Lettuce aphid populations were so low in the Salinas Valley this summer that UC Cooperative Extension entomologist Bill Chaney had to go the Hollister area to run his control trials.
However, there were more than enough leafminers, Western flower thrips, potato psyllid and a new long-life pest called a garden centipede to keep Chaney and pest control advisors busy this summer.
Chaney detailed these pests at a continuing education course sponsored by the Monterey Bay chapter of the California Association of Pest Control Advisers (CAPCA) at the recent Salinas Valley Farm Show.
For the past several years, lettuce aphid has caused problems, but this year they were no where to be found even for Chaney to test control products.
However, in tests in Hollister area where aphids reached moderate levels of 60 to 70 aphids per plant, Chaney found all of the registered pesticides continuing to control the pest. These include Provado, Assail, Fulfill, MSR, a new formulation of Mustang called Mustang Max. “All materials looked pretty good if you get good coverage,” he said, warning PCAs to rotate or tank mix products with different modes of action to ward off resistance.
However, leafminer was back in lettuce in “bigger numbers than we have seen in the past couple of years,” Chaney said. Leafminer also damages onions and spinach in lower numbers than they do in lettuce.
Chaney continues to evaluate control data this season, but he said he is finding shorter residuals for most products. “That is the first indication we may be looking at a breakdown in control” and the beginning of resistance buildup, said Chaney.
Western flower thrips also were plentiful this season. Thrips can cause damage, but just as important, they can halt vegetable exports.
“If one Western flower thrip is found in a shipping container, it is red lined,” said Chaney. “The western flower thrip does not damage broccoli, but if one is found in a container, it does not move.”
However, the tiny thrip did cause damage this year — “more than we have seen in the past” — especially on onions, celery and lettuce.
Root maggots can be a yearly problem, but this season because of a cool wet spring, they were found above ground in head lettuce, cabbage and other crops.
“Maggots in a food product are never a good thing, and they are difficult to manage when flies lay eggs in the plants,” said Chaney. It is almost impossible to time a spray that will catch the flies.
Potato psyllid is a pest most producers had not seen before this year, said Chaney. It has been an occasional pest in peppers in the Gilroy, Hollister and King City area. This year it was found damaging other crops, including tomatoes and onions.
They are tiny. It takes a 10X magnification hand lens to see them, but immature and adult potato psyllid can cause damage by injecting a toxin that causes plants to yellow. In severe cases they can cause yield losses of 20 to 50 percent.
Chaney also said potato psyllid can ruin a crop by depositing honeydew causing sooty mold.
They can be difficult to control because they are usually found on the underside of leaves.
UC IPM Guidelines indicate that if psyllids are easily detected and yellowing symptoms are observed in the plants, an insecticide treatment can provide control, but coverage is essential.
Chaney newest challenge is to control an arthropod commonly called “garden centipede.” It has its own arthropod class Symphyla.
Crop damage from garden centipede generally occurs in the same field location each year and damages both direct seed and transplant crops. Chaney has photographs of large areas devoid of any plants because of the garden centipede feeding on plants beneath the soil.
As its common name implies, it looks like a centipede with 15 body segments and 11 to 12 pairs of legs. It moves quickly in the soil, and in the lab; live up to five years. There may be as many as two to three generations produced each year.
They are about a third of an inch long. They are general feeders and can eat up to 15 times their weight each day.
They move through soil pores, but do not tunnel. They can move up to 10 feet per day and can move as deep as three feet vertically in the soil.
Damage is evident, but until now it has been difficult to determine treatment thresholds. Chaney has developed a sampling technique using a potato slice to attract the garden centipede.
“What you do is lay a slice of potato on the surface of wet soil and gently cover it up with dirt. Don't use a shovel. It disturbs the soil,” and the pest will not move to the potato he said. Cover the buried potato with a plastic cup to keep light out.
“Come back within 24 to 48 hours and count the symphylans just below the potato and on the potato itself,” said Chaney. “There is a strong correlation between how many we caught and crop damage. We have caught as many as 40 during the trapping period. If you catch as many as 60 to 70, that is enough to completely destroy a crop.”
With a valid sampling technique, Chaney said he will begin working on improving control.
The only control methods listed in the UC IPM site is flooding for two to three weeks, but that may not be effective since the garden centipede retreats down to the three-foot level.
Infested soils may be treated with insecticides, but the retreating pest could escape that as well. However, if done early in the season this may allow plants to become established before suffering too much damage by the pest.
Chaney cautioned that some PCAs have confused the more common springtail with the garden centipede. There are similarities between it and the garden centipede. Both can damage crops. Springtail are also attracted to potato slice bait.
However, springtail is a surface feeder while the garden centipede is a below soil feeder.