Species, distribution and pest
status: In the late 1970s the pandemis
leafroller (PLR), Pandemis pyrusana Kearfott, was known
as a pest primarily of apple in selected areas of Washington.
These included the region near Wapato in the lower Yakima valley,
the Sunnyslope area north of Wenatchee, and East Wenatchee from
Baker Flat to south of Pangborn Field (personal communication,
S. C. Hoyt). By the early 1980s the problem had spread to include
orchards in the Mattawa area, Royal Slope and Babcock Ridge, as
well as orchards along the Snake River west of Pasco. By the late
1980s many more orchards in the Yakima valley were experiencing
higher leafroller densities than historically had been the case.
In the 1990s the number of orchards with PLR populations requiring
specific controls continued to increase.
The obliquebanded leafroller (OBLR), Choristoneura
rosaceana (Harris), was not known as a problem in most Washington
orchards in the early 1980s, however, orchards around Milton-Freewater,
Oregon, and in some orchards near Mattawa it was the dominant
leafroller species. The Brewster area and north along the Okanogan
River has experienced increasing problems with OBLR in recent
years as well as cherry orchards near The Dalles, OR.
Life history and identification:
The life history and identification of leafrollers is given in
the new book, Orchard Pest Management: a resource book for
the Pacific Northwest , published by the Good Fruit Grower.
Here I will briefly summarize certain aspects of leafroller (LR)
biology important to its management in Washington apple orchards
along with new information on control tactics which look promising.
Leafrollers overwinter as a young larvae in a hibernacula
on the tree in crevices of bark or pruning cuts. Leafroller larvae
begin leaving the hibernacula as soon as green tissue pushes from
flower buds. To determine when all PLR larvae had left their hibernacula
buds were samples at regular intervals in spring and the percent
infested was determined. When the percentage of buds infested
within an orchard did not continue to increase it was assumed
that all larvae had left their hibernacula. No PLR larvae are
found in samples prior to the appearance of green foliage in expanding
buds. By half-inch green tip (HIG) the percentage of infested
buds (flower clusters) reached a peak and remained about the same
after HIG. These data indicate that most PLR larvae have left
hibernacula and begun feeding in buds by HIG. The emergence of
OBLR larvae from hibernacula has not been examined in detail in
WA. It is possible that OBLR larvae continue to emerge from hibernacula
The beginning of leafroller moth flight varies from
year to year and may differ slightly between PLR and OBLR. The
earliest recorded capture of PLR moths in a pheromone trap was
May 19, 1992 and the latest was June 8, 1984. The peak of PLR
moth flight is often very difficult to identify. In most years
a peak of activity is observed about 14 days after the beginning
of the flight, typically between June 15 and 25. However, there
are some years, or even locations within the same year, where
the peak activity is much delayed. Another fairly common characteristic
of PLR and OBLR moth flight is a considerable amount of moth activity
that occurs late in the first flight, often taking the form of
a second peak. This second peak usually constitutes only a minor
portion of the first flight but in some years will be almost as
large as the first peak. The origin of moths which make up the
second peak or the extended period of moth activity is not well
understood. Orchards showing such activity do not appear to have
resident populations of leafroller larvae and pupae which would
contribute to a second peak of moth activity. It is possible that
these moths originate from sources outside the orchard (or another
nearby orchard) where development has been slower. The second
peak of moth capture, or extended period of moth activity, is
confusing to growers and fieldmen when trying to manage leafrollers.
In most orchards this moth activity can be ignored.
The start of the second generation flight is often
difficult to determine. In most years there is not a clean break
in moth activity between the flight of moths of the overwintering
generation (May-June) and those of the summer generation (July-September).
The average beginning of the second flight is July 27, with the
earliest being July 21 (1992) and latest August 6 (1993). The
peak of moth activity of the second flight is even more difficult
to generalize about than that of the first. Usually the peak of
activity occurs about three weeks after the start of the flight
but high levels of activity can occur over a two week period or
Egg hatch is difficult to monitor in most orchards
because egg density is so low. Studies in orchards where PLR populations
were high has provided data to help characterize this important
event. Hatch of the summer generation begins about three weeks
after the beginning of the moth flight and lasts about three weeks,
though the duration of hatch can be longer in cool summers. In
1992, the first moth was captured on May 19 and hatch started
on June 4, 16 days later. In 1993, moth flight started on June
7 and hatch started on June 30, 23 days later. The duration of
the hatch period varies depending on temperature. In 1992, hatch
took 19 days. In 1993, the hatch period was long due to an unusually
cool July and took 36 days.
