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Friday, 23 December 2016

‘Tis the season...or is it?

By Helen Roberts

As I sit at my desk this morning, staring out the window, the weather is dire. There is slanting torrential rain and high winds, a typical December day perhaps.

Here in the UK, the seasons are changing and we are experiencing extremes of weather. For example, we have had wetter, milder winters in the southwest over the last couple of years along with increased flooding, particularly on the Somerset Levels. And then there was the very slow start to spring this year, with temperatures well below average in April. This was followed by a very hot end to the summer and warmer-than-average temperatures throughout autumn.

These changes to the seasons are linked to global climate change and are throwing the UK’s wildlife into disorder and affecting the fine balance of habitats and ecosystems. This is not a good scenario for biodiversity in the UK. Seasonal timing is off. When seasons start and end is shifting, and the length of the season itself is changing, making ‘growing seasons’ a more fluid concept. There is also increased risk for most gardeners of a ‘false spring’. Many plants and animals are changing their geographical ranges in order to adapt to these changes.

One of the most significant effects has been the disruption of lifecycle events and these are manifesting themselves in different ways. Bird migration, insect emergence, incidence of pests and diseases and flowering times are being thrown out of kilter.  

Researchers from the University of East Anglia recently analysed 37 years worth of data from the UK Butterfly MonitoringScheme (UKBMS) and found that extreme weather events were causing population crashes of butterflies. Uncommonly high rainfall events during the cocoon life stage affected 25% of UK butterfly species. And more than half of species were affected by extreme-heat during the overwintering life stage, possibly due to the increased incidence of disease or the effect of a ‘false spring’, causing butterflies to emerge too early only to be decimated by a return to cooler temperatures.

Warm temperatures are not all bad for butterflies though, as they will benefit from hot temperatures over the summer months when they are in their adult form and resources are plentiful. However, if populations crash more frequently than they expand, these extreme weather events could threaten UK butterflies.

The spider orchid (Ophrys sphegodes).
Photo: Jacinta Iluch Valero via Flickr [Creative Commons]
Changes in seasonal timing are also knocking the relationships between plants and animals out of sync, including the delicate balance between plants and pollinators. This can be detrimental to the balance of entire ecosystems. An elegant study carried out by scientists from Kew and the University of East Anglia found that earlier springs brought about by rising temperatures are affecting the relationship between a rare spider orchid (Ophrys sphegodesand its sole pollinator, the solitary miner bee (Andrena nigroaenea).   

This particular orchid has a flower that resembles and smells like a female miner bee and it uses this deceit in order to lure the male miner bee in. The male attempts to mate with the flower and by doing so, pollinates the flower. The plant has evolved to flower at the same time as the male bees emerge, but before the females do.

What the researchers discovered, by looking at the data set going back to 1848, was that rising temperatures are causing the relationship between orchid and bee to break down. Although rising temperatures cause both the bee to emerge and the orchid to flower earlier, the effect on the bees is much more pronounced. The male bees emerge much earlier and the orchids now flower as the female bees emerge. This means the males are not “pseudocopulating” with the flower because the real thing is already available and so the rare spider orchid is having fewer pollinations.

However bleak this picture may seem, plants and animals do have the ability to adjust to seasonal changes caused by climate change, it is just whether they can adapt quickly enough for these intricate ecological relationships to remain intact.

Helen Roberts is a trained landscape architect with a background in plant sciences. She is a probationary member of the Garden Media Guild and a regular contributor to the University of Bristol Botanic Garden blog.


Friday, 2 December 2016

Nematodes: the natural nemesis to slugs and other garden pests

By Alida Robey

Nematodes pop up from time-to-time on gardening programmes, but usually as something of an afterthought: “Oh, and of course if you don't want to use pesticides you can always try nematodes.” A certain air of mystique has surrounded nematodes for some years now, but these environmentally friendly pest controllers warrant far more consideration than a mere afterthought!

Nematodes are in fact one of the most successful and adaptable animals on the planet. They are second only to the insects in their diversity of species, geographic spread and the range of habitats they can occupy. There are more than 15,000 known species of nematodes, more commonly known as roundworms, and likely thousands more that are yet to be described.

