Category: glasshouses

Sowing Victoria

Posted on by andy.winfield

By Nicola Temple

A photo taken a couple of years ago – I
have a grasp on my son as he leans
over into the tropical pool to get a
good look. Victoria cruziana is in flower
as is the lotus above us.
Photo credit: Shelby Temple

For me, one of the highlights at the University of Bristol Botanic Garden is the giant waterlily (Victoria cruziana) that lives in the pond in the tropical glass house. Its enormous leaves, which can reach 2 metres in diameter, are studded with spines on the underside and always provide ample wow factor for visiting children (my own included).  

The plant is found in slow moving waterways in Brazil, Argentina, Paraguay and Bolivia – in places such as the Pantanal. Its pollination story is an interesting one in that it is pollinated by a beetle (Cyclocephata castaneal). Its white flowers give off a strong scent that attracts the beetles in the evening. The flower then closes around the beetles, trapping them in the flower overnight. The flower produces heat (thermogenesis), raising the temperature as much as 9oC above the ambient temperature outside, which means the beetles can maintain a high level of activity without using as much energy. It’s a thermal reward and the plant benefits as the active beetles will pollinate the flower. The pollinated flower opens the next evening, revealing a new light pink colouration to its petals. The beetles flee the flower and make their way to the next unpollinated flower.

Of course, this species of beetle isn’t found in the Botanic Garden, which makes pollination a bit more challenging. However, there are other insects in the Garden that have filled this niche and the plants have set seed over the last few years. However, this is the first year that staff at the Botanic Garden have tried to sow this seed and, so far, things are going well!
The seeds of Victoria cruziana are kept wet.
Photo credit: Andy Winfield

Replicating the natural environment

In its natural environment, the seeds from Victoria cruziana would be buried in the sediments, stirred up perhaps by grazing capybara and swirling river currents. It wouldn’t be until the high water levels following the rainy season had receded that the water temperature and the amount of light penetrating to the sediments beneath would be sufficient to prompt germination.

In the Botanic Garden, botanical horticulturist, Andy Winfield, first primed the seeds by scratching the tough seed coat with secateurs. The seeds were then sown into topsoil and covered with a layer of horticultural grit. The pots with the sown seeds were then placed in a container of water to a level about 10 cm depth above the seed. This replicates the approximate water depth in the natural environment. The water is heated to a temperature of between 30oC and 32oC; this is critical to start the germination process.

Andy scores the seeds with secateurs
before sowing.
Photo credit: Nicola Rathbone

Victoria cruziana grows around the edges of water bodies and in wetland areas where there is no forest canopy. In order to replicate the amount of daily sun it would be receiving in the tropics and sub-tropics, grow lights on a 12h on/12h off cycle were hung above the pots. Then the whole contraption was covered in plastic film to reduce evaporation and maintain humidity.

Andy had read that germination time is generally about 2-3 weeks in this type of scenario, but within a few days he noticed that the seeds were starting to send out roots and when I visited a week after sowing, the hypocotyledonous stems were clearly emerging from the seeds and shooting upwards toward the surface of the water. 

Preparing to plant Victoria out

At the moment, the water temperature in the pool in the tropical glasshouse is only about 14oC, far too chilly for Victoria. In the coming weeks, however, these plants are

likely to grow quite quickly. Andy and the rest of the team at the Garden will pot them on several

times, gradually reducing their water temperature. At the same time, Bristol temperatures will be increasing and the tropical glasshouse will start getting warmer, as will the pool. By the time the Victoria plants have a few decent leaves, the temperatures between the tropical pool and the plants will have become similar enough that Victoria can be put into the planters in the pond.

The annual light intensity here in Bristol is considerably less than Victoria cruziana would receive in South America. However, the long summer days here mean that during those months more solar radiation is received here in a single day than in tropical South America. This helps Victoria cruziana flourish in the Botanic Garden tropical pool over the summer and it will be worth a visit to see it in flower. See the series of photos below taken the day the seeds were sown.

