By Christine Souza
Assistant Editor, Ag Alert
(This story is reprinted with permission of Ag Alert, a publication of the California Farm Bureau Federation)
April 10, 2003 - -
University of California Cooperative Extension farm advisors and other exports
told almond growers in Stanislaus County at a Jan. 29, 2003 field day, sponsored
by UCCE and the Almond Pest Management Alliance, they should be aware that the number of
San Jose scale in the area has increased in recent years; therefore, growers
should learn how to control the pest before it becomes an even greater problem.
"I am getting a bit concerned about San Jose scale. This is an insect that we
have not had to deal with much because we have very low populations and because
of the annual dormant sprays we put on in the past," said Roger Duncan,
University of California Cooperative Extension farm advisor, Stanislaus County.
"This past summer, I saw more scale here than I ever have. Most orchards were
down far below any damaging levels, but there was more scale than there ever
was. It is something we need to be aware of."
Kent Daane, University of California, Berkeley entomologist, discussed control
of San Jose scale, Quadraspidiotus, a pest that sucks plant juices form twigs
and limbs and injects a toxin that results in the loss of tree vigor, growth and
productivity and death of limbs. He recently addressed a group of almond growers
at the almond day for the North San Joaquin Valley, at the Stanislaus County
Agricultural Center in Modesto.
Daane focused on the predators and parasites helpful in reducing scale
populations such as the commonly seen Encarsia perniciosus, and less common
Aphytis vandenboschi and A.aonidiae. While these natural enemies parasitize the
scale, insecticides used during the grower season for other pests disrupts this
natural control, and scale populations can build as a result. Low winter
mortality due to mild temperatures will also permit a buildup of scale
populations.
"San Jose scale is one of those insects that I cannot give you a clear picture
as to why it is being controlled by parasites in some areas and not in others,"
Daane said.
The Aphytis is an external parasite whereas Encarsia is internal. Excretion
pellets underneath the scale next to a pupae means that the parasite is Aphytis,
not Encarsia. The Encarsia is inside the scale and underneath the scale
covering, so when an organophosphate and oil are applied during the winter, the
parasite is mummified and protected.
In a study by Daane to determine if a wintertime application would lower
parasite density, the orchard showed a reduction of the parasites and an
increase of San Jose scale after organophosphates and oil were applied. In
another field, results showed a larger first flight of San Jose scale, this is
probably due to the fact that the oil was not applied, Daane said. Typically
when oil is applied, a larger first flight of scale does not occur. In this
trial, Daane also saw a large first flight of Encarsia, but its populations
dropped in both the organic and conventional fields where the studies were
conducted. In both fields, there was not much of an effect of the dormant oil in
terms of numbers of Encarsia.
Daane shared information taken from other trials of 12 fields of both almond and
stonefruit trees conducted in 1999, 2000, and 2001. In these trials, Daane
noticed a general pattern that each year the number of San Jose scale increased
in the traps. Its populations always starts out very low, except for 2001, when
two organic growers had fairly moderate levels of scale, about 4-to-5 percent
damage. because that was acceptable to them, they decided to eliminate the
dormant oil. They were not using any organophosphate during the dormant season.
When they pulled out the dormant oil, the San Jose scale population was probably
around 600-700 in that first flight, which is why Daane said, "You have got to
put on that dormant oil."
"That second flight generation almost three to four weeks later, depended on
temperature. That is why in some years, some varieties get hit harder than they
do other years," Daane said.
The Encarsia population remained about the same regardless of the
organophosphate and dormant oil treatments. In stonefruit in March, April, May
and June, an organophosphate and an oil treatment were applied and there was
still a fairly strong second flight of Encarsia. In June, July, and August, the
Encarsia population dropped dramatically regardless of the insecticide
treatment. Its numbers picked up again in September and October.
"We started to realize that when everyone was looking at the first flight, that
first flight of the parasite is really the population from the year before.
Those dormant treatments and early spring treatments do not seem to have any
effect," Daane said. "Notice that the parasites are always under the San Jose
scale and at the end of the season we have over 600 scale per week in the first
flight."
To follow up, Daane also compared pheromone traps and sticky traps. Where the
traps are placed in the tree can significantly impact the kind of data
collected. When the traps are placed in the upper canopy, Encarsia counts reach
high numbers. Results are just the opposite when comparing San Jose scale. More
San Jose scale are collected when the cards are placed high than when the cards
are low.
"Encarsia is out there in large numbers in all of the fields regardless of the
insecticide treatment. It is an internal parasite so it has that protection.
Aphytis is the one we think its numbers get knocked out with insecticides during
the summer," Daane said. "When looking at manipulating these by putting more
parasites back into the system, we are going to focus on the Aphytis instead of
Encarsia. With Encarsia there are so many out there you really can't change its
numbers."