An Effective Strategy for the Management of Greenhouse Thrips in 'Hass' Avocado Orchards
California Avocado Society Yearbook 76 (1992): 99-107.
Phil A. Phillips, Ron S. Bekey, J. Blair Bailey
Cooperative
Extension, Division of Agriculture and Natural Resources, University of
California, Riverside, California.
An early harvest in one year can reduce scarring and loss of grade on both that season's crop and the following season's crop.
Greenhouse thrips, Heliothrips haemorrhoidalis (Bouche), is so
named because it is often a pest of greenhouse ornamentals. However, it has a
very wide host range which includes many subtropical species such as citrus,
cherimoya, guava, mango, and sapote, as well as avocado.
In the humid coastal counties of southern
California, the greenhouse thrips (GHT) is primarily a pest of avocados grown
within about 15 miles of the ocean. The most severe infestations have been
reported in Santa Barbara, Ventura, and San Diego Counties. GHT has been
causing problems in Santa Barbara county avocados for over 40 years.
On avocado trees, the location of GHT
populations and their feeding injury depends on the variety of avocado. On
varieties of Mexican origin, including Bacon and Zutano, the GHT feeds
primarily on the leaves and rarely on the fruit. By contrast, fruit is the
primary feeding site on Guatemalan varieties such as Hass, which makes
up approximately 80% of California's commercial production. Epidermal fruit scarring
caused by thrips surface feeding, results in noticeable aesthetic blemishes and
downgrading of fruit quality by the packinghouses. In general, smaller fruit
rind scars of up to 1/2-inch to 1/4-inch in diameter are often tolerated in the
fresh avocado market. Although mostly aesthetic in nature, fruit scars
exceeding 3/4-inch in diameter resulting from prolonged GHT feeding can result
in economic losses of up to 50% or more to growers. These larger scars cause
the fruit to be graded down from fresh market grade to processing grade. The
lower processing grade fruit generally doesn't provide sufficient return to the
growers to pay for growing the fruit (~ $0.50/lb.).
There is no evidence at this time to suggest
that leaf scars resulting from GHT feeding cause any reduction in quantity or
quality of fruit. It is believed that most healthy avocado trees produce
considerably more foliage than necessary to produce high yields of top quality
fruit.
Malathion has been used to control GHT for
many years in Santa Barbara County. Extensive malathion treatments have only
been required to attempt control of this pest since the mid-1970s. In the last
year years, 75 % to 80 % of Santa Barbara's orchards have been sprayed
annually, with some growers reporting over 70% fruit damaged by GHT feeding
despite the pesticide treatments.
Since this chemical control strategy has been in
place, several additional arthropods have risen to pest status, notably the
six-spotted mite, avocado brown mite, amorbia, and omnivorous looper.
Disruption of their respective natural enemies by the malathion applications is
suspected, but at this time there is no good evidence to support this.
Although promising biological control agents
for greenhouse thrips have been identified and imported, effective control has
yet to be achieved on a large scale basis. The parasitic wasp Thripobius
semiluteus Boucek, introduced from Australia in 1986 and Brazil in 1988,
has shown the greatest potential for biological control of GHT and will
ultimately be the long term solution. An additional strategy, however, is
needed in the interim.
Several observations suggest that by manipulating
harvest dates, avocado growers might be able to reduce GHT populations and the
fruit scars they produce. Historically, damage to avocado fruits has been
greatest in the northern-most avocado producing coastal counties such as Santa
Barbara, where cooler temperatures allow the fruit to be held on the tree
longer in anticipation of higher prices. Where fruit has generally been picked
earlier in the year, such as in San Diego County, the damage has been less. The
predominant variety affected by GHT is Hass. Since GHT resides primarily
on the fruit in Hass orchards, a large portion of the population is removed
each year at harvest. Thus, a cultural technique such as advancing the harvest
date might be advised to control GHT.
To test this hypothesis, a trial was
conducted during 1988 at the Elwood Canyon Ranch in Goleta, Santa Barbara
County. The objective was to determine whether by harvesting earlier in the
season GHT populations could be lessened, resulting in fewer GHT feeding days
on the fruit and ultimately reducing fruit scarring levels-both in terms of
percent crop scarred and in the severity of individual fruit scars.
Methods
Three different harvest dates served as the
treatments: an early-season, a mid-season, and a typical late-season harvest.
The test plot design consisted of six single tree replicates per treatment in a
randomized complete block design. Treatments were assigned to test trees based
on pre-trial GHT fruit infestation levels on the test trees assessed on June 9.
Percent GHT infestation was determined by examining 13 fruit per quadrant for
52 fruit per test tree between 18 inches and 72 inches above the orchard floor
for the presence of an active GHT infestation. Each test tree was surrounded by
four buffer trees which were to be harvested at the same time but separately
from the test trees to eliminate any possible influence
1st instar (smaller, below)
and pre-pupal (larger, above) stages of greenhouse thrips. Note fecal droplet on
abdomen of1Ist instar nymph.
Photo by Jack Kelly
Clark
Adult greenhouse thrips.
Photo by Jack Kelly
Clark
GHT colony and feeding scar
at bottom and sides of fruit.
Photo by Jack Kelly
Clark
Blair
Bailey assisting with harvest sampling
Photo by Phil A.
Phillips
of GHT populations adjacent to the harvested test trees.