Andy prepares the loamy mix for sowing.
Photo credit: Nicola Rathbone

The seeds are sown into a loamy mix.
Photo credit: Nicola Rathbone

The soil is covered with a horticultural grit.
Photo credit: Nicola Rathbone

The seeds are sown and are ready for immersion
in a nice warm bath.
Photo credit: Andy Winfield
The pots immersed in the warm bath.
Photo credit: Nicola Temple
Only one week after sowing, the embryonic stem
has emerged and is stretching for the surface.
Photo credit: Nicola Temple

Sources:

Seymour, R.S. and Matthews, P.G.D. 2006. The role of thermogenesis in the pollination biology of the Amazon waterlily 
     Victoria amazonica. Annals of Botany 98(6): 1129-35.

Seed sowing at the Botanic Garden

Posted on by andy.winfield

By Helen Roberts

Last month I met up with Penny Harms, Glasshouse Co-ordinator at the University of Bristol Botanic Garden to explore the many different plants they grow from seed. At the end of May the garden staff were nearing the end of a mammoth seed sowing frenzy in preparation for the summer ahead, but Penny explained that seed is continuously sown throughout the year depending on a particular plant’s germination requirements. 
We entered the glasshouses via the potting house, which contains a whole range of pots, growing media and tools for propagation. There are different shaped tampers for firming down compost, and numerous dibbers and widgers to gently prick out seedlings. The growing substrate typically used for most seed germinated at the Botanic Garden is a neutral peat-free compost composed mainly of composted bark, coir and composted green material. 
Penny explained the finer details of the growing media used, “This compost can be altered by sieving out larger chunks for those seeds that require a finer grade tilth, such as poppy species. Fine and medium grades can be produced depending on the particular size of the sieve pores with perlite often added to help with drainage and aeration.”

Some seeds need constant warmer temperatures

Seed sowing occurs in a number of places within the gardens. Some seed needs a dose of warmth to get germination underway and are sown in propagators in the glasshouses to ensure a stable warm temperature of about 22°C. Others can be directly sown into prepared soil and include many of the ballast seed garden species like Calendula officinalis and Amaranthus caudatus, which form a display in the gardens. Those species destined for the grain barge are grown under glass and have recently been ferried across to the barge and planted. I spied seedlings destined to make the watery journey including Avena sativa (oats) and Eruca sativa (rocket). 
Some of the Amaranthus caudatus Helen has grown from seed
at home. Photo credit: Helen Roberts.
Back in the warmth of the propagation glasshouses, Penny pointed to a number of seedlings planted at the start of April including the beautiful but very poisonous half-hardy annual Ricinus communis var. gibsonii, otherwise known as the castor oil plant. This species is grown for its dark red metallic foliage and is planted out in the hot border once all risk of frost is past. A dark purple bronze variety of this species, equally as lovely, is ‘New Zealand Purple’. 
Penny explained how to grow Ricinus communis, “The species is easy to germinate from seed, but does require a temperature of 20-25°C, so it is best grown in a propagator case in a cool greenhouse where the temperature can be kept stable. Once big enough, it is carefully pricked out and hardened off to then be planted out in June.”
Growing steadily under cover of glass are a number of seedlings destined for the hot border that act as effective border fillers. They include the lovely canary creeper, Tropaeolum peregrinum, a half-hardy annual climber with pale green stems, leaves and yellow flowers, and Tropaeolum majus ‘Black Velvet’, another half-hardy annual with beautiful almost black flowers. Other climbers sown in the glasshouses are the common but wonderfully scented varieties of Lathyrus odoratus (sweet peas). The sunny yellow flowers of Tithonia rotundifolia ‘Yellow Torch’ and Helianthus ‘Mongolian Giant’ currently growing in the glasshouses are also useful border fillers. The latter species is a giant that grows up to four metres high, making it a favourite amongst children. More delicate looking blooms also need the warmth of the glasshouses for germination, such as Digitalis lanata (Woolley Foxglove), which has woolly spikes of fawn coloured flowers with a pearlized lower lip. 

