Bt and leafroller control
New insecticide chemistry for leafroller control
Resistance management and new chemistries
Attract and kill
The codling moth, Cydia pomonella L., is the "key"
pest of pome fruit in Washington. An average of nearly 3.5 applications
of insecticides is applied per acre each year to manage codling
moth. This means that some orchards are using 4 or 5 or more insecticide
applications to keep this pest under control. At the same time
growers and crop consultants report that fruit damage by codling
moth has been increasing. The introduction of codling moth mating
disruption has resulted in about 15% of Washington's apple acreage
using this selective product, resulting in reduced sprays of broad-spectrum
insecticides to control this pest. Often, however, supplemental
cover sprays are required in mating disrupted orchards because
codling moth densities are too high for pheromones alone to provide
adequate control. Currently, broad-spectrum insecticides are the
growers' only alternative to supplement codling moth mating disruption.
These broad-spectrum insecticides, primarily Guthion and Imidan,
have negative impacts on some natural enemies present in orchards.
The development of new insecticides with different modes-of-action
and with high selectivity promises to provide chemical control
tools that can be used in conjunction with mating disruption,
or as "stand alone" products, to control codling moth
and retain full activity of natural controls.
Leafrollers are the second most important pest of apple in Washington
and in some regions exceed codling moth as the "key pest"
in orchards. Two leafroller species are important in commercial
fruit orchards in eastern Washington, the pandemis leafroller
(PLR), Pandemis pyrusana Kearfott, and obliquebanded leafroller
(OBLR), Choristoneura rosaceana (Harris). The life history
and identification of leafrollers are given in the book Orchard
pest management: a resource book for the Pacific Northwest,
published by the Good Fruit Grower. Pheromones for each species
can be used to monitor their seasonal activity and assist in the
timing of summer control applications.
There are two generations of PLR and OBLR per growing season.
Both species of leafroller overwinter as young larvae in hibernacula
in crevices of bark or pruning cuts on the tree. By half-inch
green tip (HIG) most larvae of PLR have left their hibernacula
and can be found feeding in buds. The emergence from the overwintering
hibernaculum appears to be more extended in the OBLR because older
and younger larvae may be found together during bloom. Moth flight
varies from year to year and may differ slightly between PLR and
OBLR. In general, first moths are detected in late May or early
June, with peak flight between June 15 and 25. 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 beginning
of the second flight typically occurs in late July with the peak
of moth activity about three weeks later.
Egg hatch of the summer generation begins three to four weeks
after the beginning of the moth flight and extends for a period
of three to four weeks, longer in cool summers. There are degree
day (°D) models for both leafroller species that can help
to time treatments against the summer generation. Models for each
species use slightly different lower thresholds, 41°F for
PLR and 43°F for OBLR. Following Biofix, or the capture of
first moths of the overwintering generation in pheromone traps,
egg hatch for each species begins after an accumulation of about
400°D. Egg hatch of the overwintering generation begins about
mid-August and can extend through much of September, typically
beginning about three weeks after the start of the second moth
flight. It is difficult to apply controls against leafroller larvae
at this time because of the nearness of harvest and because it
is often impossible to drive spray equipment through the orchards.
Lorsban continues to provide the best control of overwintering
leafroller larvae compared to other registered alternatives, with
the possible exception of synthetic pyrethroids, e.g., Asana,
which are not recommended because their use can result in mite
outbreaks. The level of control with Lorsban has diminished markedly
in some orchards since the early 1980s. For example, in the early
1980s when Lorsban was first recommended for leafroller control,
the average percent suppression in field tests was nearly 96%.
In the period from 1985 to 1990 the average percent control of
leafrollers with Lorsban was only about 80%. In recent field tests,
control was only about 70% or less. While these data do not prove
that leafrollers have developed resistance to Lorsban, they certainly
add to the anecdotal data suggesting that this product is not
providing the same level of control experienced when first introduced.
Penncap-M (encapsulated methyl-parathion), which has been relied
upon as a summer control of leafroller populations, has not provided
acceptable suppression in many orchards in the past three years.
Where this has occurred, consultants should avoid recommending
Bts and leafroller control: The softest
leafroller control programs relied upon an application of Lorsban
in the delayed-dormant period followed by bacterial insecticides
(Bt products) applied from pink through petal fall and again in
summer when needed. Bt products are stomach poisons and, as such,
are highly sensitive to variations in weather and spray coverage
(concentrate sprays are a good method of application if foliage
is well covered). Bt products have short residual activity, lasting
only about 7 days in the spring and even a shorter time in the
summer. Figure 1 gives data on the residual activity of Dipel
2X following application in the spring and summer. There were
significant declines in leafroller mortality after only 4 days
in summer, while mortality in this test remained relatively high
through 8 days in the spring. These data are typical of the residual
activity of most Bt products. There has been little difference
observed in the ability of different Bt products to control leafrollers.
