One female navel orangeworm that survives the winter inside a pistachio nut can produce as many as 85 to 100 eggs in the spring. That’s why a relatively few mummies left on the trees or the ground after harvest can create a big NOW problem for the following crop, including last year, when California’s pistachio orchards were under unusually heavy pressure from the insect.
That’s also why Bob Beede, University of California Cooperative Extension farm advisor, emeritus, continues stressing the critical role of proper winter orchard sanitation -- blowing the berms and destroying over-wintering nut mummies – in controlling NOW.
“Cover sprays help in reducing in-season NOW, but they are no substitute for winter destruction of residual nuts from harvest,” Beede reports in his December, 2013, Pistachio Task List newsletter. “Winter sanitation is still the cornerstone of effective NOW control.”
He recommends knocking the mummy nuts onto the ground as soon as possible to expose them to as much rainfall as possible for decomposition. Winter sanitation includes removing mummy nuts remaining in the tree; blowing them out of the crotches where the limbs arise from the trunk; blowing, and if necessary, raking the berms clean, and then destroying the nuts by disking or flailing with a roller behind.
“This cultural practice breaks the developmental cycle of NOW without the use of pesticides.” Beede says. “It is also a community effort. This may be one reason why some areas are more prone to high damage.”
NOW pressure is probably increasing in the southern San Joaquin Valley due to more acres of pistachios and other host crops, like almonds walnuts and pomegranates, he adds.
Winter sanitation is a tougher job in pistachio orchards than in almonds and walnuts. This reflects the smaller size of pistachio nuts. For example, one pound of pistachios contains about 300 in-shell nuts compared to about 184 in-shell almonds and 50 walnuts. Pistachios are also harder to destroy. Due to their lighter weight, they resist being sucked up and broken by the flail mower. The high air velocity of some equipment used to blow the tree berms free of trash and overwintering nuts can deposit some of them into the adjacent tree row just cleaned.
Pistachios also get imbedded into the soil around the base of the tree where the ground cracks loose from shaking. Depending on the amount of wear on the rubber guards on the shaker frame, the number of nuts left during harvest at the base of every tree can range from a few to as much as a couple of handfuls.
NOW have been found surviving during the summer on nuts left on the orchard floor from the previous season.
Nuts damaged by NOW and other insects are a major concern to foreign buyers because of their unsightly appearance and greater susceptibility to aflatoxin-producing molds. Over the last 20 years, the number of nuts damaged by NOW and other insects each has averaged about 1 percent. Last year, as in 2012, almost 2 percent of the crop was lost to insects.
In addition to controlling NOW, orchard sanitation also helps reduce Botryosphaeria inoculum, which can splash up into the trees during heavy storms, Beede notes.
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