The hatch of the overwintering generation eggs occurs
in August and September and typically beginning about 3 weeks
after the start of the second moth flight. In 1992, hatch of the
overwintering generation eggs began on August 19 and lasted 36
days. In 1993, hatch began unusually late due to the cool summer
on September 7 but due to a warmer than normal fall was completed
in 23 days.
A degree-day model will also be helpful in predicting
key events in the life cycle of leafroller pests. The developmental
thresholds for PLR larvae and pupae have been estimated at 41
and 85 °F and, though thresholds for egg development may
be slightly higher, these values should provide a reliable base
for calculating degree-days to predict PLR development. The average
number of degree-days from January 1 to the capture of first PLR
moths in pheromone traps is 968. Using 950 as a BIOFIX (biological
fix point) to initiate a degree-day model subsequent events can
be predicted. The average number of degree-days from first moth
to peak activity of the first flight is about 280. The average
number of degree-days from first moth to the beginning of the
egg hatch is about 440. The duration of the egg hatch period of
the overwintering generation is about 510 degree-days. The duration
of the egg hatch period of the summer generation is about 450
degree-days. The average degree-days for the beginning of the
second moth flight is about 2380.
At this time there is no degree-day model for use
in predicting life history events for the OBLR. It is possible
to use the PLR model to estimate when OBLR egg hatch would begin
BUT developmental thresholds are different for OBLR and predictions
may not be accurate. An OBLR degree-day model is being developed
and should be available for evaluation in 1996.
Pheromone traps are useful for monitoring the seasonal activity
of leafrollers, especially the beginning and the peak flight.
While moth capture in pheromone traps has some value as a relative
estimate of codling moth (CM), Cydia pomonella L., populations
it is unreliable for leafrollers. Male leafroller (PLR or OBLR)
moths migrate long distances (several hundred meters) and are
highly attracted to pheromone traps. Thus, it is possible to capture
high numbers of moths (30 to 50+) in traps each week and still
have too few leafroller larvae in the orchard to cause significant
fruit damage. Low moth capture in pheromone traps usually indicates
that the orchard is not threatened. The placement of pheromone
traps for leafrollers does not appear to effect moth capture in
the same manner it does with CM. Traps placed at almost any height
within the tree canopy will capture approximately the same number
of moths. Pheromone traps are useful for determining which species
is present in the orchard or most common in a region. Research
on reducing pheromone load in lures used in pheromone traps is
beginning and may result in method of estimating levels of local
(in orchard) leafroller populations.
There are no easy method to indirectly estimate leafroller larval
densities, such as, beating trays or traps. Direct visual inspection
of foliage is the only reliable method to detect the presence
and estimate the density of leafroller larvae. Monitoring leafroller
larvae is the only precise means of determining their presence
and relative density within an orchard. The density of leafroller
larvae is not necessarily difficult to determine but it is time
consuming and requires a certain amount of training. There are
two times of the year when monitoring larval densities is important,
the spring from pink through petal fall and again in mid-summer,
late July and August.
It is very difficult to accurately assess the density
of larvae in buds before the pink stage of flower bud development
without the aid of magnification, such as a hand lens. By the
HIG stage of flower bud development feeding activities of leafroller
larvae are more noticeable but a hand lens needs to be used and
even with the aid of a hand lens the accuracy of density estimates
is questionable. It is easier to discover leafroller larvae from
tight cluster through pink because feeding damage and webbing
is more visible. The number of fruit buds or flower clusters which
need to be sampled depends on the density of larvae in the orchard.
While treatment thresholds for leafroller larval densities have
not been completely worked out some guidelines are suggested.
