Pest Control with Mesh Crop Covers

The tomato potato psyllid TPP is the insect pest from hell, attacking all the Solanaceae family and causing downgrading of fruit and tubers and sometimes the death of plants.  Covering vulnerable plants with a fine mesh prevents infestation and can improve crop yield by changing the climate around the plant and filtering out wavelengths of light that foster fungal growth.  The Future Farming Centre is supporting growers by carrying out research, specifying the best meshes to use, and assisting with importation of mesh. 

Dr Charles Merfield, known as “Merf”, came to NZ from the UK with a practical degree in agriculture and experience in managing organic vegetable farms. In the mid-90s he went to Lincoln University and studied for a M.Appl.Sci and PhD.

“I started out at the muddy boot end of farming and horticulture, worked my way into the research side, ended up at the Biological Husbandry Unit and set up the Future Farming Centre. I’m interested in finding solutions for farmers through practical research and then getting that information out to farmers,” he says.

“The first problem posed to me when I started here was how to combat the tomato potato psyllid. If you wanted to design a perfect insect pest the TPP would come pretty close. It came from the United States and it was discovered in Auckland around 2006. Within a few years it had spread the entire length of the country.”

The TPP will attack everything in the Solanaceae family –  tomatoes, potatoes, capsicums ,aubergines, chillis, peppers and tamarillos and it will also host on most of the Solanaceae weeds.  Consequently it will survive in an environment even when there are no valuable crops present. In addition it carries a bacterium and phytoplasma, the former causing “zebra chip” discolouration when potatoes are processed into crisps and chips.  Both organisms can cause significant crop damage, yield losses and downgrading of fruit.  TPP can also kill tamarillo trees.

Chemical sprays are effective but require excellent coverage, including the underside of leaves, which is difficult to achieve and there is a risk of the pest developing resistance.  Biological controls and IPM techniques, if developed, would probably struggle to reduce pest numbers sufficiently because only very low numbers of TPP are required to cause significant crop damage.

It is a hot climate pest, so the further north you go the more devastating the problem.  Organic potato growers in Hawkes Bay have given up and tamarillo growers are tearing their hair out, says Merf.

“From my experience in Europe I was aware of mesh crop covers that are widely used for pest control for everything from small insects up to vertebrates, deer and rabbits, and it seemed to be an obvious solution to field crops like potatoes. However, the mesh hadn’t been marketed over here so I imported some for testing,” he says.

“It acts like a fly screen on a house and I was pretty sure it would work but I was concerned with potatoes, that we would get a huge outbreak of blight because it would be all warm and moist under the mesh covers.  However, in a small trial we found that it not only controlled the psyllid but also reduced the blight under the covers dramatically – there was lots of blight around and the potatoes that were not covered turned black, but the ones that were under cover remained green.”

It was a complete reversal of what Merf was expecting, so last season he did a more controlled field trial with meshes from New Zealand and Europe, got very good control of psyllid and still got good control of blight.

“Although this is still limited research the fact that we got good control using another type of mesh indicates that it is a real effect. If you cover the plant completely you get 100% control of psyllid, which can’t be achieved using chemicals, so there are none of the associated problems of zebra chip and yield losses plus there is this reduction in blight,” says Merf.

“The blight reduction was a mystery and I suspected that it was a climate effect, so in the second trial we put some data loggers under the mesh sheet to measure temperature and humidity.  There was no significant difference between measurements under the mesh and outside, so it doesn’t appear to be a climate effect.”

Merf now suspects that the cause is spectral filtering by the mesh. Farmer observation followed by research in Europe, the UK and Israel has shown that the filtering effects of poly tunnels can have profound effects on both crop growth and quality and the incidence of pests and diseases.

“In Israel they found that some insects need UV light to orientate themselves, so blocking UV disorients them so they can’t navigate in the tunnels or they can’t identify the plants.  It has also been found that a number of fungi can’t sporulate unless there is UV light,” he says.

“My gut feeling is that that is what we are seeing on potatoes under mesh. Early blight is an Alternaria species and late blight is a Phytophthora, and these mesh crop covers have a guaranteed ten-year lifespan so they put some very strong UV sunscreen into the sheets to protect the plastic.  It is likely that those materials are absorbing the UV light and therefore we are getting low UV levels in the crop.”

“We are talking with one of the world’s top UV spectral experts, Dr Jason Wargent at Massey University and so what we are hoping to do is to get some funding and look at the spectral effects on the two potato blight species in the lab and then do some fieldwork with plastic polytunnels, covers and meshes that are UV transparent and UV blocking.  Once we identify the cause we will have the potential to increase the level of UV blocking, or whatever spectral effect is having the effect on the potato blight, and increase the effects so we can get better blight control.”

There is also the potential for meshes to protect potatoes from other insects such as aphids and potato tuber moth, the latter being difficult to deal with because once it is in the tubers, it can be controlled only through systemic insecticides that are generally toxic.  Temperature control through variations in mesh may also be possible, and this could allow better yields in hot climates where potatoes do not grow so well.

Merf says that the technique could be applied to other crops.

“In Europe the only control method available to both organic and conventional producers of cabbages, turnips and swedes is a mesh cover to control cabbage root fly, and they are seeing a significant improvement in quality and a reduction in downgrades,” he says.

“Under that mesh you get a slight lift in temperature and a reduction in wind speed, and growth rate may be better in a protected environment, so we could be looking at a reasonable increase in yield from the direct effect of meshes. More research is required.”

The cost of mesh for a commercial grower could be around 60 cents per square metre, which is a considerable amount, but the mesh should have a ten-year life and being able to avoid pests and diseases, get an improved yield and market spray-free potatoes should allow growers to command a premium.

In the longer term, says Merf, what happened in Europe with the withdrawal of chemicals from sale or for use on food and fodder crops will probably happen here.  And if we have other insect pests arrive, instead of going for chemicals we can look at the use of the same mesh as a means of control.