Biological control of garden pests by insects, mites and parasites

Using beneficial insects, mites and parasitoids to control pest arthropods, and other biological problems such as fungi, pathogens and weeds, is known as biological control (biocontrol). It’s gaining popularity as a viable, non-toxic solution to many biological issues on horticultural sites, in farming applications, and in home gardens. Chris Thompson of Bioforce, New Zealand’s largest supplier of biological control organisms shares his knowledge.

“Anyone with a pest problem able to be resolved by the application of biocontrols (beneficial organisms) can access this safe, effective mode of pest eradication or at least achieve a healthy balance,” says Chris. “Eradication isn’t always advisable or cost-savvy. It’s more economical to strike a balance. If you have good bugs living on the site and you’re okay with a few pests which are under control, you don’t need to introduce new beneficial insects all the time. It’s easier to aim for biocontrol rather than spraying to eradicate when the pests may just move to your neighbours for a while where you can’t get to them.”

Potential biocontrols also exist for weeds such as ragwort.

Biocontrol development

Biocontrols are discovered through field observations in a crop or in nature where a pest is observed to see what arrives to eat it. The number eating are observed and assessed for viable commercial breeding. The pest is also observed for its capacity to viably control the target pest. For instance, parasitoids exist for Green Shield Bugs but they only parasitise 10-20% of the target population. They subdue it, but not well and the pest can still flourish.  By contrast, Enforce (a tiny wasp) will parasitise 80 percent or more of Whitefly nymphs.

Biocontrol in New Zealand

Two biocontrol companies operate in New Zealand. Bioforce, and Zonda Beneficials.

Bioforce has operated for twenty-six years having taken over Crop and Food research from the DSIR, and in 1997 also taking DSIR over.

Chris’s father worked for DSIR and had worked closely with Crop & Food (now Plant & Food Research) researchers. The Thompsons ran a Bumble Bee business, but friendly pesticides allowing bees to live in the tomato glasshouses didn’t exist, so they developed alternatives.

“It was common to use a hormone agent to ‘set’ tomatoes before the commercial use of bees,” says Chris, “however spraying the setting agent on the plants wet them, causing pathogens, botrytis, etc. Then fungicides became necessary, which was a major driver in going away from hormone sprays.”

Bioforce does a lot of research but is also involved in joint projects. An initiative known as A Lighter Touch, co-funded by MPI with Hort NZ and industry groups, includes a Bioforce research project to reduce pesticide and fungicide use on tomatoes with the aim of leading the world in sustainable horticultural production.

“More than 90% of New Zealand tomato production was under biocontrol until the tomato/potato psyllid arrived and wrecked things in 2005,” says Chris. “Psyllids can vector a bacterium which eventually kills tomato plants or causes zebra-chip in potatoes with just a few minutes of feeding on a plant. Commercial growers found that spraying crops for psyllids killed them but not the Whitefly which is resistant to most agrichemicals. Incompatibilities between agrichemical targets and the species actually affected by them create huge issues for growers but we’ve found native insects that seem able to work together to kill psyllids and eat Whitefly. That saves having to spray so we can keep clean crops all year. A Lighter Touch has two years to run but some products are already available.”

Spray choices should be made that don’t affect other organisms or cause alternative pests to flare up. Bioforce lists compatibilities on their online database, and are happy to provide direct recommendations.

Advantages of biocontrol

Chris says research on the unintended consequences of pesticide and fungicide use shows alteration in sugars and nutritional content of fruits. Like humans, plants contain beneficial fungi which help sustain, them but fungicides can damage these necessary fungi which plants require to perform their daily functions.

  • Biocontrol agents are non-toxic so can be distributed without PPE (Personal Protective Equipment).
  • No withholding period so harvesting can be immediate.
  • Bugs are lighter than spray-packs making them easier to deploy.
  • No spray damage or burning to crops.
  • Bugs work consistently so repeated laborious spraying regimes aren’t required.