The distribution of leafroller larvae within an orchard
tends to be highly clumped. This distribution pattern increases
the number of trees which must be sampled in order to accurately
estimate populations. To accurately estimate a population density
of 1-2% infested buds it is recommended that 10 buds be examined
on at least 100 trees in a block (10 to 20 acres). Trees sampled
should be uniformly scattered throughout the block. One to two
percent infested buds is a reasonable estimate of a treatment
threshold for leafroller larvae in spring. Checking 10 buds
or flower clusters per tree takes only a few seconds but checking
a total of 100 trees is time consuming. Sampling between tight
cluster and pink will provide necessary information on leafroller
larval densities to determine if additional treatments are required
between pink and petal fall. PLR larvae are not distributed uniformly
in trees. In trees taller than 10 feet more, sometimes as many
as 10 times more, PLR larvae will be found in the upper half of
the tree compared to the lower half. It is therefore necessary
to sample buds in this portion of the tree if accurate estimates
of PLR density are to be made. The distribution of OBLR larvae
has not been studied as much as PLR but its larvae seem to be
more uniformly distributed through the tree canopy so that it
may not be necessary to search high in the tree.
Sampling leafroller larval populations after petal
fall but before larvae begin to pupate can provide some valuable
information about the level of control achieved by spring treatments.
It may also be the best measure of the need to apply summer controls
as sampling leafroller larvae in summer before they cause fruit
damage is all but impossible. After petal fall most leafroller
larvae move to growing shoots and feeding damage is relatively
easy to see. To obtain an accurate estimate the density of
leafroller larvae in summer it is necessary to examine 10 shoots
per tree on 100 trees per block.
Sampling leafroller larvae in summer is easier than
in the spring but generally provides less useful information.
It is not possible to obtain an accurate estimate of larval density
for a period of time following egg hatch since young larvae tend
to remain in the fruiting canopy of the tree and do not role leaves.
Larvae, especially PLR, are not easy to find until they are nearly
half grown and have moved to actively growing shoots to feed.
The majority of fruit injury by PLR larvae is done by young larvae
in late June and early July. Therefore, the July-August sample
of infested shoots comes too late to be used as a treatment threshold.
The information obtained from shoot infestations
is, however, useful for some management decisions. If larval densities
are high (>5% infested shoots) then a curative treatment of
Penncap-M or Lorsban should be considered to prevent further crop
loss and reduce the leafroller population that would threaten
fruit during harvest. If lower larval densities are observed (1-2%
infested shoots) softer controls, such as Bacillus thuringiensis
(Bt) sprays could be used. To obtain an accurate estimate the
density of leafroller larvae in summer it is necessary to examine
10 shoots per tree on 50 trees per block.
Understanding the behavior of leafrollers helps in the design
of control programs. The stage usually responsible for dispersal
is the adult. Codling moth females can readily move 50 to 100
meters from a source into an orchard, though most stop at the
border where infestations are typically highest. Observations
on the movement of PLR and OBLR females suggested that their movement
is very limited.
In a study to examine the movement of female PLR
an average of 95.2% of all egg masses were found trees indicating
that females had pupated on. A few egg masses were found on trees
one row away and fewer still two rows away from trees where females
had pupated. A similar result has been found with OBLR where nearly
90% of egg masses were found on trees where the larvae had pupated.
In the OBLR study there was more movement of females down a tree
row than across. It appears that female leafrollers have a strong
tendency to remain within a few trees where they developed as
larvae, choosing to stay put and call males to mate.
Leafroller larvae are known to disperse by ballooning.
They move to the edge of a leaf or shoot tip and drop on a silken
thread until a gust of wind carries them away. This is a very
risky means of dispersal fraught with dangers and resulting in
high larval mortality. The colonization by young leafroller larvae
of the uninfested trees was determined for PLR and OBLR in separate
studies. for PLR 94.0% of the total larvae found were on trees
where egg masses had occurred. One tree away almost 50% of the
trees had one larva but two trees away from where egg masses occurred
very few larvae were found. OBLR larvae also disperse by ballooning
but again the number that successfully colonize trees is low and
most larvae that leave a tree with an egg mass are found only
one tree away. Leafroller larvae may disperse several meters by
ballooning and it is probably larvae that are responsible for
infesting or re-infesting clean orchards.
