Category: pests

Nematodes: the natural nemesis to slugs and other garden pests

Posted on by andy.winfield

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.

The wondrous creatures that share our gardens part 2: feathers and fur

Posted on by andy.winfield

By Alida Robey

This week’s post continues to separate the facts from the fallacies concerning the creatures that cross our paths while we garden. In my last post I looked at invertebrates and this week, I look at creatures with feathers and fur.

Is that little robin back again this year?

Robins live just over a year.
Photo credit: Linda Stanley [via Flickr CC by 2.0]

Unfortunately, this is highly unlikely. Robins are relatively short-lived birds, with an average lifespan of 1.1 years. The oldest known robin lived to 12 years. Compare this with blackbirds (2.4 year average; 20 year max) or starlings (2.5 average; 22 year max). This sadly means that the robin you see in your garden year after year is unlikely to be the same one.

How long do foxes live?

Whilst in captivity, foxes can live as long as 14 years, but they usually live 5 years in the wild. In rural areas where fox control is practised, up to 80% of the fox population may be less than 1 year old.

Foxes can dig their own dens, but sometimes they prefer to simply renovate vacated rabbit holes, or even co-habitate with badgers, amicably occupying the other end of their sett.

Foxes have been known to travel 5-10 miles from their den on their nocturnal hunt for food.

If you are troubled by foxes – and increasingly in urban areas such as Bristol this can be the case – I am reliably informed that rural chicken keepers get the males in their family to take a pee around the hen house. This being one of the best-known deterrents to these wily poachers!

Are moles really blind?

Moles can create havoc in the garden.
Photo credit: Stephan Caspar [via Flickr CC BY-NC 2.0

Moles can be the bane of your life if you get them tunnelling in your lawn or garden. These amazing little creatures can shift twice their body weight in soil in a minute, which amounts to 540 times their body weight in a day. This can have a rather disruptive effect on your lawn if you are unfortunate enough to have them as neighbours – and given there are 35-40 million of them in the UK, the chances of that are fairly high! Moles are solitary except when mating and they only surface at night to forage for food and nesting material. They live mainly on worms, grubs and larvae.

Contrary to the popular myth, moles are not blind. They have very small eyes as they spend so much time tunnelling. Their eyes are light-sensitive, but don’t detect colour; they rely heavily on sound and smell. They are the only mammal that lives totally underground. Their blood composition allows them to cope with significantly less oxygen than other mammals require.

How far do hedgehogs travel? 

Hedgehogs travel up to 3 km a night in search of food. They can swim and even climb the lower branches of hedges in search of bugs and caterpillars. Their spindly little legs can help them reach speeds of up to 4.5 mph if they need to, which is more than twice my average pace as a steady walker (with significantly longer legs)!

Hedgehog numbers are in decline.
Photo credit: Milo Bostock [via Flickr CC BY 2.0]

We do love hedgehogs, yet we don’t seem to love them enough, as their numbers are in drastic decline due to human lifestyle choices. Whilst hedgehogs can live 10 years with a bit of care and attention, their average life in today’s urban environment is only  2-3 years with 20% of baby hoglets dying before they even show their little noses out of the nest. A large number then die during their precarious first hibernation. Roadkill, pesticides and urban development contribute to mortality, so less than 0.4% ever reach 10 years of age.

A hedgehog diet is mainly slugs, snails and beetles, but they also eat worms and spiders, and occasionally carrion and birds eggs. Milk and bread that is left out for these carnivores can actually be fatal to them. And to dispel yet another myth, hedgehogs do have fleas, but these are a variety exclusive to hedgehogs and will not transfer to pets or humans. It is also the case that if you should try to treat any hedgehogs you find with other animal flea treatment you will again almost certainly  kill the poor hedgehog.

Hibernation gets the hedgehog through winter when the availability of its normal diet is scarce. To cope, its body temperature drops from 35 degrees Celsius to around 4 degrees Celsius. It breaths only every 6 seconds and drops its heartbeat to one tenth the normal rate. A hedgehog can lose around half its body weight in the process of getting through the long cold winter.