Most experience has been gained with Dipel 2X and Javelin, but
other products used in appropriate concentrations have provided
adequate suppression of leafrollers. Generally, leafroller population
suppression with Bt products requires more than one application
to achieve desired results.
The potential to enhance Bt product efficacy against leafroller by adding a "feeding stimulant" to the spray mixture was investigated in the laboratory and field. In the laboratory the feeding stimulant Coax® was tested to determine if any increase in mortality could be observed when it was mixed with a Bt product. The test consisted of dipping leaves in a solution containing a Bt product diluted to the recommended field rate or a solution containing a Bt product plus Coax. In most tests the addition of Coax increased leafroller larval mortality, suggesting it might also provided better control of leafrollers in the field. In field trials, however, results of adding Coax to Bt products have produced highly variable results so specific recommendations on adding Coax to Bt treatments have not been made.
In the spring, applications need to be applied when daily maximum
temperatures are at or above 65°F for at least 3 days. This
ensures that larvae are actively feeding and therefore consuming
toxic doses of the product. One negative side effect of using
Bt products noted in recent years has been that some larvae surviving
exposure to Bt products have a delayed development and thus produce
moths and larvae of the next generation out of sequence with the
"normal" or "expected" time. This may make
the leafroller degree day models of little use in timing summer
control sprays. Dr. Alan Knight (USDA-ARS, Wapato) has conducted
research into the delayed development of leafrollers exposed to
Bt products and suggests that a delayed development of the summer
generation of about 7 days can be expected with each Bt application
made in the spring. Continued use of Bt products is recommended
as a means to keep leafroller control programs "soft"
and to reduce overuse of Lorsban or new insecticides in a resistance
New insecticide chemistry for leafroller control
The Food Quality Protection Act of 1996 (FQPA-96) will affect
the availability of chemical control tools used for codling moth
and leafroller control in tree fruits for nearly 30 years. This
will likely occur within the next three to five years. How FQPA-96
will specifically affect different insecticides is unknown, and
speculation could take up hours of fruitless discussion. However,
there is hope in the form of some new (novel) insecticide chemistries
that are reviewed in this article. Three new insecticides, Confirm,
Comply and Success, show promise as controls for leafrollers and
codling moth, and registration is anticipated within the next
one to three years.
CONFIRM® (tebufenozide, Rohm and Haas) is a new chemistry that stimulates a premature molt in the larvae of Lepidoptera. CONFIRM binds to the ecdysteroid receptor in Lepidoptera almost exclusively. Because it is a novel chemistry it should be effective against insect species that have developed resistance to traditional insecticides.
The molt that is initiated by CONFIRM® is not completed, and the larva becomes trapped within its old skin, is unable to feed and eventually dies. Death is often slow, and there is some indication that sublethal doses that allow larvae to survive result in adults that are not able to reproduce. In addition, it has been shown that for some insects exposure of adults to residues of CONFIRM can result in reduced fecundity or egg viability.
The targeted stage against codling moth for CONFIRM is the newly hatched larva. The timing is the same as that traditionally used for Guthion or Imidan, i.e. at 250°D following BIOFIX (first moth capture in pheromone traps in the spring), and the treatment is repeated at about 21 days following the first spray to cover the entire first generation hatch. In a timing study conducted in 1996, the traditional first cover timing used for Guthion or Imidan seemed to work best for CONFIRM. This test was conducted in an orchard where codling moth densities were extremely high, and CONFIRM does not perform as well as more traditional products under high pressure situations.
The size and structure of the CONFIRM molecule (Fig. 2) make it difficult for it to move readily through the insect skin and therefore it must be consumed to have activity. This means that spray coverage with CONFIRM on the target site, the fruit, is crucial to efficacy. A uniform distribution of a CONFIRM residue is probably the best way to ensure maximum codling moth control. Dilute spray volumes are recommended, and the addition of a surfactant that will evenly distribute the product over the plant is the best strategy to gain the greatest effect. Several years of experience with CONFIRM against codling moth suggest that it should not be rated as a "superior" product for controlling this pest. Factors reducing its effect are the uniform distribution of residues, that is spray coverage, and timing. Because only the first stage larva is controlled, residues must be in a place where the larva will eat a toxic dose.