Lorsban became the main product used for leafroller control in
the early 1980s when poor control was achieved with other chemicals.
However, the pest status of leafrollers began to change in the
mid-1980s as growers and crop consultants reported that chemical
control was less effective in many orchards than it had been in
the early 1980s. These experiences are supported by results from
field tests conducted at the WSU Tree Fruit Research and Extension
Center (TFREC). The level of control relative to an untreated
plot in 10 tests conducted between 1979 and 1985 averaged 94.6%
(89-100). In 12 tests conducted between 1987 and 1993 control
averaged 83.4% (64-99). All these tests were replicated small
plot trials and against spring and summer PLR populations. Treatments
were applied by handgun as dilute sprays so that coverage and
control should have been maximized.
Resistance development to organophosphate insecticides
was investigated by comparing the mortality of different PLR strains
(populations) to azinphosmethyl (Guthion, Azinphos-M). Dose-mortality
curves were used to estimate the LC50 and LC90, the concentration
of a chemical that causes 50% or 90% mortality, respectively.
The most susceptible PLR strain (PLR-S) originated from an organic
orchard in the Yakima valley. Other strains originated from the
TFREC and commercial orchards in Wenatchee and Yakima.
The most susceptible OBLR strain (OBLR-S) originated
from a laboratory colony obtained from the Agriculture Canada.
The LC50 of azinphosmethyl for PLR-S larvae was 17 ppm while LC90
was 54 ppm. The LC50 of azinphosmethyl for PLR larvae from commercial
orchards ranged from 64 to 100 ppm while the LC90 ranged from
190 to 288 ppm, near the field rate. Field collected PLR strains
were 3.8 to 5.9 times more tolerant (LC50) of azinphosmethyl than
the susceptible strain. The concentration of a full rate of azinphosmethyl
is 315 ppm. Thus, even at a full field rate the best level of
control expected for some PLR populations with this product would
be about 90%.
Lorsban remains the most effective control for PLR
larvae in the spring. Limited testing indicated that PLR-S larvae
were about 10 times more susceptible to chlorpyrifos (Lorsban)
when compared to azinphosmethyl (LC50-chlorpyrifos = 1.8 ppm vs.
LC50-azinphosmethyl = 17 ppm). Tests against field strains of
PLR larvae with Lorsban have not been completed but preliminary
data show that they remain susceptible to concentrations far below
the recommended field rate.
The LC50 of azinphosmethyl for OBLR-S larvae was
5 ppm while the LC90 was 16 ppm. The LC50 of azinphosmethyl for
OBLR larvae from two commercial orchards was 39 and 62 ppm while
the LC90 was 105 and 667 ppm. Field collected strains of OBLR
were 7.8 and 12.4 times more tolerant (LC50) of azinphosmethyl
when compared to the susceptible strain. Thus, azinphosmethyl
would be expected to kill little more than about 50% of OBLR larvae
in some Milton-Freewater orchards . In fact, where azinphosmethyl
was used in three field trials conducted in Milton-Freewater the
average control was only about 70%.
Lorsban still provides better control of overwintering leafroller
larvae than alternative materials (except possibly synthetic pyrethroids
whose use is not recommended because their use can result in mite
outbreaks). However, the level of control has diminished since
the early 1980s, a trend which if continues will eventually lead
to control failures in the field. Lorsban along with Penncap-M
(encapsulated methyl-parathion) represent the two most effective
chemical control products against leafroller in summer.
The primary tactic for leafroller control remains
the use of conventional insecticides. In spring, Lorsban, in combination
with oil, is the most common insecticide used. Oil does not enhance
leafroller control but is important for control of other pests.
Growers have used an additional application of Lorsban later in
the spring when populations were high but this tactic can have
a negative impact on leafminer biological control. Penncap-M along
with Lorsban have been the best conventional insecticides for
controlling leafrollers in summer. Both are good at controlling
adults or larvae, though Penncap-M has a longer residual activity
and may provide better control where leafroller populations are
Both Lorsban and Penncap-M are highly toxic to the
main parasite which provide control of the western tentiform leafminer
so should be used in summer only if absolutely necessary. Research
looking at using low rates of synthetic pyrethroids shows some
promise for leafroller control, however, at a cost of mite outbreaks.