An early-season harvest of six test trees and
their respective buffer trees was conducted on June 20th, 1988. A second,
mid-season, harvest was similarly conducted on August 13th, while a third,
late-season, harvest was conducted on October 10th. A final harvest of the
following year's crop on all 18 test trees was conducted on March 29, 1989 to
measure any carryover effects from the 1988 season harvest treatments. These
harvests were all conducted by the regular three-man picking crew employed by
the cooperating ranch. Fruit from the test trees were deposited into standard
picking bins, where a random sample of 100 fruit per tree was assessed for GHT
rind scars. Each sample fruit was scored for presence or absence of and
severity of GHT feeding scars. Each fruit was given a scar severity rating on a
scale of 0-5, 0 being no scar, I being up to a "dime" sized scar, 2
being "dime" to "quarter" sized scar, 3 being
"quarter" sized scar to half the fruit surface scarred, 4 being over
half the fruit surface scarred, and 5 being completely scarred. Although not of
economic size, the small scars were recorded because with continued GHT feeding
at these scar sites the scars would eventually attain a size of economic
importance. After scoring individual sample fruit from each test tree during
each harvest at the ranch, fruit from all six test trees at each harvest was
pooled and sent to a commercial packer, Calavo Growers of California, for grading.
This procedure was intended to provide additional full tree crop quality
information as backup to our 100 fruit sample taken from each tree's harvested
crop. Since GHT tends to be very clumped in its distribution, with little
lateral movement between trees in an orchard, it was felt that the buffer tree
design of the experiment was not essential. Therefore, as additional backup
information, fruit from the harvested buffer trees was sent to the ranch's
packer, Eco Farms, for grading.
Results
During 1988, on what was the current season's crop, even though the
percentage of fruit having GHT scars was similar for all three harvest dates,
the late-season harvest resulted in significantly more severe fruit scarring (i.e.,
larger individual fruit scars) than either the mid-season or early- season
harvests (Figure 1). Unfortunately, the backup packing house grading data from
the full crop on each test tree were incomplete due to the loss of the grading
information for the final late-season harvest. However, the early-season
harvest produced an 8.6% loss from GHT on the full crop from our test trees for
the high value fresh market grade fruit, while the mid-season harvest resulted
in an even higher loss of 15.1% fresh market grade fruit due to GHT feeding
scars (Table 1). With a significantly greater percentage of fruit scarred in
our late-season harvest test trees, it would be logical to assume the greatest
loss in fresh market grade would have resulted at the packing house from this
harvest. Again unfortunately, these packing house grade data were lost.
Current Season GHT Scars –1988
Hass Avocado
1988 Harvest Date
Damage Rating* % Fruit
Scarred
* DAMAGE RATING SCALED UP BY FCTOR OF 10
** SIGNIFICANTLY
LARGER SIZED SCARS
Figure 1.
When the carryover effects on the following year's
crop are considered, the early-season harvest resulted in not only
significantly less new crop scarring, but also in considerably less new fruit
with active GHT early in the season (Figure 2). Moreover, the severity of
individual scars on fruit harvested the following year was considerably greater
with each delay in harvest date during the previous year (Table 2).
Discussion
Immature GHT, and to a lesser extent even adults, tend to feed in
concentrated areas, with their populations building and expanding over the
season out and away from the initial infestation point on the fruit. This
behavior creates "islands" of activity on the fruit surface. Since
GHT cannot successfully feed on older, previously fed-upon, and scarred areas
of the fruit rind, these "islands" of actively feeding GHT
continually move outward from the initial point of infestation onto new tissue
at the periphery of the developing scar tissue. As a result, fruit rind
scarring resulting from GHT feeding is cumulative over the season, ever increasing as the GHT
population expands. If conditions are optimum for GHT growth and increase
through the season, avocado fruits may become 100% scarred, causing the GHT to
move up the fruit stem to locate new feeding sites.
Considering this GHT feeding behavior, it
would seem reasonable for an early-season harvest to produce fewer
thrips-scarred fruit than a harvest later in the season, as the thrips are
actively feeding longer on the fruit which is harvested last.
By harvesting avocados earlier in the season, the period of crop
overlap between the current crop and next year's crop is minimized. The greater
the crop overlap period, as with late-season harvests, the greater the period
of time allowed for GHT to establish colonies on the new crop at a very early
stage in its development. This would understandably increase the damage to the
following year’s crop, given favorable conditions for GHT development.
Conclusion
For the Guatemalan avocado varieties such as Hass,
where GHT populations are concentrated on the fruit, an early harvest
strategy can accomplish both a significant reduction in GHT-scarring on the current
season's crop, as well as on the following season's crop. Where chronic and
severe areas of GHT infestation can be identified in an orchard, the early
harvest strategy should be applied to these areas, leaving the remainder of the
orchard available for harvest at the optimum time where the highest market
prices prevail.
Acknowledgments
The authors gratefully acknowledge the
California Avocado Research Committee for partial funding of this research and
Mark McKee, manager of the Elwood Canyon Ranch, for use of the orchard, the
ranch's harvesting crew, and their enthusiastic cooperation.
Table 1.
10/10/88 (DATA LOST FROM
PACKINGHOUSE)
1988 Harvest Date's Influence on GHT's
Scarring of the 1989 Avocado Crop
Table 2.
GREENHOUSE THRIPS MANAGEMENT:
IMPACT ON HARVEST DATE ON NEXT YEAR'S CROP*
*Harvests (treatments) conducted in 1988 for impact on 1989 (3/23/89) harvest (strip picked)
** Average
scar rating per fruit for entire crop.
Scars
rated 0 = 0, 1 = up to dime size scar, 2 = dime to quarter size,
3 =
quarter size to less than h surface scarred, 4 = more than &@ scarred,
5 =
completely scarred
*** Average scar rating for
fruit with scare