In a study where CONFIRM was used in a Delicious orchard as the sole control for codling moth, suppression was never as good numerically as the conventional program; however, in one of three years, crop loss was statistically the same as the untreated control. It must be recognized that in this test CONFIRM residues were put to a severe test because the adjacent untreated plot had an extremely high codling moth population.
In a study where apples were produced without the use of neuroactive insecticides CONFIRM became a critical tactic used to protect the crop from both codling moth and leafrollers. In the first year of the project, mating disruption was the primary control used for codling moth and it was supplemented by oil sprays where populations were too high for codling moth alone. Leafrollers were controlled primarily with Bt products. At four sites after the first year of the study, codling moth or leafroller populations had reached levels where it was feared that the ability of available tactics to provide control in the second year would not be adequate to protect the crop. CONFIRM was made available as a fresh-fruit EUP in 1996 on limited acres and used at these four sites because it had no neuroactivity associated with its mode-of-action. At sites B1 and W2 codling moth damage at harvest in 1995 was too high to expect mating disruption and available supplemental controls to provide crop protection in 1996. Similarly, at sites W2, Y1 and D1, leafroller damage to fruit was high and there was concern that Bt products alone would not provide adequate crop protection in 1996. With the use of CONFIRM codling moth and leafroller populations were controlled in 1996, with the exception of site W2 where a spray was missed and where damage was all located at the top of a slope where mating disruption had least effect on codling moth. In 1997 (data not shown) CONFIRM was again used to supplement codling moth mating disruption and to control leafrollers. This combination of tactics provided excellent protection of the crop at all locations.
A fresh fruit-only experimental use permit (EUP) has allowed the testing of CONFIRM on 50 to 100 acres of apple in Washington in 1996 and 1997. It is expected that the fresh fruit EUP will be available again in 1998. Full registration of CONFIRM is anticipated for 1999.
CONFIRM is highly selective, acting only on larvae of moths, and
thus is safe to bees, predator and parasitic insects, and mammals.
In the figure below, the relative toxicity of CONFIRM and Guthion
against mammals (rats), birds, fish and bees is shown. CONFIRM
is essentially nontoxic to all these organisms. This should make
CONFIRM an ideal tool to use in IPM systems as a highly selective
control against Lepidoptera while preserving natural enemies.
COMPLY (fenoxycarb, Novartis) has been used for several years in pear for psylla control under an emergency exemption (Section-18) to control pear psylla. The chemical structure of COMPLY is shown in Fig. 3. This chemical is classified as a carbamate insecticide although it does not act anything like a carbamate. Carbamate insecticides are nerve toxins that block the transmission of nerve impulses at the synapse, the junctions between nerves, in a manner very similar to the action of organophosphate insecticides. While COMPLY looks like a carbamate, it acts as an insect growth regulator. In fact it mimics the action of the insect's juvenile hormone. In the normal process of insect development, juvenile hormone is at a high concentration in the younger life stages and declines in concentration as the insect matures; that is, the juvenilizing effect of the hormone becomes less, allowing mature structures to be expressed. When COMPLY is introduced at a time in the insect's life cycle when juvenile hormone is supposed to be low it causes abnormal development to occur, and usually this results in the insect's death.
COMPLY is another large molecule and thus is most effective when consumed by the insect. It has little or no contact activity. The exception to this is activity COMPLY has on insect eggs. COMPLY kills the eggs of codling moth and leafminer though it has not been shown to be toxic to leafroller eggs. COMPLY disrupts the normal embryonic development of codling moth and leafminer eggs and is most effective when the eggs are deposited on top of the residue so it important to have control sprays in place prior to egg laying. Several studies have been conducted testing the efficacy of COMPLY against codling moth. Table 7 shows data from one such test. Note that the COMPLY treatments were applied early, at petal fall, just as codling moth were beginning to lay eggs, and treatments were repeated in 21 days to provide coverage of the entire oviposition period. The recommended timing of the first codling moth spray with COMPLY is between 75-100°D. Fruit damage in this test was high, owing to the extreme pressure of the pest population, and still COMPLY provided control similar to Guthion even at low rates. While this test was carried out for the entire season, COMPLY use will be restricted to the early spring so the opportunity to use it against the second codling moth generation will not exist.