Some seed is worth the wait

Some seed germinates very quickly if conditions are right – sometimes within a week – but other seed can be extremely difficult and requires a great deal of molly-coddling in order to get germination success. Penny carefully pointed to a seedling of Cardiocrinum giganteum var. yunnanense, the Giant Yunnan Lily, which as an adult is a beast of a plant and can grow up to 2.5 metres with huge fragrant nodding creamy inflorescences. This species is normally propagated from either seed or by bulbs and can take as long as 4 to 5 years before it flowers. It is a monocarpic species that will die after it flowers leaving offsets, which will then become subsequent plants. This is certainly a species for a patient gardener; it takes a long time to flower but it also takes a considerable time to germinate as Penny emphasised.
Blooms of the Cardiocrinum giganteum var yunnanense
– the Giant Yunnan Lily.
Photo credit: Col Ford and Natasha de Vere
[via Flicr CC licence 2.0]
“This species can be tricky to germinate,” explained Penny, “This one has taken over a year to germinate so it is quite special.”
This is a species that appeals partly due to the fantastic inflorescences, but it has freaky (and rather scary) looking seedpods that resemble vegetative heads with fangs. In my opinion, the lengthy germination and time to flowering is worth the wait. 
Tropical and subtropical plants that only survive as mature specimens in the glasshouses can be even trickier to propagate by seed. The seed from Passiflora, a large genus of mostly vines needs to be sown quickly when fresh as dried seed takes much longer to germinate. The subtropical vines of the Aristolochia species, aptly named Dutchman’s pipe, require similar treatment. Species grown at the gardens are A. labiata and A. trilobata, and have beautiful ornate blooms of about 15cm. A. labiata flowers resemble the mottling and coloration of a rooster’s comb. Although tropical and sub tropical species can be a bit trickier to grow up from seed, most species can be sown throughout the year. 
Some species do not need the cosseting of warmth and will happily germinate outside although some seedlings, like borecole (kale), are protected with wire mesh to prevent bird damage particularly from pigeons. Species that have germinated and are growing happily outside at the Garden include the mixed colours of Salvia viridus, more commonly known as the Clary Sage. This produces small spires of lovely flowering bracts loved by pollinators. These are intended for the Mediterranean beds along with the tall spires of Echium italicum, the Pale Bugloss, a beautiful pyramidal plant belonging to the Borage family and Viola arborescens, a pretty violet with large lavender coloured flowers. 
Other plants need to be sown at different times of the year and some species have enough flexibility in this that you can sow depending on when you want a plant to flower in the subsequent year. For most gardeners, autumn and spring sowing are the busy sowing months. At the Botanic Garden, for example the open faced flowers of Papaver somniferum, (Opium Poppy), are sown in the autumn.
“These poppies are treated differently to other poppy species,” said Penny, “in that they have been sown in September and will be good strong plants by the time they are planted out the following year, producing flowers earlier than if seed had been sown in the spring.”

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.

Undergraduates get their first glimpse at the garden

Posted on by andy.winfield

By Alida Robey

I’ve been promising myself a visit to the University of Bristol Botanic Garden since I arrived in Bristol four years ago. Life has intervened. Yet when the opportunity came to join the new intake of students from the University on their first practical of their 3 year undergraduate degree, I leapt at the chance. 
Once there, the thrill of the plants, garden, stories and mysteries within, were hard to resist!  I joined the briefing given by the Garden’s curator, Nick Wray, as he introduced the day’s second group of 70 students (over 250 students attended the practical over two days) to their PhD student demonstrators – there to inspire the undergraduates about different aspects of the gardens.  

An introduction to the day

These biology and zoology students were visiting the garden as part of their ‘Diversity of Life’ module – taking a first-hand look at some of the adaptations that have enabled plants to diversify into the more than 400,000 species that exist today. Beyond this, however, the practical offers an opportunity for the students to get to know each other and learn to work collaboratively, gain confidence in sharing knowledge,  as well as orientate themselves to this incredible resource available to them.
Nick and the demonstrators were up against time and the logistics of manoeuvring 70 students around 6 ‘work stations’. Students were split into manageable groups and two volunteer guides were brought in to assist moving the groups swiftly through the rotation of topics presented around the garden.
Off we went. As a newcomer myself, I shared the sense of wonderment and awe one student expressed as she exclaimed at how much more there was at the Garden than she had expected. She pointed out how interestingly organised the gardens were, which effectively revealed the story of plant evolution – a set-up that Nick had explained was unique to the University of Bristol Botanic Garden.

