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Wednesday, 31 July 2013

Biological battles in the glasshouses

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. ervi has a longer life cycle, is larger and will select larger hosts.

The Aphidius species 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 2nd and 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.