Some experimental insecticides are promising as controls
for leafrollers and registration is anticipated within the next
one to three years. Confirm (Rohm & Haas) is a new chemistry
that stimulates a molt in the leafroller larva when exposed to
the product. However, the molt is now completed and the larva
dies within its old skin unable to feed. Death is not fast and
there is some indication that sub-lethal effects on larvae that
survive result in adults that do not reproduce very well.
Another insecticide that has been used in pear for
psylla control under an emergency exemption (Section-18), the
insect growth regulator Comply (Ciba-Geigy), is a very effective
leafroller control. One or two sprays applied one to two weeks
after petal fall, when larvae are in the last larval stage, can
provided high levels of control. Comply is an insect juvenile
hormone mimic and acts only against the last larvae stage of leafroller
but kills the egg of CM. Comply is also an effective control of
CM and leafminer without begin detrimental to most natural enemies
found in orchards.
Dow-Elanco is developing a new insecticide, Spinosad.
This is a fermentation product and has shown excellent control
of leafroller larvae and leafminer. It is relatively short lived
and does not appear to disrupt biological control of most pests.
based on strains of Bacillus thuringiensis (Bt), have shown
promise as controls for leafroller larvae. Bt products are stomach
poisons and as such are highly selective, effecting mostly Lepidoptera
but not natural enemies. Several Bt products have been tested
in the past three years and little difference in their efficacy
has been noted.
The precise timing of Bt products against a specific
stage of leafroller larvae in spring seems to be less important
than targeting periods of good weather. Because Bt must be consumed,
applying products when weather forecasts predict 3 to 4 days of
warm, dry weather should increase leafroller larval feeding activity
and thus efficacy. Bt products have been effective used as late
as petal fall though leafroller larvae are in the last two instars
by this time. Usually 2 applications of a Bt product in spring
or summer has provided as good of leafroller control as a single
conventional insecticide, providing excellent coverage of the
foliage is achieved and conditions following treatment are conducive
to larval feeding. The use of feeding stimulants to enhance leafroller
control with Bts has been studied and Coax has shown some promise.
Coax-Bt combinations have had longer residue activity compared
to Bt alone in field-aged bioassay tests. However, in large field
trials the benefit of these combinations has not as clear. There
is some evidence that adding silicon surfactants (e.g. Kinetic)
to Bts enhances leafroller control probably by providing for more
uniform coverage of the product.
The softest leafroller program is a Lorsban at HIG
followed with two Bts applied between pink and petal fall when
weather is most favorable. Summer controls of Bts may be needed
and are very important for the conservation of natural enemies
The use of pheromones to control leafrollers holds promise but
research using hand-applied dispensers has not effective suppression
of populations. The main component of the PLR and OBLR pheromones,
Z11-14Ac, is also present to some degree in the pheromones of
the fruit tree leafroller (FTLR), Archips argyropilus,
and European leafroller (ELR), Archips rosanus. Suppression
of PLR has been possible when initial densities were low (<
one larva per tree) using only a single pheromone component (Z11-14Ac)
released from the plastic "twist-tie" type dispenser.
Control has not been achieved when initial leafroller densities
were moderate or in orchards with large trees (>10 feet).
Research is continuing into new formulations of leafroller
pheromone, especially a sprayable pheromone formulation that showed
some promise in preliminary tests in 1995. Larger tests are planned
for 1996 with the ruling by EPA giving sprayable pheromones the
same tolerance exemption enjoyed by hand applied pheromones in
dispensers. There is also some potential to produce a dual CM-leafroller
dispenser that would release pheromones of both species.
Biological control of leafrollers has not been a realistic control
tactic primarily because parasites which attack leafrollers are
susceptible to conventional insecticides used to control CM and
other fruit pests. With the introduction of mating disruption
for CM control many of the insecticides traditionally applied
during the summer can be greatly reduced opening an opportunity
to establish and conserve natural controls for leafrollers.
Two parasites hold promise as biological controls
for leafrollers. A very small parasite, Trichogramma sp.