Into the glasshouses for plants that eat and are eaten

I followed a group into the glasshouses where Edith showed us the adaptations plants have evolved to cope with extreme habitats. Plants from very different families share common features that are adaptive in similar conditions. Euphorbia, for example, which grows in the deserts of Africa is so similar to the form of cacti found in the deserts of America that they are often misidentified – this is an example of convergent evolution.
The striking Haemanthus coccineus – a native of South Africa -flowers and then sets seed in autumn to coincide with the first rains, giving the seedlings a full rainy season to develop. The leaves appear well after the flowers to reduce the amount of moisture lost prior to the rains. Edith pointed out carnivorous plants that have adapted to nutrient poor habitats. She showed us a plant that produces citronella to deter insects and a species that looked half eaten to make it less attractive to herbivores.
The group was then passed along to Nick who ushered us into the tropical greenhouse to reveal further wonders, such as the orchids of Mexico that require pollination by moths to produce vanilla pods. When commercially produced in the Comoros Islands, pollination is done by hand for every flower – a task often given to children in this struggling economy. We saw the giant lily pads of Victoria cruziana. Reminiscent of triffids, Nick pointed out that in summer they have to be cut back every three days to prevent them growing out of the pond.
Nick Wray shows the students the largest seed in the world.
Photo credit: Nicola Temple

Hmmm… time to escape back into the fresh air where things were growing at a more manageable pace for me, but Nick continued to show the group other commercially important plants, such as lotus, bananas and cotton. He held up a specimen of the world’s largest seed – that of the sea coconut or coco de mer (Lodoicea maldivica), which can weigh up to 30 kg.

The students were then taken into an area of the glasshouses that’s not open to the public and shown some very rare and unique plants, including Amborella trichopoda, which is of particular interest because molecular analyses suggest this is one of the earliest flowering plants. It is the last remaining species of a group that first appeared on Earth more than 140 million years ago, when dinosaurs still dominated the animal kingdom.  A sprawling shrub native to New Caledonia, Amborelladoesn’t cope with changes in humidity very well, so it is kept behind plastic to control the humidity.
Some students scribbled madly, while others just chose to listen as Nick enthusiastically explained what a unique experience this is for University of Bristol students.  ‘Until last year, Bristol was the only botanic garden in the UK growing this plant,’ said Nick. (The University of Cambridge has recently acquired one.)

New Zealand garden – survival of the species

In the New Zealand garden, Dave showed the radical ways plants survive difficult conditions; in this case, the attentions of the now extinct Moa bird. This was graphically illustrated by Pseudopanax, which starts off its first 10 years or so as a sapling with hard, spiky, downward facing sword-like leaves. Once considerably taller – namely beyond the reach of 3m tall Moas – the trees don’t invest as much energy into being unpalatable and transform into an unrecognisably different form, with soft and safely inaccessible leaves reaching to the light.

Angiosperm phylogeny explained

A group gathers around the pond to learn about angiosperm
phylogeny. Photo credit: Nicola Temple

I moved on to hear about angiosperm phylogeny; a new term for me, but more exciting and less daunting than it sounds. In the past, plants were classified into family groupings based on their physical characteristics. With the advent of DNA sequencing in the last 20 years, we can use genetic relatedness to help us understand how plants have evolved. James, our demonstrator, pointed out some of the oldest species of flowering plants, including star anise (Illicium verum). This area of the garden is organised into the two major groups of flowering plants monocotyledons (seed has single embryonic leaf) and dicotyledons (seed with two embryonic leaves). The monocots include plants such as orchids and grasses, including agriculturally important species such as rice, wheat, barley and sugar cane. The more familiar garden plants, shrubs and trees, and broad-leafed flowering plants such as magnolias, roses, geraniums, and hollyhocks are dicots.

Learning in the garden beats a textbook any day

Speaking with the students, they said they enjoyed being able to touch and feel the actual plants, make comparisons and learn within this physical context. They could see as James explained how even though Protea, lotus, Banksia and London plane tree (Platanus x acerifolia) looked very different, their DNA suggests they are more closely related than they appear. Genetic relatedness is traditionally illustrated using a cladogram – a branching tree with scientific names at the end of the branches, with no sense of what these species look like. What an opportunity to see what the diversity at the end of those branches can look like!
Students use pens to see how flowers are
adapted to distribute pollen on the
pollinators that visit them.
Photo credit: Nicola Temple

My time ran out before I could get as far as the sessions on pollination and plant evolution!  With my head spinning from this intensive and whistle-stop tour of some of the delights and extraordinary features of this garden, I sat on a bench in the autumn sunlight to reflect on the afternoon with fellow blogger, Nicola Temple, who had invited me take part in this day.