Biocontrollable Pests

Biocontrollable pests include: Chicken-Mites, Spider-Mites, Whitefly, several Aphid varieties, Thrips, Grass Grubs, Psyllids, Mealy-Bug, Botrytis, Grey Mould, Fleas, Beetles, Cutworm, and more.

Chris says, “When Landcare Research (the Crown Research Institute conducting science and research focused on environmental issues, opportunities and solutions) selects organisms to control weeds they take great care to ensure those organisms cannot cross to other potential host plants which are native or of ecological value in New Zealand.”

“We don’t want another cane toad situation. They were imported to Australia to control a sugarcane insect but tests to see if they could survive, and if they had natural enemies to keep them under control, weren’t carried out. It’s a nightmare example of the importation of a foreign organism going wrong.”

The importation of cane toads into a country where they thrived beyond anyone’s imagination has created a devilish problem in Australia. They are poisonous to predators and have spread over huge areas of Queensland, into the Northern Territory, New South Wales, and north-western parts of Western Australia.

“Pyrethrum spray can kill one pest,” he says, “but destroy several beneficial ones as well.  It may be organic, but it’s still a toxic product.”

Guava Moth

A study of Guava Moths which can jump onto other plants species in multiple generations per year, is in train. Chris says Nemastar will kill them at various stages but there’s more work to do.

Fall Army Worm

Get Fall Army Worm early with BTK sprays. Native Trichogramma wasps will help as biocontrollers but their ovipositors are not long enough to penetrate more than the first of several layers of eggs so are inefficient.

Codlin Moth

Annual applications of Nemaplus is the main method of control in Europe and is also in use in New Zealand.

Black Spot

Using bacillus subtilis products such as Superzyme, Fulzyme, Serenade, etc., will control black spot.

Biocontrols under research

Bioforce is working on a tomato project already running for two years, and also on a Thrips control in strawberries with a native Pirate Bug. The strawberry project was plagued by constant rain with growers losing fruit to water damage and pathogens rather than Thrips.

Fall Army Worm and Shield Bugs continue in their pestiferous ways with no options yet apparent but for most other pests, controls are either in the pipeline or already available.

New pests are generally ignored by birds and insects because they don’t recognise them so the pest multiplies in the first two years, but eventually they’ll sample the intruder and predation begins. Psyllid is less problematic now because it is predated by many things.

Replenishing biocontrol agents

“Some biological agents like Mite E for spider-mites are highly selective and walk past anything else that would fit in their mouth in search of the perfect spider-mite meal,” says Chris. “Others, like Mite A or Tasman Lacewings, are generalist and will try most things. Most biocontrols require the pest to be present even at low numbers, and generally persist until the following spring when they’ll build up under their own steam. Pests build up and peak over spring, while predators peak in summer. If you release a small number of beneficial insects early when the pests are present and beginning to be problematic, the beneficials stamp them out before they peak. Ideally for a home garden, buying once or twice a year works well, and sometimes you might get a season when the pests don’t return. Aphids and Thrips biocontrols cost between $18 and $22, and most are under $40.”

Biocontrol successes

Hops are a leading New Zealand export crop. Chris believes only one grower used an insecticide this season (2023). Growers supplying Japan are not allowed to use pesticides. The majority of fresh-cut orchids are also under biocontrol. Poultry giants, Tegel and Ingham’s, constitute 90% of the market and use biocontrols to manage chicken-mites. If the chicken-mite biocontrol runs out of food, most of the control will die, but if it’s introduced into a modest population of mites the predator population tends to endure for the life of the flock.


“Biocontrol definitely has a viable future if you look around the world,” says Chris. “Pesticide development peaked in the 1990s, yet from 2010 to 2020 less pesticides were registered globally than during the 1950s when they were newly invented. Every year it’s becoming harder to grow using chemicals i.e. the avocado industry has seen the pesticide application intervals of several sprays (between spraying and harvesting) increase from ten to 180 days.

Shorter version first published in Organic NZ – Sep/Oct 2023 – Vol.82, No.5

© Bioforce all images – with thanks for use

© Theresa Sjoquist – July 2023