Add plant diversity to your gardens. Also make sure you have flowering plants year-round. Those are key recommendations of ecologists for enlisting natural predators to combat pests in our garden and orchards.
These tips can even help ward off pests such as the Asian citrus psyllid (ACP), which is, alarmingly, moving into our citrus trees.
Nicola Irvin, PhD, is a specialist in biological control at UC-Riverside. To describe biological control succinctly, it’s using natural enemies to manage pests and their damage. Irvin, a native from New Zealand, began work 15 years ago with the sharpshooter insect that was devastating the grape industry. Not surprising, her research today is focused more on the ACP.
“Hoverflies and lacewing have been identified as the most important predator of ACP nymphs in California.” Dr. Nicola Irvin
Enlisting our natural predators
This article doesn’t have room to discuss the many amazing aspects of beneficial insects that not only help in pollination but also kill pests. What’s important to know is thousands of overlooked insects native to our locales play vital roles. Some work by parasitizing the pest, that is they lay eggs in them so their hatched larvae then feed on the pest. Others just eat them outright.
Also important in understanding how they work, many beneficial insects and most pests go through many forms or instars (much like the monarch caterpillar). For example, the Asian psyllid goes through five forms, or instars before it’s an adult that can carry the bacterium that causes citrus greening disease.
Irvin explained that having an abundance of different types of predators and parasitoids can prevent the pest from achieving its adult size, because these natural enemies will attack them in their various forms.
Native flies and wasps are particularly disregarded insects that are receiving more notice these days. For example, many adult female wasps perform double duty. They parasitize the host pest by laying eggs in them, and also kill hosts, such as scales and whiteflies, by feeding on them. It’s important to note that none of these native wasps sting people, and they don’t build group nests. Some are tiny – the smallest the size of a grain of sand – while medium sized ones such as tachinid flies get mistaken for houseflies.
Dr. Irvin carries out research at UCR investigating the benefit of native flowering plants on the survival of parasitoids of ACP. “When I planted some red California buckwheat in my yard, I also noticed it attracted a lot of hoverflies. Hoverflies eat scales, thrips, aphids, and even the larvae form of ACP. They’re generalists, so will eat many soft bodied pests, and are very ravenous,” she said.
While ladybeetles seem to provide the most control of ACP in Florida, this is not mirrored in southern California. “Hoverflies and lacewing have been identified as the most important predator of ACP nymphs in California. Their combined effect can kill 86% of ACP nymphs,” Irwin said.
Other natural predators of pests include spiders, beetles, predatory mites, and a group called true bugs. Then there are natural predators such as birds, reptiles and even mammals such as the opossum.
Aphids among the peskiest pests
Irvin views aphids, mites, and caterpillars as the most prevalent pests for backyard gardeners in the inland areas, adding that mites can be particularly problematic in our dusty environments. “However, if you forcefully spray the leaves with water and keep the dust down, that will help reduce mite populations. Be sure to provide adequate irrigation as water-stressed trees and plants are also less tolerant of mite damage,” she said.
For cutworms, loopers, and hornworms, three common types of caterpillar, she recommends Bacillus thuringiensis, commonly called Bt, a targeted organic insecticide that only affects caterpillars. Remember, however, that the despised tomato hornworm turns into the beautiful sphinx moth.
Adding diversity and year-round flowering plants
Irvin says the most important thing you can do is add different types of plants to your yard. “Increasing plant diversity in your garden attracts more beneficial insects by providing a wider variety of habitats and alternative foods to predators, parasitoids and pollinators. Introduce lots of native plants if you can, because they are adapted to the environment and require less water. They also attract native birds and insects, like native beeflies and robber flies, that will reduce mosquitos and insect pests.”
Also, make sure you have flowering plants year-round to provide them nectar and pollen. Otherwise beneficial bugs will move off. “It’s especially good to plant flowering plants that are composites or from the Asteraceae family, which have shallow flowers that most natural enemies can feed from,” she said.
The various varieties of California buckwheat, goldfields and tidy tips are good examples of flowering plants to attract beneficial predators. Another Irvin mentioned is California poppies, because this heavy pollen producer attracts ladybugs, lacewings and hoverflies. Plus, they reseed and come back year after year. Sweet alyssum and common buckwheat also work well if you prefer non-native annuals.
She also recommends letting herbs go to flower such as coriander and dill, because they attract a lot of beneficial insects.
Use targeted approaches over general sprays
Ken Kupfer is a popular speaker on using biological controls for pests. His main home is in Florida, where he has personally witnessed the psyllid infestation. As the developer of a targeted bait called KM AntPro for the argentine ant, his work with organic and sustainable growers finds him in California nearly half the year.
Kupfer’s work on argentine ants has also made him a strong proponent of natural remedies for pests. “With argentine ants, if you can take their crazy colonies away, then the natural pollinators and predators can come in,” Kupfer said. As most people have witnessed, Argentine ants form supercolonies that skillfully forage on the honeydew that is excreted by aphids, scales and mealybugs. They are so protective that they swarm and kill any natural predators in their midst.
Both Kupfer and Irvin explained that if there is overkilling of natural predators that are present in a given environment, whether it’s by the argentine ant, or by overuse of insecticide sprays that kill anything it comes in contact with, pest levels stay high or in the case of sprays, actually rebound.
Kupfer is hoping that California can do a more reasoned approach to combatting the psyllid, especially since the psyllid is showing up in public neighborhoods. “In Florida, the use of systemic pesticides and foliar pesticides is mandated for the citrus crops. It’s really saturated the environment and it of course kills 99% of other bugs it comes in contact with,” he said.
Kupfer says employing the parasitoid wasp, a natural predator for the psyllid overseas, is a good idea, but he cautions that “it is like using a fighter plane in the battle, when we can’t forget about all the other predators.”
In 2011 Kupfer discovered the psyllid on his Florida property. He began a five-year study using 70 trees in the vicinity and various kinds of flowering plants to introduce large numbers of natural predators. He also knocked down the Argentine ants on his property. His positive results included minimal tree mortality and no need for spraying.
“We need to use the natural predators in Florida or California and feed them into our ecosystems. By taking a small area on your property and having some native plants, whether its native milkweed or buckwheat, that will usher in the assassin bugs and lacewings,” Kupfer said.
In addition to adding diversity, both experts said moderation is important in biological control. “First, if you have large diversity in your garden it will recover more easily from pest infestations,” said UCR’s Irvin. “Second, if I find aphids and kill all the aphids by spraying them, I may have a worse outcome. Instead, get out the water hose and hose off some of the aphids, but you don’t have to get rid of them all because you want to keep some of their natural enemies around.”