of leaf miner that is specific to that plant so it won't grow on any crops, it won't move into the native habitat at all. We then used that as a bioassay for the health and the parasitoid communities on each of the farms. With the following years after we put the plant in we sampled the leaf miners and saw how many species of parasitoid we got on the organic and conventional farms and also the percent morality - how well it was controlled. Specifically, what was the bioptic resistance to a novel pest on the two types of farms. We got two levels of information concerning the results. The first one is that there is more biodiversity on the organic farms than the conventional farms, significantly more species are found at all three trophic levels on the network so more plants, more herbivores and more parasitoid. That didn't translate into an increase in eco-system service test of pest control though on the organic farms which was really interesting. The results might be different if we were to repeat the work in a more intensively farmed area but here in the South West of England which is a hotspot for organic farming we got the same level of bioptic resistance, as we call it, to a novel pest on the conventional farms and that means the conventional farms really do have these services in tact and perhaps learning to manage that network of interaction better we might be able to reduce pesticide use in the long term.

The organic farms can certainly be reservoirs of biodiversity in the landscape. We don't know that much about the movement of parasitoids at the farm scale so it is a tricky one to answer and you could actually say that organic farms providing reservoirs of biocontrol agents, so they move onto conventional land, that's one approach. You could also say they are actually gaining from the pest control or neighbouring farms. We don't know which way the benefits are going at the moment. But having organic farms around is certainly good for biodiversity in the landscape.

ENDS