(probably platneri), was found in attacking 17% of the leafroller
egg masses in 1992. Trichogramma platneri is available
from commercial insectaries and research into mass releases for
CM control is begin conducted in California and Washington. Trichogramma
adults search out eggs of Lepidoptera and deposit one of their
eggs within the host egg. Development of Trichogramma is
completed within the host egg. Some or all of the eggs within
a leafroller egg mass can be attacked by Trichogramma.
Eggs which are attacked turn black as the parasite completes its
Another parasite, Colpoclypeus florus, a gregarious
ectoparasitic Eulophid, was discovered for the fist time in Washington
in 1992. This parasite is common in Europe where it is often the
most important parasite attacking leafrollers in orchards. Research
in 1992 and 1993 has shown that C. florus can be reared
on either PLR or OBLR. When collected in the field 13 to 17 parasite
adults are produced from each leafroller larva, similar to what
has been reared from PLR and OBLR larvae in the laboratory. Females
predominate making up about two-thirds of the parasite reared
from leafroller larvae collected in the field or reared from parasitized
larvae in the laboratory.
C. florus attacks later
stages of leafroller larvae. The host larva is stung by the parasite
in the head capsule. The leafroller larva is not paralyzed but
stops feeding and spins a dense webbed shelter within a rolled
leaf. The webbing of this shelter is several times more dense
and tough than the webbing found in leaf shelters produced by
unparasitized leafroller larvae. C. florus eggs are laid on the
webbing and when they hatch the small parasite larvae crawls onto
the leafroller larva and begins feeding. C. florus larvae
are a bright green and can be seen covering the body of the host
larva. When mature C. florus larvae form pupae within webbed
shelter. Male C. florus emerge slightly before females
and wait within the webbed shelter until females to emerge when
mating occurs immediately. After all females have been mated,
usually one day after adult emergence begins, adult C. florus
leave the webbed shelter and search for new host larvae to attack.
Complete development, adult to adult, requires about 18 days at
a constant 77 °F and 26 days at 68 °F.
Low levels of parasitism of PLR by C. florus
has been noted in the spring. However, in late summer C. florus
has parasitized 70 to 95% of PLR larvae within unsprayed apple
orchards. C. florus uses late instar larvae as hosts and,
according to European literature, overwinters as a mature larva.
The main leafroller species that are pests of fruit trees in the
western US overwinter as eggs (FTLR and ELR) or young larvae (PLR
and OBLR), neither being acceptable hosts for C. florus.
The lack of suitable hosts in late summer to support overwintering
C. florus explains its disappearance from orchards in the
spring following a year in which high levels of leafroller parasitism
occurred in late summer. Low overwintering survival of C. florus
may mean that it will have to be introduced into orchards annually
to maintain effective leafroller control at low levels.
Experiments conducted to evaluate the potential of
C. florus to control PLR larvae were conducted in the spring
and summer of 1993. The percentage of leafrollers larvae parasitized
by C. florus ranged from about 50% to almost 100%, lower
in the spring and higher in summer. Spring dispersal of C.
florus appears to be limited mainly to within the row in which
they are released while much broader dispersal has been noted
in the summer. Large block releases of C. florus are planned
for the spring and summer of 1996. There is some potential that
C. florus will be produced commercially so that growers
will be able to purchase it and release it annually in the field.
Studies in 1994 with C. florus showed that high summer temperatures could be detrimental, suppressing adult activity. Subsequent investigations in the laboratory showed that C. florus adults do not survive for more than a few hours when exposed to constant temperatures above 92°F. If this parasites is to be useful for biological control of leafrollers a better understanding of how high summer temperatures effect its behavior and survival will be necessary.
Parasitism by parasites other than C. florus
have ranged from 0% in the spring to between 10% and 50% in summer.
A species of parasitic fly in the family Tachinidae can be the
most common parasite in unsprayed blocks where C. florus
has not been released, especially in northeast Oregon and the
Columbia Basin. Combined leafroller parasitism on trees where
female C. florus have been released has reached as high
as 99.4%. The ability of C. florus, and other parasites,
to find and kill leafroller larvae when present at low densities
will determine just how valuable biological control will be in
the management of this pest.