Front Page of Mercury News: How a blood thirsty parasite is taking down a major strawberry pest
In undisturbed areas surrounding strawberry fields of Monterey Bay, there’s a battle waging between good bugs and bad bugs. And the good bugs seem to be winning.
Scientists are witnessing the “fruits of their labor” as a European parasite introduced 15 years ago is helping to take down some notorious native pests called Lygus hesperus that threaten the strawberry crops.
The parasitic wasps are tiny, delicate-winged insects, but they’re anything but gentle. Instead, like the terrifying creature in the film “Alien,” they insert their eggs into their victims using long stingers. The eggs develop and hatch, and then the young parasites eat their victims from the inside out, exiting upon reaching adulthood.
As this predatory life cycle repeats again and again, scientists say the European Peristenus parasites are helping to reduce populations of Lygus bugs in the Monterey Bay. When scientists recently collected samples of the remaining Lygus bugs from parasite release sites, 71 percent of the critters had parasites inside.
Strawberry production is a $1.83 billion industry in California. Lygus bug damage contributes to around $18 million to $37 million annually in economic losses.
On the Central Coast, Lygus bugs attack strawberries, but they also feast on a wide range of other crops, such as cotton and chickpeas in the Central Valley. The bugs invade flowering strawberry fields in the spring, launching their assaults from surrounding undisturbed vegetation. They use their long mouthparts to pierce the seeds on the outside of the strawberries and inside flowers to suck their contents. This causes fruits to become “cat-faced,” or severely wrinkled and misshapen.
Typically, scientists introduce predators to attack invasive foreign bugs that have become serious pests, but Lygus bugs are native to California. For scientists to bring in a foreign insect to go after a native insect is highly unusual. It turns out that Lygus bugs have no native predators, which is one reason they’ve been so troublesome since the 1930s.
Farming in California is the likely reason native Lygus bugs have become a major pest.
“If there was no agriculture, the Lygus bugs would be around in lower numbers in the wild and hard to find,” said Charlie Pickett, an entomologist with the state Department of Food and Agriculture.
Whether the introduced parasites will cause strawberry-loving Lygus bugs to continue to diminish over time remains to be seen. In the meantime, there is something that growers can do to encourage the process, and it could mean big savings from reduced fuel and pesticide use.
Some farmers are now planting alfalfa, a crop usually grown for cattle, in large patches away from their strawberry plants. Alfalfa is more attractive to the Lygus bugs, pulling them away from strawberry plants in large numbers — a practice known as “trap cropping.”
If the alfalfa crops are left unsprayed for the parasites, they can do what they do best: Eat Lygus bugs and lots of them.
Another strategy growers use to manage Lygus is to run vacuums mounted to the front of tractors to clean the pesky bugs from their fields. By incorporating alfalfa trap crops that pull the Lygus bugs to one place, growers can manage them better with the vacuum — without running the costly tractors for hours over an entire field.
But the parasites still play an important role.
“The Lygus bugs are smart. They learn to detect the vibration of the vacuum and dive down into the plants. You need the parasites to attack the remaining Lygus that the vacuum can’t pick up,” said Larry Edding from Pacific Gold Farms, a Naturipe organic strawberry grower, who collaborated with the scientists to develop the scheme.
Despite years of trial and error, more time is needed for the parasites to reduce the Lygus bugs in the Monterey Bay area to a manageable number. According to entomologists, when an introduced predator parasitizes a host at a rate that exceeds 35 percent, it is a good sign that the pest can be brought under control. When you compare that to 70 percent parasitism of Lygus bugs recently seen in the Monterey Bay, there is a possibility that the scheme will succeed.
Edding has seen good results at Pacific Gold Farms with the alfalfa trap-crop parasite scheme. The density of Lygus bugs at his farm have declined 95 percent over the past 13 years.
“You don’t meet a grower like Larry Edding every day; this is an information-rich approach,” Pickett said. “He knows what to do with this stuff and how to make it work.”
Introduced organisms must go through rigorous testing and quarantine to ensure they will not have undesired effects on other organisms.
“You have to look at the positive impacts and weigh it against the negative impacts,” Pickett said. “Your farm labor does not have to be exposed to pesticides. That is huge.”