The stark fact is that without drastic measures by householders and others, hedgehogs are heading rapidly towards extinction having fallen in number from some 30 million in the 1950s to 1.1 million by 1995 and further loss since then means there are now fewer than a million left in the UK.

Slug bait and pesticides kill hedgehogs, so if you want to do your bit, then please do consider nematodes and other hedgehog-friendly pest control methods. Nematodes can be bought online and simply watered onto your garden a few times a year. Doing so as an alternative to slugbait should help both your bird population and any of the few remaining hedgehogs in the country – both of which will help reduce slug devastation in your garden. Worms, birds, hedgehogs and many other wild creatures perform vital ecosystem services for us – they are workers in our gardens and countryside. The more we protect them, the more help they give. So when you are fighting the battle to take control of the  unruly creatures in your garden remember to keep some room for ‘the wild side!’

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. 

The wondrous creatures that share our gardens part I: The creepy crawlies

Posted on by andy.winfield

By Alida Robey


As I peacefully garden, random thoughts about the wildlife I encounter waft through my head. How far does a hedgehog travel in a night? If you throw a snail over the fence, is it likely to find its way back into the garden? How long do worms live?  Rather than continue repeating this cycle of speculation,  I decided to get to the bottom of these little mysteries.

Do those snails make it back over the fence?

There are 43,000 species of Gastropoda (the taxonomic group better known as slugs and snails). There are 220 non-marine species of these mucous crawling critters living in the UK, so it’s small wonder we have such problems with snails in our gardens. Snails can live up to three years. Each snail has both male and female sex organs (they are hermaphrodites) and mating occurs when two entwined snails each shoots a small sperm-carrying dart at the other; each then goes off to lay about 40 eggs.
Garden snail.
Photo credit: Daniela [via Flickr, CC license]
And that burning question….what happens to all those snails that I catapult over my fence? Well yes, snails can happily find their way back over fences and walls, up to a distance of 10 metres. Professor Dave Hodgson, Professor of Ecology, University of Exeter, revealed this fact when he used LED lights and UV paint to track the movement of snails at night in a British garden. The study revealed that snails can travel distances of up to 25 metres during a 24-hour period and reach top speeds of one metre per hour. So you will need to do more than lob them discretely over the wall if you are hoping to do more than just make them work a bit harder for their supper! 

What is the lifespan of a slug?

Most slugs evolved from snails, losing their shells through evolutionary processes over time.
Depending on the species, slugs can live between 1 and 5 years. If you are squeamish about slugs, count your blessings you don’t live in North America where there is the banana slug, which grows to 10 inches long!
The banana slug (Ariolimax columbianus).
Photo credit: Oregon Department of Agriculture [via Flickr, CC license]
Much as we may berate slugs, they do serve an important function in nature, munching up rotting debris and recycling the nutrients back into the food chain. 
However, they also like to munch the tender leaves and stalks of our most treasured little seedlings, and it is for this reason that many British gardeners are keen to shorten the lifespan of these troublesome pests. Use of slug baits is highly controversial and there is insubstantial and conflicting information regarding how these poisons affect other creatures, such as birds and hedgehogs, which are natural predators of slugs. 
Despite being in widespread use, slug pellets are considered by some to be a relatively ineffective method of slug control, some say killing no more than 10% of the slug population in the average garden. Slug pellets fall into 2 main types; those containing metaldehyde or the less common methiocarb.
Metaldehyde is the most common and less toxic form of slug poison and if not taken in too large quantities less likely than methiocarb products to be fatal to other animals – the suggested dose being one pellet every 10 cm (4”). 
Methiocarb is about ten times more poisonous than metaldehyde, and therefore of greater danger to other animals. It breaks down more slowly too, making it a longer lasting hazard in the environment with the potential to affect more animals (targeted or not).
The RHS has some research underway that is due to report in December 2017 on the relative effectiveness of different methods of slug and snail control, including the use of nematodes. This research will help them improve their advice about slug and snail control. 

Does cutting a worm in half make 2 worms?