The stage of leafroller controlled by COMPLY is the last larval instar. Younger leafroller larvae are not affected by the consumption of COMPLY. Targeting the last larval instar of leafrollers means that COMPLY treatments should be applied at petal fall and possibly repeated in 14 to 21 days to cover the entire development of leafroller larvae in the spring. It will be important to not delay COMPLY treatment too long because leafroller larvae stop feeding and pupae are not affected by COMPLY near the time when they pupate. In this test both rates of COMPLY were similar in their control. Table 9 shows results of a similar test conducted against OBLR larvae. In this test control was better than Lorsban against a very high leafroller population. Note that efficacy in both of these tests was evaluated by counting the number of leafrollers in the summer generation, that is, in the generation following the one actually treated. This is often the means used to evaluate products like COMPLY since their mode-of-action results in a slow death of the target stage and it is difficult to assess effects on the generation that is actually treated.
COMPLY is generally not toxic to insect natural enemies and thus
is very compatible with pest management approaches that encourage
biological control of pests. One caution is that COMPLY can be
toxic to bees if pollen is contaminated. When contaminated pollen
is taken back to the hive and fed to bee larvae they do not develop
normally; therefore, COMPLY should never be used when bees are
present in the orchard. COMPLY registration on apple is anticipated
for 1998 or 1999. Its use will be restricted to the early postbloom
period, probably with a long PHI of 70+ days. In the figure below,
the relative toxicity of COMPLY and Guthion against mammals (rats),
birds, fish and bees is shown. COMPLY is essentially nontoxic
to all these organisms with the exception of fish and bees. COMPLY
should be an ideal tool to use in IPM systems as a highly selective
control against Lepidoptera while preserving natural enemies.
SUCCESS® (spinosad, DowElanco) is a new insecticide discovered by DowElanco scientists. You may have heard this product referred to as "spinosad," which is the proposed common name for the chemical. The name "spinosad" comes from the active chemicals in the insecticide called spinosyns. Spinosyns are a naturally derived set of molecules with insecticidal activity produced from a new species of Actinomycetes, Saccharopolyspora spinosa. Two of the most active spinosyns, A and D, make up the product called SUCCESS.
Like the other insecticides discussed above, it is a large molecule and is not readily absorbed across the insect's skin. Its main effect then is through ingestion, carrying with it all the requirements of good spray coverage already discussed with the other products.
The actual mode-of-action of SUCCESS has recently been determined. It acts on the insect's nervous system at the nerve synapse as described in the following quote from a DowElanco publication: "Spinosad depolarizes insect neurons by activating nicotinic receptors, causing widespread hyperactivity in the nervous system, which leads to involuntary muscle contractions and tremors. Eventually, insects become prostrate with tremors and, after prolonged exposure, become paralyzed from neuro-muscular fatigue" (from "The Modes of Action of Spinosad and Other Insect Control Products" by V. L. Salgado, Senior Scientist, DowElanco, Down to Earth 52: 35-43). Following paralysis, the insect dies, usually after a relatively short time period.
SUCCESS has been shown to provide excellent control of leafrollers and leafminer. For leafrollers, the target of SUCCESS is the larval stage, and good control of all stages has been achieved. Both spring and summer control trials have demonstrated that SUCCESS is highly effective against leafrollers applied as either a dilute or concentrate spray as long as good coverage of foliage is achieved. The best timing of spring applications to control the overwintering larvae is at petal fall. In this trial SUCCESS provided better control than Lorsban when both were applied at petal fall or when Lorsban was applied at a more traditional timing, half-inch green tip (HIG).
Summer applications timed at young larvae following egg hatch have provided the best control. SUCCESS has been reported to have a relatively short residual life and, under moderate to high leafroller population pressure, two applications in the summer may be required to achieve maximum effect. Two applications of SUCCESS provided better control than one, and control was comparable to traditional summer leafroller control products.
SUCCESS is used at very low rates and appears to have very little
negative impact on the environment of human health. In the figure
below, the relative toxicity of SUCCESS and Guthion against mammals
(rats), birds, fish and bees is shown. SUCCESS is essentially
nontoxic to all these organisms with the exception of honey bees.
Test have shown, however, that dried residues of SUCCESS have
little, if any, effect on honey bees and it should be possible
to use SUCCESS during or near the blossom period in a manner similar
to how Carzol is used. SUCCESS should be an ideal tool to use
in IPM systems as a fairly selective control of leafroller and
leafminer while preserving natural enemies. SUCCESS has been shown
to be toxic to some parasitic hymenoptera and may affect biological
control of certain insects if it is used when adult parasites
are active in the orchard; however, the residues of SUCCESS are
toxic to parasitic wasps for only a relatively short time, 5 to
7 days. Full registration of SUCCESS is anticipated for 1998.
