Management of Leafrollers

in Washington Apple Orchards

Jay F. Brunner

Washington State University

Tree Fruit Research & Extension Center

Wenatchee, WA 98801

Species,distribution

Life history

Adult monitoring

Larval sampling

Behavior

Resistance

Chemical controls

Bacterial insecticides

Mating disruption

Biological control

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 after HIG.

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 more.

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.

Adult Monitoring: 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.

Larval Sampling: 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.

Behavior: 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.

Resistance: 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%.

Chemical Controls: 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 high.

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.

Bacterial insecticides, 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 in orchards.

Mating disruption: 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: 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 development.

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.