Like many of the students I spoke with as we went from location to location, I was delighted to have had the opportunity to understand the great thought behind the layout of the gardens.  There was far and away more here than I had bargained on.  I wanted to keep going but knew I could only take in so much on my first visit.  As we had gone around I had been surprised as an observer to note how quiet the students were, very few asking any questions.  Having stood back from it though I wonder if, like me, they were overwhelmed by the hidden depths to this exceptional garden. I’m certainly going to seek every opportunity to spend more time here, whether learning or simply enjoying the peaceful and stunning surroundings.
And I daresay I will come across many of the students from this day, pursuing their studies and enjoying the sheer delight and boundless wonderment that nature continues to shower upon us and that this garden so beautifully illustrates.

Children take a ‘walk through time’ at the Bristol Botanic Garden

Posted on by andy.winfield
It’s 1 pm, the sun is shining and the volunteer guides are starting to gather near the welcome lodge in anticipation of 60 Year 4 children arriving at the Botanic Garden for a tour. It’s my son’s school, Horfield CEVC Primary School, and so I’ve decided to come along for the tour and get a glimpse into how the Garden is viewed through the eyes of eight and nine year olds.
Anne is one of the volunteer guides at the garden and she and I get chatting while we await the children’s arrival. She was a teacher for 40 years – teaching at GCSE and A levels. She laughs as she tells me she was a bit nervous she would find touring younger children challenging when she started giving these school tours at the Garden. She soon found, however, that though it was different from teaching upper level students, it was also just good fun.
Volunteer guide Tony gives a talk to Year 4 students
from Horfield CEVC Primary School prior to their tour.
“I’m not responsible for making sure they learn the curriculum, I’m here to entertain them with interesting stories about the plants we have here in the garden – to get them excited and inspired by what they see,” Anne says from a shady bench.
The guides have come prepared; they know the Horfield children have been learning about Egypt and different habitat types. As well as discussing the logistics of touring sixty children around the garden in small groups, they check in with each other about plants that might be important to point out that will link to the topics and themes they’ve been learning in the classroom.
Then the coach arrives.

A tour through the glasshouses

Before the children break into small groups to go around the garden, volunteer guide Tony gives a very brief talk about what plants need to survive. The children enthusiastically put up their hands in response to Tony’s question of what plants need to grow. Horfield Primary is lucky enough to have a garden and most of the children will have grown plants in the classroom at some stage (my son brought a runner bean home from school a few weeks back that’s doing splendidly). So, although photosynthesis hasn’t been taught by Year 4, there are other opportunities where the children are learning the basic needs and processes of plant growth.
Students have a look in the pitcher plants in the sub-tropical
zone of the glasshouses.
Baking sun and a tight schedule keeps the introductory talk brief and I follow Anne’s small group down into the glasshouses. She points out the Deadly Nightshade along the way and talks about the large black poisonous berries – a good wow factor for the kids right off the start!
In the sub-tropical zone, the children talk about the challenges of plants growing in a rainforest beneath a heavily shaded canopy and some of the adaptations they’ve made to get alternate sources of food. They have a look into the pitcher plants to see whether any wayward insects have fallen into the plant’s pitcher-shaped trap. As Anne walks by the lichen, she talks about how lightning changes the nitrogen in the air into a form that’s easier for plants to use – lichens need a continual supply of nitrogen to survive. Lightning helps feed plants? This has the children’s attention.
Having a look at the giant lily pads in the pond in the
tropical zone of the glasshouses.
In the tropical zone the giant lily pads (Victoria) impress the children immediately. Then Anne points out the papyrus that’s growing at the corner of the pond and the children quickly make the link between this plant and the papyrus paper that they’ve been learning about in their Egyptian studies. As I switch between the different groups I hear one of the other guides tell a story from Egyptian Mythology about how the Scorpion-godess, Selkis, protects the child Horus by hiding him in a papyrus thicket.
The lotus plants (Nelumbo nucifera) are also linked to Egypt as there is a Nymphaea lotus that grows in the Nile. Anne encourages the children to splash some of the pond water onto the leaves of the lotus plant and I watch as two girls are astonished at how the leaves repel the water.
Water beads off the leaf of the lotus plant.
Some of the other highlights in the tropical zone were the cocoa plant, vanilla and cotton. The Madagascar periwinkle (Catharanthus) is also pointed out for its utility in treating leukaemia.
As the children stand beside the cacti in the glasshouses, they are told that some of these plants will go 10 years without ever experiencing rainfall – longer than these children have been alive. Anne and the other guides talk about all the adaptations these plants have made to go for long periods without water.
Students are given an opportunity to experience what
happens to Mimosa plants when you touch their leaves.

