Everything you want to know about Biocontrols

These days, it seems as if there’s a new invasive species every time we turn around! Just this summer, I’ve found the dreaded jumping worms in my garden, and I’m seeing many new invasive species on my walks along the floodplain of the Ottawa River. This past spring, I spent a good part of one morning removing garlic mustard from the small area of the riverbank at the end of my street. By midsummer, I’d found another new invasive – a rather pretty vine with heart-shaped leaves and little yellow flowers. This turned out to be red hailstone (also known as golden creeper, tuber gourd, wild potato or Thladiantha dubia). It is another of the many ornamental plants introduced by the nursery trade that has proven to have thuggish tendencies. Like kudzu, red hailstone will happily scramble over native vegetation, achieving such dense colonies that it suffocates or strangles anything in its path. On days when I despair at the damage these imports are wreaking on our environment, the thought of biocontrols brings me some comfort.

What are biocontrols?

Cornell University defines biocontrols broadly as “using beneficial organisms to reduce populations of pest organisms, or to maintain them at sufficiently low levels.” Biocontrols can be plants, animals, fungi, or microbes, and they are an important part of the arsenal of tactics employed in Integrated Pest Management.

Introduced species become invasive in a new environment that lacks the pests, diseases and / or predators with which the species evolved in its original environment. So, it makes sense that introducing one or more of these biological controls from that environment will help to reign in the tendencies of an introduced species to become invasive. However, without appropriate precautions, this approach can go badly wrong.

The mad case of the Australian Cane toad

An early and disastrous example of a poorly thought -out biocontrol agent was the introduction of the cane toad (Rhinella marina) to Australia. In the early 20th century, Australia was emerging as a major sugar cane producer. However, larvae of native beetles ate the roots of the sugar cane, making a serious dent in production. In 1935, after about a decade of government-funded research, an Australian government entomologist imported a breeding cane toad and subsequently released 2,400 toads near Cairns, Queensland. The release was done with no studies to find out if the toads would eat the target native beetles. It turned out they didn’t. Nor were there any studies on potential environmental impacts of the introduction. Unfortunately, cane toads proved to be an environmental nightmare. Native animals that eat the cane toads often succumb to a poison they exude. Populations of these native animals have since declined. Meanwhile, populations of cane toads have continued to expand and spread across Australia.

How do we make sure biocontrols are safe?

Using biocontrols properly takes a lot of study, laboratory work, and testing before any bio-agent is released. It can take decades and millions of dollars to find the right biocontrol and ensure it will be appropriate for the environment where a release is planned. The introduction of the cane toad to Australia missed two of the most important steps – making sure the biocontrol would be effective against the species it was supposed to target, and undertaking environmental studies to ensure there would be no unintended consequences.

The study of new biocontrols begins in the home environment of the target invasive species. What ecosystem interactions keep that target species under control? Is there an animal or insect that eats it? Is there a pest or disease that limits its spread.

Once candidate control species have been identified, the next step is to see how these control species might fare in the new environment. Scientists begin by asking questions: such as, how specific is the relationship between the control species and the intended invasive species? Or, is the relationship exclusive in the home ecosystem, will it remain so in the new environment?

For example, are there native species similar to the invasive species that might also be harmed by the introduction of a biocontrol? This is the case for Phragmites species which have become so invasive in our roadside ditches and wetland areas. The native Ontario Phragmites species is important to healthy Ontario wetlands. Work to introduce a biocontrol for the invasive species must ensure that any biocontrol will be specific to the invasives, and not harm the natives.

How do biocontrols work?

Given the time, cost and effort involved in properly introducing a new biocontrol, it’s understandable that not every invasive species has a biocontrol. Nor should we expect that research will be conducted into biocontrols for every invasive species. The focus is usually on finding controls for invasives that do the most economic damage. Even then, there’s no guarantee that an effective biocontrol will be found. Sometimes, chemical or cultural controls are more effective.

Once a promising biocontrol has been identified through laboratory and greenhouse tests, it must be capable of surviving and multiplying in the wild – but not too much! This past spring, researchers from University of Toronto and Agriculture and Agri-FoodCanada identified promising biocontrols for both phragmites and garlic mustard (Alliaria petiolata).

Will biocontrols save biodiversity?

Invasive species are recognized as a major contributor to loss of biodiversity. Along with habitat destruction and over-harvesting, the introduction of a novel species that lacks competitors and has aggressive behaviours, can be a significant threat to native species.