Sadly no – but thankfully at least one half should still survive. 
Earthworms
Photo credit: Wormwould [via Flickr CC by-NC 2.0]
I always say the common earthworm is the hardest worker in the garden, and Charles Darwin called them ‘nature’s ploughs’. They do an extraordinary job of aerating and fertilising the soil, pulling leaves down beneath the ground at a rapid rate and bringing valuable minerals to the surface with their wormcasts. One acre of land can hold around 3 million earthworms, which are capable of bringing around 9 tonnes of soil to the surface through their wormcasts in just one year. The tunnels they make and live in can go as deep as 1.8 m (6 ft), which enables them to find moisture in times of drought. Worms are capable of digesting even leaves that are generally poisonous, owing to the presence of a chemical in their gut called drilodefensins. Scientists at University College London identified this molecule, which breaks down the toxic chemicals plants produce to deter herbivores. Without this ability, leaves would pile up on the surface of the soil.

Ladybirds
Photo credit: danielweiresq
[via Flickr CC by-NC 2.0]

How many spots does a ladybird have? 

Well, actually, there are 40 different species of ladybirds in Britain, with differing numbers of spots and some have yellow wing cases rather than red – the invasive harlequin ladybird (Harmonia axyridis) can also have a black wing case. And no, ladybirds do not gain more spots as they get older. The ladybird is a most welcome addition to the garden as its larvae munch their way through vast quantities of aphids prior to pupating.  However, they have to eat heartily as aphids are so fast at reproducing, they get through three generations for every single generation of ladybird. For this reason, the ladybird’s ability to truly help control aphid infestations has been brought into question.

Is the common wood lice harmful to plants?

An unfortunate name for a little creature that is more closely related (admittedly from way back), to crabs and lobsters! It quietly beavers away eating rotten wood, fungi and garden debris and protects itself from drying up by only coming out from under stones and logs at night. 
Woodlice
Photo credit: Mark Hilditch [via Flickr CC by-NC 2.0]
Woodlice cause little or no damage to plants. Large numbers can be found in compost heaps, where they help break down the plant material and are a useful part of the composting process. But you may well have seen what you think is the same thing around your house. This is a slightly different species of wood louse to the garden variety and is called the pill bug.  It can be distinguished from the woodlouse by its tendency to roll into a tight ball when threatened. Pill bugs are not thought to do any harm in the house although their presence may indicate an issue with dampness, as this is their preferred environment.

How can you tell  the difference between toads and frogs?

I have often wondered how to tell which was which. Frogs generally have a wet, smoother skin, while that of toads is dry and ‘warty.’ Toads walk rather than leap and are less nimble movers than frogs. I regularly help with a garden in Herefordshire where I am frequently surprised by the lump of soil that turns out to be a toad that then ambles off unsteadily down the garden – a frog would have leapt off at the least disturbance. 
Toads return each year to the same pond to spawn their eggs – as many as 7,000 in a sitting.
Frogs come in a much wider range of colours than the steady brown toad with its darker brown blotches; they can be anything from yellow, brown, orange, red, grey, and different shades of speckling. 

What ARE Nematodes?

Caenorhabditis elegans is a nematode that is studied
extensively by scientists.
Photo credit:snickclunk [via Flickr CC by-NC 2.0]
Nematodes are microscopic worms that are naturally present in the soil. They can be purchased in concentrated volumes as biological pest control. There are specific nematodes for each different  garden ‘pest’ such as slugs, vine weevils, ants, chafer grubs, leatherjackets, caterpillars, codling moth etc. An advantage of nematodes is that they don’t persist in the environment like chemicals do – when their prey item depletes, they naturally die off to natural population levels. 
As this is a ‘live’ product, it is currently only available from online suppliers as nematodes have to be used within a few days of purchase. They are sent in a little pack that resembles fresh yeast and one simply has to dilute this to the required level and then water them into the garden. I have used nematodes myself very successfully and believe that with increasing regulatory pressure on toxic sprays and treatments that are harmful to pollinators and other wildlife, nematodes are the way of the future for pest control.
Please leave a comment if you have any questions about the wildlife you encounter in your garden and we’ll do our very best to find an answer. Part 2 will have some common questions about our furred and feathered garden friends.
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. 