There are parasitic nematodes that live in the gut of animals, humans, birds and mammals. Other species are free-living in the soil, feeding on bacteria and garden waste. Some are parasitic on plants and may cause disease and crop devastation. But, as a gardener, I’m most interested in those species that are free-living in healthy soil and those that parasitise common garden pests.

Free-living garden nematodes are microscopic thread-like worms, which are scarcely visible without a microscope. (This is in marked contrast to the 9 metre long species, Placentonemagigantissima, which can be found in the placenta of the sperm whale!). In good nutritious soil there could be as many as 3 billion individuals per acre. They eat fungi, bacteria and algae. So, much like ordinary earthworms, they have a useful role in decomposing and recycling nutrients.

Biological control with a specific target

Parasitic species have an equally important role in the garden. With such a diversity of species, it is not surprising to find that there are nematodes that specifically parasitise slugs, ants, vine weevil, leather jacket, chafer grub – you name it! This means that a slug nematode won't have any impact on anything but slugs - this isn’t always the case with other biological controls and rarely the case with chemical controls.

A wax moth pupa can be a host to thousands of
nematodes. The parasitised cadavers can be placed in
orchards to protect crops from pests such as citrus root and black
vine weevils.
Photo credit: Peggy Greb, US Department of Agriculture
It works like this: the juvenile nematodes are in the soil looking for a specific host. Once found, the nematode enters the body of the host and gives off  bacteria inside the host's body. These bacteria multiply and cause blood poisoning and, eventually, death. The nematodes then feed on the body of the creature and multiply, sending a new generation off into the soil to find another host. When hosts are scarce, the nematodes naturally die off.

The practicalities of using nematodes

As nematodes are living organisms they have a very limited shelf life. They therefore need to be bought online, stored according to instructions and used very soon after delivery.
There are several UK suppliers of nematodes.

It is important to choose the correct nematode species for the right type of pest and to use them in the right conditions. The soil temperature has to be above 5oC (and remain so) and they should be applied only when the pests or their larvae are active. Nematodes are also light sensitive, so use them early morning or dusk, when light levels are low.

They come as a thick paste in a little sachet, which you need to dilute with water. Repeat applications may be needed.

The specifics:

Ants : Drench the nests between April and September.

Chafer grubs: Apply nematodes in August and early September.

Fruit flies, carrot root fly, onion fly, gooseberry sawfly and codling moth: All of these pests can be treated with a generic nematode mix called Nemasys Natural Fruit and Veg Protection Pest Control. You can use it as a general treatment after planting out and when the soil has warmed up, or to target specific pests when you see them, such as gooseberry sawfly caterpillars. These (and other caterpillars) need to have direct contact with the spray while they are on the leaves.

Leather jackets:  These are the larvae of the crane fly or daddy longlegs and attack the roots of grass in the lawn. Treat with nematodes in the autumn, when the adult daddy-long-legs are laying.

Slugs:  The nematode for slugs was discovered by scientists at the University of Bristol! An application early in spring will tackle the young slugs growing under the ground, which are feeding on humus. A single application should last for at least 6 weeks, which allows time for tender seedlings and young plants to get established. They can be applied until early Autumn.

If using on potatoes, apply them 6-7 weeks before harvesting , when the tubers are most likely to be eaten by slugs.

Slug nematodes are very efficient, enjoying the same wet environment so loved by the slugs themselves.

Vine weevils: An application in March will give much greater control of larvae when they are present - either March to May, or from July to October.

I have heard the anecdotes from many gardeners who have had good results using nematodes for ants, vine weevils and slugs. But in May 2016, the Royal Horticultural Society and BASF, the only UK manufacturer of nematodes, announced a one-year research project to put slug nematodes to the test.

So in May 2017, we should see just how well this little creature stacks up against the chemical and other treatments in tackling arguably our most annoying garden pest.

Alida Robey has a small gardening business in Bristol. For several years in New Zealand she worked with others to support projects to establish composting on both domestic and a ‘city-to-farm’ basis.