Time travel through the evolutionary dell

I leave the heat of the glasshouses to join Tony’s group as they begin their walk through the evolutionary dell. Tony is telling the children about horsetails (no friend of the gardener) and pointing out the nodes of the plants. He tells them that 350 million years ago this little snippet of a plant would have been the size of a tree! The kids crane their necks up imagining and as we walk toward the tree ferns one of the girls says “It feels like time travelling!”
Tony takes the children on a walk through time in
the evolutionary dell.
Indeed it is like time travelling in the dell. In the 100 m span of the dell, we travel 200 million years from the horsetails (350 mya) to the first flowering plants (150 mya), such as the magnolia that’s on the left as you leave the dell. Surrounded by ferns, moss, horsetails, Wollemia and other conifers, the guides tell the children about how plants reproduced before the evolution of flowers and pollinators.

It’s never long enough

Somehow an hour seemed to fly by and before long the guides were rushing through the last few displays before sending the children off on their coach. As I had the opportunity to hop between the different groups I got the great sense that each group would have left the garden with a different experience as each guide has their own style and favourite stories associated with the garden. It’s never possible to see everything, but hopefully that means some of the children will encourage their parents and guardians to bring them back for another visit!
Tony holds up a horsetail and talks about plant nodes.

Linking to the curriculum

Mrs Amy Parkin, one of the Year 4 teachers at Horfield Primary, was kind enough to speak with me the next day after the tour about how tours such as this link with the classroom curriculum. This is the first time Horfield Primary has done the tour at the Botanic Garden and it was prompted by Curator Nick Wray giving a talk earlier this year to the Key Stage 2 children.
“We had two weeks where we talked about prehistoric Bristol, dinosaurs and fossils,” said Mrs Parkin. “Each class did a science trail with various outside activities and we also had speakers come in to talk to the children. Nick spoke about what plants would have been around 160 million years ago and he brought in some different species to show the children.”
As well as learning about Egypt, the Year 4 children have also covered the topic of habitats under their science curriculum and there are also cross-curricular links with their geography topic of water.
“The tour at the Botanic Garden helped extend the children’s knowledge on habitats,” said Mrs Parkin. “We focused on animals in different habitats in the classroom and in the tour we saw how plants adapt to different habitats as well.”
This tour will also give the Year 4 students a taste of what lies ahead as they will have plants as a topic in Year 5.

Talking with the students after the tour

After the tour I had a chance to speak with Megan and Henry about what they thought of the Botanic Garden. Megan said “I really liked the giant lily pads, especially since a small child could sit on one!”, while Henry really liked the giant lemon that was in the glasshouse.
When I asked Megan and Henry what the most interesting thing they learned was, Megan said she couldn’t believe that some plants can live for 10 years without water. Henry, on the other hand, learned something new about pollination, “There are lots of different bugs that pollinate plants – blowflies and beetles – and birds too!”
The Botanic Garden will run about 15 school tours during the months of June and July, with the help of their dedicated volunteer guides. These tours are in keeping with the Garden’s mission to promote education and awareness as well as to encourage and foster interest in plants within the Bristol community. In fact, the garden would like to run more school tours, so if you are involved with a local school and are interested in a trip to the Botanic Garden, please contact them via:   www.bristol.ac.uk/botanic-garden

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.