If his advanced field studies pay off, a University of California, Berkeley insect biologist will have new ways of managing aphid pests of the California dried plum industry.
Nicholas Mills told the recent Dried Plum Day in Tulare of his research, supported by the California Dried Plum Board, on the use of parasitic wasps and life cycle disruption.
Leaf curl plum aphid, more common to the Sacramento Valley, and mealy plum aphid, widely seen in the San Joaquin Valley, infest new shoots and are associated with splitting across the sides of fruit.
Mills said he believes high temperatures, followed by irrigation in early June, cause most of the splitting along the suture of plums. However, he conceded, splitting, both suture and side, does occur more often on trees that also have aphids.
Over the past three to four years of his CDPB trials, he has reared and released two parasitic wasp species he brought back from Europe's Mediterranean Basin.
The wasps lay eggs in aphids, and the larvae consume the interior of the aphid leaving mummified remains. Both species are active against leaf-curl plum aphid, one throughout the year and the other only during the early spring.
“When these parasitoids are active very early in the season, they seem to be able to prevent leaf-curl plum aphids from building colonies along shoots of plum trees.”
Mealy plum aphids also have a corps of natural enemies in lacewings, ladybird beetles, flies, and aphid midges that claim their share of control, but, Mills said, “those predators can't manage aphid populations on their own, so we've been looking for parasitoids in Europe.”
Mealy plum aphids occur on almond, apricot, peach and plum trees in Spain but certain body parts are different shaped on the different crops, suggesting they may be separate species.
Mills learned that parasitic wasps gathered from almond trees in Spain produced the most mummies, leading him to suspect they likely would be more successful on California plums.
Mills plans to release these species, reared in his laboratory in Berkeley, this season in plum orchards and monitor the effects on aphids.
In another approach to aphid management, Mills said he has data suggesting it is possible to interrupt the insects' complicated, multi-stage lifecycles by defoliating plum trees in the fall.
Aphids hatch in March and start developing on the trees. After a couple of generations, leaf curl aphids morph into a winged form that goes to feed on daisies and other Compositae plants during the summer before returning to plum orchards.
Summer on cattails
Mealy plum aphids migrate from their native plum trees to spend the summer on cattails, then again sprout wings and migrate back to plums.
“If we could break the life cycle at the point when they are migrating back to prune trees,” Mills said, “we might be able to prevent their egg laying in the fall and populations in the following spring.”
One way to accomplish that is to defoliate trees with zinc sprays at the end of summer, perhaps in mid-October, according to Mills' trapping studies of the aphids. The switch to migratory, egg-laying forms is triggered by day length, which would be monitored to guide the timing of the sprays.
Brown rot is a perennial headache for several tree crops, and Themis Michailides, UC plant pathologist at Parlier, has unveiled a new, simple, interactive Web site to aid dried plum growers in deciding when to spray fungicides, irrigate, or thin fruit for brown rot control.
The fungi Monilinia fructicola and M. laxa infect Prunus blossoms during rain or other wet conditions and lead to decay of fruit.
Over the past three years of CDPB-supported research, Michailides told the Tulare group he has determined the optimum temperature and other conditions that foster blossom rot.
“The susceptible time for blossoms is from the popcorn stage to early petal fall. Late petal fall and fruit set are resistant to infections. Longer periods of wetness create more blossom blight.”
The temperature range, he added, is from about 50 degrees to 86 degrees, with most infections occurring at 70 to 75 degrees.
After reaching the Web site at http://tjm.uckac.edu, the grower responds to questions from the decision support system for IPM of prune brown rot, known as the DDS.
The predictive computer model takes over and issues either a “note,” “warning,” or “recommendation,” based on the calculations it makes from the grower's responses.
Michailides said the system offers the added benefit of records of the disease, control history, and decisions made for specific orchards.
Gary Obenauf, the CDPB's research coordinator, said the board's integrated prune farming practices (IPFP) project was carried out on more than 800 acres of prunes as a part of its 34 grower plots during 2001.
Responding to the board's interest in reducing the impact of potential pesticide limits imposed by the Food Quality Protection Act, the project encourages use of reduced-risk pesticides and use of irrigation scheduling and optimum use of nutrients.
Among the practices was zero use of organophosphates for dormant applications. Asana was used where needed, but monitoring showed that about 60 percent of the orchards did not need treatment. That alone demonstrated that monitoring can be an important money saver, Obenauf said.
The plots also showed that as much as 40 percent could be saved in irrigation costs, while fertilizer applications were reduced in several orchards.
“In most of our comparisons, we saw no significant difference between conventional and reduced-risk practices in yield or quality of the product.”
Reminding of the pesticide monitoring under way in the Sacramento Valley, Obenauf said San Joaquin Valley growers must prepare for it. “We can argue about numbers, but the fact is, OPs are being found in water and it is an issue we've got to deal with.”
The key is to demonstrate that agriculture is making some progress in controlling pesticide run-off. If not, the Department of Pesticide Regulation is apt to require changes in registrations or restrictions on labels. If those fail, the eventuality would be cancellation of organophosphate products.
“So that's why we are involved in trying to reduce off-site movement of these materials. It is very evident if we had not become engaged in this process, cancellations of OPs in California would have started three years ago.”
Growers are concerned about water quality questions and they do not want to cause problems in the environment, but, Obenauf stressed, solutions must be cost effective.
“Agriculture is going through significant change, whether we want it to or not. Change takes time and money, and we need the information to make the change. We can't just do away with something in a biological system and not affect something else.”