Biocontrol is fragile
Invasive stinkbug further upsets already delicately balanced IPM systems.
These four creatures have survived pesticide treatments to become the most important biocontrol agents in eastern apple orchards. There are two species of predatory mites (far left and far right pictures); the “mite destroyer” ladybeetle Stethorus punctum (center left); and the woolly apple aphid parasitic wasp, Aphelinus mali.
The arrival of the brown marmorated stinkbug in the eastern United States has upset efforts by growers to effectively use biological agents to control secondary pests.
It has also stalled efforts toward developing more ecologically based integrated pest management practices using mating disruption and more pest-selective insecticides. These would conserve biological control agents and help achieve biological control of primary pests such as codling moth and oriental fruit moth.
Biocontrol—the control of pests by their natural enemies—has always been fragile whenever pesticides are used, Dr. David Biddinger, an entomologist at the Pennsylvania State University Tree Fruit Research and Extension Center in Biglerville, explained during the Great Lakes Fruit, Vegetable, and Farm Market Expo in Michigan in December.
Most pests develop resistance to pesticides much more quickly than the natural enemies, he said. Even well-intended actions, such as phasing out older neurotoxic
Pesticides under the Food Quality Protection Act of 1996 and the introduction of more than 20 safer pesticides in the last 15 years, could upset the balance. Fruit growers were adapting to setbacks caused by the new pesticides and the new rules, and were moving towards enhancing the role of biological control when the stinkbug came along and set them back again.
Biddinger’s advice to growers facing the stinkbug: Do the best you can until better methods come along. Preserve IPM tools that have been effective in the past, but watch out for possible side effects in the next few years from using broad-spectrum insecticides to control the brown marmorated stinkbug. One of these effects could be a reduction in beneficial biological control agents and outbreaks of secondary pests like mites, woolly apple aphids, scale, leafminers, and leafrollers.
“We have already seen a move away from pheromone mating disruption, which was an effective nonpesticide alternative for controlling important pests like codling moth,” he said.
Biddinger traced the history of IPM programs that were developed beginning in the 1960s.
“Experience in many crops has shown that if pesticides are used without consideration of the natural enemy complex, the management system shifts to a ‘pesticide treadmill’ syndrome where pesticides are used for dealing not only with the initial pests, but also with the secondary pests whose natural enemies are destroyed,” he said.
This occurred in many apple orchards, where organophosphate and carbamate insecticides, with their broad-spectrum activity, were the main insecticides of choice directed mainly at codling moth.
Fortunately, a few—but only a few—beneficial mites and insects became resistant to these insecticides. These included some predatory mites, the lady beetle Stethorus punctum, and the woolly apple aphid parasitic wasp, Aphelinus mali. These developed resistance to organophosphates and were incorporated as key components in apple IPM programs, Biddinger said. They learned to live with materials like Guthion (azinphos-methyl), Lorsban (chlorpyriphos), and Sevin (carbaryl), allowing growers to use them and still develop IPM programs that included biocontrol of secondary pests.
Still, these IPM programs were fragile, a hybrid system, he said.
“They typically have a greatly reduced natural enemy complex compared to situations where no pesticides are used,” he said. “Most pests are controlled by a complex of many different species attacking different life stages at different times of the year. Relying on only a single species of biocontrol is not stable.”
“The IPM system based on resistant beneficial biocontrol agents that took decades to develop proved to be rather fragile and began to fail in Pennsylvania orchards in the mid-1990s,” he said. “Then, most of our moth pests began to develop resistance to organophosphate and carbamate insecticides, and higher doses or mixtures were required for pest control.”
There was also a shift due to the Food Quality Protection Act.
The U.S. Environmental Protection Agency’s mandated increased concern for human health and water quality effects led to development of so-called reduced-risk insecticides. While the risks were reduced for people and the environment, they were in many cases not reduced for beneficial insects—honeybees, parasites, and predators, he said.
Eliminating or restricting the cheaper broad-spectrum insecticides did help spread eco-friendly, targeted mating-disruption systems. “While mating disruption was markedly more expensive,
it dramatically reduced insecticide applications aimed at codling moth and, indirectly, the need for spraying to control secondary pests as well,” Biddinger said.
Mating disruption is a targeted, species-specific, nonlethal method to control primary fruit-feeding pests. Used in combination with pest-selective, reduced-risk insecticides, fruit growers were poised to move toward a more ecologically balanced approach. They could rely more on natural enemies and less on pesticides that need to be continually replaced as resistance develops, he said.
“Insecticide sprays in the five years before brown marmorated stinkbug became a widespread problem had been reduced by over 50 percent in many orchards and acaricide use by over 80 percent.”
It is this beneficial outcome that brown marmorated stinkbug now threatens. “Where stinkbug is a problem, insecticide sprays have more than doubled,” he said.
The stinkbug’s effects
“As with many introduced pests, effective biological control agents are not available currently, so short-term solutions rely on pesticides for control,” Biddinger said of the brown marmorated stinkbug. “This is a truly difficult pest to control since it spends over 80 percent of its time outside of orchards, and we do not yet have a truly effective means of monitoring the adults with either pheromone, light, or bait traps.”
In field and laboratory bioassays conducted by U.S. Department of Agriculture researchers in Kearneysville or by Penn State entomologists, the broad-spectrum insecticides that FQPA was trying to phase out and those that are the most detrimental to biocontrol agents are the most effective against the brown marmorated stinkbug, he said.
Organophosphate insecticides are surprisingly ineffective, he added.
The pyrethroids, Lannate (methomyl), the chlorinated hydrocarbon Thionex (endosulfan), and some of the neonicotinoids are the best products evaluated in the lab and field so far. All these materials are disruptive to IPM.
The pyrethroids and Lannate are broad-spectrum insecticides that are highly toxic to many natural enemies found in tree fruit, he said. Least disruptive to IPM are Thionex and the neonicotinoids; they are not disruptive of the phytoseiid mite predators that are most important for biological mite control in Pennsylvania. But they are toxic to many other biological control agents and to honeybees and native bees.
“While these predatory mites have some tolerance to Lannate at lower rates, higher rates of this compound and multiple applications of pyrethroids would basically eliminate all biological control in apple orchards and set IPM back forty-plus years,” he said.