Forests may be more vulnerable to pests and disease in the future

Posted on by andy.winfield
As I sit in my home office watching the autumn rains and winds strip the last remaining colourful leaves off the trees outside, I find myself in awe of the tree. There’s a primary school across the street from my house and there are several huge beautiful chestnuts in its grounds where I watched the children shelter from the sun on hot days. There’s also the spindliest little apple tree that one could imagine, which despite its size produced at least a dozen enormous apples this year!
Trees affect every aspect of our lives – they provide food, timber, pulp and fibre, but beyond this they have important ecosystem functions in the natural landscape. Trees help to regulate our climate, they store and sequester carbon (about 30% of global CO2 emissions are absorbed by forests), they store water helping to prevent floods, they purify water and they provide habitat.
However, widespread pests and diseases have taken their toll on natural forests over the past century with outbreaks seemingly becoming more frequent and widespread in recent years. There has been considerable focus on the devastating effects of these outbreaks on trees with large economic value – orchards and timber plantations for example – but what are the consequences of the widespread death of our forests in terms of ecosystem services?
Oak in its autumn colours.
A review published recently in the journal Science considers this exact issue. UK researchers from the Universities of St. Andrews, Cambridge and Oxford reviewed the consequences of tree pests and diseases on ecosystem services around the globe.  The authors concluded that our current approaches to pest and disease management do not take into account the ecosystem services or the beneficiaries of these services provided by forests and that new approaches are needed, particularly as the likelihood of pest and disease outbreaks increases as a result of global climate change and globalisation.

Who’s attacking our forests?

Trees are attacked by any number of pests and diseases, including bacteria, viruses, invertebrates, water molds and fungi. The effects of these pathogens may be compounded as well; trees that have been defoliated by insects may be more vulnerable to disease.
Millions of years of co-evolution have generally allowed trees to build up natural defenses to the pathogens they encounter in their native environments.  However, the introduction of species or the movement of species outside their historical ranges has opened up a whole new world of pathogens that have been the cause of the most devastating attacks on our global forests in the last 200 years.
The American chestnut (Castanea dentata) was devastated by chestnut blight – a fungus accidentally introduced to eastern North American forests around 1900. In the early 20thcentury, over a quarter of the trees across approximately 200 million acres of eastern hardwood forests were American chestnuts, but by 1993 its frequency had declined to 0.5%. Today the tree is effectively extinct as very few mature trees are producing nuts.
Dutch elm disease – another fungal pathogen, which is transmitted by bark beetles – is familiar both in North America and Europe as it has eliminated mature elms (Ulmus spp.) from much of the landscape. Now there is concern that ash (Fraxinus excelsior) could suffer the same fate due to another fungal pathogen (Chalara fraxinea), which has been killing trees in Poland since the 1990s. Scientists are monitoring its spread to the rest of Europe.
The devastation wreaked on a Canadian forest by the
mountain pine beetle. Credit: D. Huber, Simon Fraser University
Public Affairs and Media Relations (Flickr CC).
As a Canadian I would be remiss if I didn’t also mention the devastating effects of the mountain pine beetle (Dendroctonus ponderosae). It has already killed several million hectares of pine species in Canada and the US and they expect over 37 million hectares of forest to be affected in British Columbia alone before 2020.
Of course, with globalisation and the widespread movement of plants and plant products around the world, the frequency and spread of pests and disease is only likely to increase. Climate change will also improve conditions for pests and disease as milder conditions in some areas may let some pathogens increase their natural range, or may permit pest populations to explode in numbers.

Attack of the Frankenfungus


When pathogens move around the globe they are not only introduced to new hosts and plant prey, they can also escape the natural predators and diseases that keep their populations under control.
This global movement also exposes pathogens to new genes that can make them even more virulent. For example, when the fungus that causes Dutch elm disease, Ophiostoma novo-ulmi, spread across the northern hemisphere, it hybridised with a native fungus species (O. ulmi) and acquired some new genes that decreased the elm’s ability to resist infection.   

What does the loss of dominant tree species mean for our forests?