Resistance management and new chemicals: As new insecticide chemistry becomes available for use in tree fruit crops, it is imperative that the industry think of implementing these new tools with a sound resistance management strategy. A direct substitution of new chemistry for the "old" insecticide chemistries currently in use will be the surest way to promote resistance development in pest populations. We have gathered baseline data on the levels of susceptibility of CONFIRM and SUCCESS to leafroller populations within the state and in reference to known susceptible populations maintained at the TFREC in Wenatchee. Table 12 gives the LC50 values for laboratory colonies of PLR and OBLR, plus values for two field populations of OBLR for CONFIRM and SUCCESS and two conventional insecticides. These values provide a reference point to which future data can be compared in order to determine if resistance levels to different insecticides are changing.
Initial resistance survey results suggest that leafroller, and probably codling moth, populations in Washington do not exhibit cross-tolerance (resistance) between COMPLY, CONFIRM and SUCCESS and organophosphate insecticides. Hopefully as new insecticides are registered they will work equally well to control leafrollers in all regions of the state. The optimum resistance management strategy to follow when and if all three new insecticide chemistries become available is to use an insecticide only against one generation of leafroller over a two-year period. Because the new insecticides should be more effective than current insecticides, populations of leafrollers in general are expected to decline markedly compared to what is experienced now. The judicious use of any insecticide is always the best resistance management strategy.
Integration of new insecticides into a pheromone-based IPM program
also represents a valuable resistance management strategy for
using these new insecticides. CONFIRM and COMPLY provide control
of both codling moth and leafrollers. They can be used as "soft"
supplemental controls in codling moth mating disruption programs
and, as pheromone delivery technology continues to develop, in
mating disruption programs for leafrollers.
Mating disruption: The potential
to use pheromones to disrupt mating of leafrollers has been a
"work in progress." A dual codling moth-leafroller pheromone
dispenser has been tested within the fruit industry, primarily
by Dr. Alan Knight (USDA-ARS, Wapato), and has shown some promise
in reducing leafroller damage while providing codling moth control.
This dual dispenser will be registered for use in 1998, but the
distribution will be somewhat limited in Washington until more
information on its performance is obtained.
Formulations of sprayable pheromone for leafroller control are
also under development, and one product produced by Ecogen has
federal registration. Research trials conducted on sprayable leafroller
pheromones to date do not provide sufficient evidence to positively
state that they do control populations of leafrollers in orchards.
However, there is promise that as these products continue to be
improved they will provide yet another control tactic to battle
leafrollers in fruit orchards.
Attract & kill: SIRENE®-CM
(IPM Technologies) is a new product for codling moth control that
uses removal of male moths as the mode-of-action. It is a combination
of pheromone and synthetic pyrethroid in a black tacky tar-like
substance that is placed as a small drop on a tree. Male moths
are attracted to the drop by the pheromone and upon touching the
"false female" receive a lethal dose of insecticide.
There is good potential that this same approach could be used
as a control for leafrollers. An active research program will
begin in 1998 to develop a SIRENE product for leafrollers.
Biological control: A large number
of parasitic insects, 15 or more species, has been identified
as attacking leafrollers in Washington. Some of these parasites
have great potential to help reduce leafroller populations in
commercial orchards, especially when reliance on broad-spectrum
insecticides is reduced as new insecticides become available.
Research by Chris Nobbs (graduate student at WSU) and Dr. Robert
Pfannenstiel (WSU-TFREC, Wenatchee) has shown the potential for
culturing a nonpest leafroller on a cover crop plant. This cover-crop
leafroller can provide a host for parasites, thus maintaining
natural enemy populations within the orchard with the potential
to greatly impact the biological control of pest leafroller species.
SUMMARY: Are there really any "magic
bullets?" I would have to say no. There are, however, some
exciting new insecticides and other pest control tactics under
development or very close to registration that hold great promise
in the fight against leafrollers in fruit orchards. The new insecticides
in particular will require growers and crop consultants to adopt
new strategies for timing of sprays and different expectations
on how fast the kill of the target will occur. If some new insecticides
with novel modes-of-action are not registered prior to the full
effect of FQPA-96 being realized, pest control in tree fruits
will indeed be challenging. With the registration of CONFIRM,
COMPLY and SUCCESS, and the availability of additional technology,
such as mating disruption and "attract & kill,"
the control of leafrollers and codling moth will be better and
will allow for a more complete expression of biological controls
in orchards than at any time since the introduction of synthetic