One way to address the harms of invasive species is by using biocontrols. Biocontrols can be plants, animals, fungi or microbes. Their important characteristic is that they prey on invasive species, eating it or infecting it so its vigour is reduced, and it becomes less of a threat to native species. In terms of Integrated Pest Management, biocontrols are at the upper end of the spectrum because preventive and monitoring strategies have already failed.

Biocontrols are not a panacea. They cannot eliminate an invasive species. They can only help to bring the environment back into balance – but it is a new equilibrium that includes some populations of the invasive species.

New biocontrols for two invasive species

Among the nastiest of invasives species in Ontario is phragmites (P. australis), which flourishes in ditches along roadways, easily outcompeting native wetland species, such as cattails (Typha spp.), wild rice and orchids. Introduced to North America as an ornamental species in the 1800s, phragmites has taken more than a century to become truly problematic. Now, at last, there are biological alternatives to labour-intensive weeding and costly chemical sprays.

The phragmites controls are two species of moths (Archanara neurica and Lenisa geminipuncta), whose larvae bore into the plant’s stems. Both moth species reproduced well in field trials and have done a good job of controlling the invasive phragmites, while leaving the native species alone. The releases mostly took place in south-western Ontario, so it will be some time before populations of these two moth species extend their range as far as the Ottawa area.

The second pernicious invasive species is garlic mustard. One of the oldest spices used in Europe, garlic mustard was probably introduced to North America by the first European settlers. While is has few predators in North America, one group of European beetles, weevils of the genus Ceutorhynchus, do feed almost exclusively on garlic mustard. One species focuses on eating the crown of the plants, while another eats its seeds.

Three challenges have been identified with the introduction of these Ceutorhynchus weevils. First, the samples collected in Europe were infected with a parasitoid, so researchers had to make sure that rearing and releasing the weevils didn’t also inadvertently introduce this European parasitoid. Second, the weevils are picky little critters and rearing them requires precise conditions. At a Master Gardener conference this past spring, Dr. Michael McTavish, who has been working with University of Toronto researchers to raise these insects, noted they had only successfully raised a few hundred of them. “We could almost name them individually!”, he said. Third, garlic mustard is in the same family as other economically important brassicas like broccoli and cabbage. So, the biocontrols must be specific to garlic mustard and not expand their diet to these other species.

The crown-boring weevils have now been introduced into south-western Ontario, but with an introductory population of only a few hundred, it will take years before they become an effective control.

Examples of biocontrols in action

There are several other invasive species for which we now have effective biocontrols. These days, I hardly see any red lily beetles (Lilioceris lilii), because they are effectively controlled by a small parasitoid wasp (Tetrastichus setifer) that is otherwise harmless.

For years, purple loosestrife (Lythrum salicaria) all but disappeared from our ditches and marshlands because two species of black-margined loosestrife beetle (Galerucella calmariensis and G. pusilla) damage its buds and shoots, and interfere with its ability to flower, seed and overwinter.

There is even a range of biocontrols for the dreaded Japanese beetle (Popillia japonica). In Ontario, the parasitic winsome fly (Istocheta aldrichi, family Tachinidae) is proving effective. I see fewer Japanese beetles and the ones I do see seem smaller and less vigorous. In the image below, note the two white dots on the insect’s upper back. These are eggs of the parasitic fly. If you catch a beetle with these white dots, don’t kill it! We need these infected Japanese beetles, more specifically their parasites, to multiply.

Limits to biocontrols

The relationship between a target invasive species and its biocontrol is essentially one of a prey and its predator. As a result, biocontrols can never eliminate an invasive species. They just keep it under control.

Many invasive plants leave seeds in the soil where they grew. When the population of the biocontrol species drops, those seeds have the chance to grow. This is the case with purple loosestrife. A plague in the late 1990s, this invasive, albeit pretty, wetland plant all but disappeared by the 2010s. Now it’s back. In another five to ten years, populations of its biocontrol species will have rebounded enough to get it back under control.

Biocontrols are not a panacea and they will never allow us to restore an environment to its original state before the introduction of an invasive species. However, biocontrols will help to keep invasive species under control, creating a new environmental equilibrium where native species once more have a fighting chance.

Finding ways to effectively reduce the number of invasive species entering Canada would be a much better alternative! As my old Mum used to say, “An ounce of prevention is worth a pound of cure.”

“A garden is like a river. It flows, it’s always moving, and it’s never the same. It never reaches anywhere other than this moment.” – Monty Don