Widespread loss of a dominant tree species can have devastating effects far beyond any economic value they may have had. A wide range of ecosystem services will initially be harmed, such as retention and purification of water, wildlife habitat and carbon storage. Large stands of dead trees also become fuel for wildfires, which are far less specific about their victims and further alter the ecosystem.
However, inevitably the lost trees are replaced by new species and as this natural succession occurs some of the ecosystem services will be restored – carbon storage and water purification, for example. Unfortunately, other ecosystem services may never be restored. New tree species will create different habitats altering the biodiversity. 
Some ecosystems are particularly vulnerable as they are dominated by a species that plays a critical role in maintaining the structure of that ecological community – known as a keystone species. Boreal forest (or taiga) is an excellent example of this. The conifers that dominate the northern latitudes of boreal regions are adapted to short growing seasons, recurring disturbance from storms, fire and floods, and growing in peatlands. Loss of any species in these regions would have a significant impact on the ecosystem structure.

Climate change packs a one-two punch for forests

Not only does climate change have the potential to increase the numbers and range of pests and disease, it can also make forests more susceptible to these infestations. Though the future is uncertain, predicted increases in extreme weather events – droughts, floods, cyclones, and extreme temperature fluctuations – are likely to put our forests under severe stress, increasing their vulnerability. 
Of course, some pests may also be hindered by climate change. For instance, species that rely on an insulating blanket of snow to overwinter may be more vulnerable if snow cover is reduced in a milder climate scenario.

What is the future of our forests?

Nobody knows the answer to this question. However, the UK authors of the Science paper bring to light the need to do more fundamental research in understanding how pathogens affect natural forest communities. To date, most research has focussed on economically important species, yet the ecological role of forests and the ecosystem services they provide have considerable value also.
The long life span of trees has been a barrier to understanding some aspects of the infection and spread of some pathogens; the time it takes for some trees to reach a reproductive stage could outlive the careers of some scientists. However, new methods in molecular biology are overcoming these barriers these days. Understanding the process behind these pathogens will help in the prediction of their spread as well as how they may respond to climate change.
The authors also call for better management approaches that identify different classes of threat, which are defined by (i) the type of disease-causing agent (e.g. fungus, bacteria, insect), (ii) how it moves (e.g. wind, water, animal, wood imports) and (iii) the type of ecosystem service threatened (e.g. keystone species, timber value).
Management practices can also help build resilience in our forests. For example, practices that help preserve the genetic diversity of species and avoid monoculture will provide the genetic foundation that will help species resist disease. Steps to mitigate climate change may help reduce the abiotic stress on forests and reduce the expansion of pest populations.

Though there remain many unknowns and the future is uncertain, the critical role forests play globally is clear. So, if you are able, get out into a local wood or forest today and appreciate it. Those trees are cleaning the air we breathe and the water we drink. They grew that apple you brought along for a snack! They’re doing a lot as they stand there, so appreciate it…dare I even say…hug a tree?!  Who knows, you might start a trend?!
The original paper is: Boyd IL, Freer-Smith PH, Gilligan CA, Godfray HCJ. (2013) The consequence of tree pests and disease for ecosystem services. Science, 342 (6160): doi 10.1126/science.1235773
The AAAS press release associated with the paper can be found here.

Biological battles in the glasshouses

Posted on by andy.winfield

By Helen Roberts and Nicola Temple


In the calm and serenity of the glasshouses, among the flowering lotus and breathtaking orchids, there is a lethal battle going on – biological warfare between predator and prey. About two months ago, Penny started to use biological control in the glasshouses as a chemical-free means of managing pests like whitefly and aphids. Parasitic wasps and beetles are released in areas of infestation and left to do what comes naturally to them…prey upon pests.
Biological control is a system that has been used by horticulturalists since the early 1800s. The University of Bristol Botanic Garden uses biological controls as it avoids the use of toxic chemicals and also controls pests that have become resistant to pesticide treatment. The method is more economical and certainly more environmentally friendly.
However, it’s not just entirely a simple matter of releasing the predators and then forgetting about it. First, the pests in the greenhouse need to be properly identified and the proper predator controls selected. Then, it’s necessary to release the controls under the right conditions and at a critical time of the season – known as inoculative release – in order for the control to be effective.

