Rearing predatory insects | Plants teeming with pest predators prove effective in saving crops
LINDELL BEACH, B.C. — U.S. scientists have developed a method to use plants as storehouses, or “banks,” to raise predatory insects that can migrate to local cash crops and feed on pests.
“(The idea is that) you rear the banker plants in the greenhouse and move them to a cash crop,” said Cindy L. McKenzie, a U.S. Department of Agriculture entomologist in Fort Pierce, Florida.
The concept has been around for a couple of decades but is still considered relatively new.
It is based on a three-pronged premise:
- The banker plant.
- The predatory insect.
- The pest insect damaging cash crops.
Predatory insects are raised on the banker plants and the insects eat what is available to them so that they can thrive and lay eggs before leaving to prey on pest insects on nearby cash crops.
Their eggs, which are left to incubate on the banker plants, set up the next generation of predators to control pests, and the host banker plant provides food for the offspring in the form of pollen.
The twospotted spider mite is one of the most persistent pests of vegetables, particularly green beans, causing leaf damage, decreased growth vigour and often death.
Spider mites have traditionally been controlled with pesticides, but their short life cycle and high reproductive potential have resulted in a resistance to a broad spectrum of pesticides.
McKenzie worked with Lance Osborne, an entomology professor and associate director of the University of Florida’s Mid-Florida Research and Education Center, and postdoctoral researcher Yingfang Xiao, who was lead author of the study, to evaluate the use of corn as the banker plant to host the predatory gall midge, which feeds on spider mites.
One study researched the use of papaya as a banker plant to raise a non-stinging wasp that lays eggs in immature whiteflies, killing the whitefly when the wasp offspring hatches inside.
The predatory wasps were given both the papaya whitefly and the silverleaf whitefly to feed on. They ate both but were especially effective at ridding crops of silverleaf whiteflies close to the banker crops.
This biocontrol system has been used effectively in commercial herb, tomato, cucumber, eggplant, lettuce, and poinsettia greenhouses in Florida.
Ideally, farmers would need to identify the pest threatening their crop to select a suitable predatory insect and banker plant to raise it.
“You can use more than one banker plant system,” said McKenzie. “In (growing) poinsettia, we used both the papaya banker plant to control whitefly and the ornamental pepper banker plant to control bad mites, whitefly and thrips. Some predators eat multiple types of pests.”
Both the western flower thrips and chilli thrips feed on foliage, resulting in distorted, puckered and twisted leaves or plant death. Chilli thrips cause $3 to $6 billion US in crop yield loss annually in the United States.
The study, which used ornamental peppers as the banker plant, evaluated the predatory mite Amblyseius swirskii for survival and population increase. The mite feeds on silverleaf whitefly and the western flower and chilli thrips.
Three ornamental pepper varieties were studied in lab and greenhouse conditions: Masquerade, Red Missile, and Explosive Ember.
The results showed all three varieties were excellent banker plants and each plant could support more than 1,000 predatory mites, especially at blooming time.
This indicated that pepper pollens were a highly preferred food for the mites. The food source supported all their life stages to complete their life cycle.
However, Explosive Ember supported the highest number of predators at more than 1,200 mites, which was attributed to the fact that it had more leaves than the other varieties.
Researchers also thought the tiny domatia chambers provided shelter for breeding, increased fungal spores for alternative food, controlled their micro-environment and provided protection from other predators.
Once they reached the adult stage, the mites dispersed to beans crop and controlled all three targeted pests.
It was the first report of ornamental peppers being used as banker plants to support the predatory mite.
It underscored the notion that the banker plant system should be considered a viable part of an integrated pest management program for environmentally friendly greenhouse farming. The report appeared in the journal Biological Control published by Elsevier.
McKenzie believes the method could benefits growers.
“We hope that a grower could use this in the field and we are testing a predator in first approach with bell peppers in the field,” she said.
With promising results from the first field trial they plan to repeat it this spring. But she says results may not be as reliable because of uncontrollable influences.
“We have gotten control as good as the standard insecticide treatment, but the beauty of the banker plant is that you can remove it, spray for the pest if necessary and then replace it. Or you can spot treat where pest hot spots show up.”
McKenzie said different banker plants have different pest spectrums and some may be more limiting than others.
For instance, the papaya banker plant is limited to Florida but the papaya whitefly is a pest wherever the papaya is grown.
This limits the papaya banker plant’s applicability, but McKenzie said ornamental banker plants may be more wide-ranging or have less limits, depending on the farmer’s crop focus.
Using banker plants is an interplay between selecting the right plant that can support and maintain healthy populations of the predatory insect and ensuring it can effectively reduce the population of targeted pests.