A grizzly end for aphids

Aphids have infiltrated the glasshouses at the Botanic Garden.


The Botanic Garden is using two species of parasitic wasps to control a range of aphid species. The story is a grizzly one for the aphid (think the infamous dinner scene in the movie Alien), but with an excellent outcome for plants in the long run! Female Aphidius colemanii and Aphidius ervi seek out their aphid hosts and with incredible precision pierce the aphid’s exoskeleton with their ovipositor and lay an egg directly into the aphid. After a couple of days the aphid dies as it is consumed from the inside by the newly hatched wasp larvae. The larvae then spin a cocoon around the aphid shell and an adult parasitic wasp eventually emerges. These wasps will also control insecticide resistant strains of aphids. Of the two species, A. ervihas a longer life cycle, is larger and will select larger hosts.

The Aphidiusspecies used at the Garden are released as newly emerged adults and are best released when temperatures in the glasshouse are between 15oC to 30oC.

Wiping out whitefly

Encarsia formosa is released on little
discs infused with parasitised scales.
To control whitefly, the Botanic Garden team are using two minute parasitic wasp species, Encarsia formosa and Eretmocerus eremicus. Encarsia formosa controls whitefly populations in much the same way as the parasitic wasps of aphids except they target the whitefly scale, which is the 2nd and 3rd nymph (immature) stage of whitefly, rather than the adult. Adult Encarsia will also feed directly on the whitefly scales. Female Encarsia can lay up to 200 eggs and only a single egg is needed to kill the whitefly. The parasitoids are sold as black parasitised scales that have been fixed onto cards and these are hung under the canopy of the greenhouse plants out of direct sunlight.
Eretmocerus eremicus is slightly different in its approach in that it lays its egg between the whitefly nymph and the leaf surface. Between the 2ndand 4th nymph stage, whitefly are sessile, and so when the egg hatches after 4 days, the wasp larva attaches its hook-like mouthparts to the underside of the whitefly scale and starts to chew. After about 4 days of chewing, the parasitoid larva crawls into the body of the whitefly scale and just sits there biding its time until the whitefly starts to pupate. When the pupation phase begins, the parasitoid releases enzymes that begin to digest the insides of the whitefly and this will be the wasp larva’s last meal before it begins its transition to adulthood – a process that takes about 12 days. The adult wasp chews its way out of the remains of the whitefly scale and the cycle begins all over again.

Making meals of mealy bugs

Cryptolaemus montrouzieri is a small ladybird species that is used in the control of mealy bug. Its larval stage looks like the mealy bugs they prey on, which is a case of aggressive mimicry. Eggs are laid in amongst the cottony egg sacks of mealy bugs and the eggs hatch after 5 days. The three larval stages of the beetle and the adults will feed on mealybug eggs, young crawlers, and the honeydew produced by mealybugs.
Adults are released onto infested plants in the evening and can be encouraged to stay in an area by using netting. These predators will also eat aphids and other scale insects if their prey of choice is in short supply.

Not all biological warfare goes to plan


As previously mentioned, the use of biological controls has many advantages, including reduced costs, reduced dependence on harmful chemicals and reduced potential for pests developing pesticide resistance. However, human interference in the predator-prey relationship doesn’t always go to plan.

One famous example is the introduction of the cane toad to Australia. These were introduced in 1935 to control the Greyback cane beetle that was destroying sugar cane crops. Essentially not enough was known about the cane toad and how it interacted with the target beetle; the two species are not compatible at all in terms of a predator-prey relationship. The beetle feeds at the top of the sugar cane stalks but the cane toad can neither climb nor fly and therefore cannot reach the beetle. The toad moved in to other areas besides sugar cane and spread like wild fire. They are productive breeders, which combined with a lack of predators due to their high toxicity, led to a population explosion. Its feeding habits are highly non-specific – it will just about eat anything that it can stuff into its mouth. Their introduction, despite the best of intentions, was an unmitigated disaster. This was one example that showed just how wrong biological control can go if not researched thoroughly.
However, rest assured the Botanic Garden will not be releasing anything but well-researched and proven beneficial insects into the glasshouses. When done properly, biological control is a highly effective strategy for managing pests.