Washington Tree Fruit Research Commission

Research Reports

Insect Responses to Induced Apple Defensive Chemistry (2004)

FINAL PROJECT REPORT
WTFRC Project #
YEAR 0/0
Organization Project #
Title:Insect Responses to Induced Apple Defensive Chemistry
PI:Peter J. Landolt
Organization:USDA-ARS, Yakima Agricultural Research Laboratory, Wapato, WA
 PDF version of report

Collaborators

Roy Navarre, USDA-ARS, Prosser

Objectives

1. Determine effects of induced defensive chemistry and leaf age on suitability of apple leaves for caterpillars.

2. Determine effects of induced defensive chemistry of apple leaves on caterpillar movement.

3. Determine effects of induced defensive chemistry on codling moth oviposition.

 

Significant findings

1.  Apple leaf quality deteriorates with the season. We confirmed in multiple seasons that L. subjuncta larval development is prolonged and pupal weight is decreased on apple foliage as the season progresses. These data indicate the apple leaves are a relatively good food material for Lacanobia larvae early in the season but deteriorate in quality later in the season.  Similar results were obtained for Pandemis leafroller larvae on apple leaves. Poor leaf quality may cause more damage to fruit by leaf-eating caterpillars.
2. Sources of variance for apple leaf quality.  The quality of apple leaves as caterpillar food was not strongly tied to leaf relative age (as leaf position on a branch).  Leaf quality was not reduced with methyl jasmonate treatments (to induce chemical defenses), or with prior feeding on the leaves by other caterpillars (also should induce chemical defenses). Leaf quality did vary with apple variety, with fastest development on Red Delicious leaves.
3. Potato is a superb host of Lacanobia larvae, compared to many weeds, annual crops, and foliage of tree fruits evaluated.  Also, in 2003, Lacanobia larvae, mixed with cutworms and armyworms, were found defoliating areas of commercial potato plantings near Othello and near Ephrata.
4. Pandemis leafroller and Lacanobia fruitworm larvae prefer apple foliage over fruit and Lacanobia fruitworm larvae preference for foliage was greatest with Spring foliage.  These findings support the hypothesis that damage to apple fruit may be exacerbated by deterioration in foliage quality.

Methods

A. Suitability of apple foliage:
Two assay methods were used to evaluate caterpillar development on foliage.  Individual neonate (newly hatched)  caterpillars were held in 2 oz clear plastic cups with leaves.  The leaves were added or replaced daily and old leaves and droppings were removed as needed. These were maintained until adults emerged and data were recorded on mortality, pupation, and adult emergence.  Pupae were weighed to the nearest milligram.The second assay involved placing 3rd instar larvae in the cups with foliage for 7 days. Larvae were weighed on day 1 and day 7 to determine weight gained, and leaves were added daily. These assay methods were used in a series of experiments to evaluate larvae performance on apple foliage.
1. Suitability of apple foliage as Lacanobia food through the season.
Neonate larvae were placed with foliage throughout much of the season, depending on the availability of Lacanobia eggs, to get data on development time, mortality, and pupal weight.  These assays were conducted over three years to accumulate replicates in May through August.
2. Effect of leaf position (as relative indicator of leaf age) on Lacanobia development.
Third instar larvae were placed with leaves that were a) 1st and 2nd  from the tip, b) 3rd and 4th from the tip, c) 5th and 6th from the tip, d) 7th and 8th from the tip, and 9th and 10th from the tip.  Leaves were replaced daily for 7 days to obtain data on weight gained versus treatment (leaf position).
3. Effect of methyl jasmonate treatment to apple leaves on Lacanobia development.
Application of methyl jasmonate to foliage was done with a plastic spray bottle containing a 0.5 mM/L methyl jasmonate solution in water.  Leaves were wetted and foliage was held for 48 hours before use in experiments.  Third instar larvae were placed with treated and untreated leaves, which were replaced daily for 7 days to determine weight gained versus methyl jasmonate treatment.
4. Effect of apple variety on Lacanobia development.
Neonate larvae were placed with foliage from Red Delicious, Golden Delicious, Gala, Fuji, and Granny Smith apples, and were maintained through to pupation in order to determine if suitability of foliage as larval food varied with variety. This experiment was conducted in August.
5. Effect of insect damage to apple leaves on Lacanobia development.
Cloth sleeves were placed over apple branches on multiple trees and 3 third instar Lacanobia larvae were placed within each of the sleeves for 48 hours to effect feeding damage to the foliage and provide time for induced biochemical responses by the tree. Foliage was then harvested for use in the assay, in comparison to foliage from another set of trees that were not so treated.  Third instar larvae were placed with either treated or untreated foliage, which was replaced daily for 7 days to obtain data on weight gain versus treatment.
6. Potato foliage as food for Lacanobia larvae.
Field grown Russet Burbank potato foliage was used as food for larvae, using both the egg to adult assay and the 7 day weigh gain assay. Using the first assay design, Lacanobia larvae were fed potato foliage daily until they pupated, with data on survival, development time, and pupal weight.  The second assay design was used to determine the amounts of weight gained by larvae over a 7 day period, provided potato foliage daily.  This was used to compare potato foliage treated with methyl jasmonate to untreated foliage, and potato foliage to apple foliage.

 

B. Behavior of codling moth.
1. Larval choices in arena assay.
An arena type assay was used to evaluate larval behavior in the presence of fruit and foliage.   A 16 oz clear plastic cup with a screened lid was provided a small apple fruit and a cluster of apple leaves.  A 3rd instar larva was placed within the cup and observed at one hour, 3 hours, and at 24 hours to determine feeding on fruit and foliage. These assays were conducted in batches of 20 replicates.   The basic experiment (comparison of Fuji fruit and foliage) was conducted using Lacanobia fruitworm larvae and Pandemis leafroller larvae,  in winter with greenhouse grown apple saplings and with cold-stored Golden Delicious apples, and again in early summer using fresh cut Fuji apple foliage and fresh picked thinning sized Fuji apples.  After comparing data at one hour, 3 hours, and 24 hours, it was decided that the 24 hour data was most useful, due to variance in the time it took larvae to begin feeding.
2. Adult codling moth oviposition in response to fruit in flight tunnel.
A flight tunnel was used to evaluate codling moth rates of oviposition when in the airstream downwind of pear fruit, and infested pear fruit, both in comparison to no fruit as a control. This experiment tested the hypothesis that pear fruit will produce greater amounts of kairomones in response to feeding damage by codling moth larvae (induction of chemical synthesis), as occurs with apple fruit, stimulating adult moth oviposition.
Other attempts to develop discriminating assays for the study of codling moth oviposition as a response to fruit or foliage were not successful.  Moths appeared to respond primarily to physical parameters, such as light and reflectance, and to surface features such as roughness.
C. Chemical defenses of Apple Foliage.
Apple foliage posseses a variety of types of compounds thought to protect against attack by pathogens, insects and other organisms.  These include tannins, terpenoid compounds, green leaf volatiles, and protease inhibitors.  We have conducted analyses of terpenoid  and green leaf volatile compounds, using organic solvent extractions followed by GC-MS, and a fluorimeter to quantify changes in levels of a protein standard with a protease standard and apple leaf extract added.  Protease inhibition is quantified as the degree of reduction in protease breakdown of the protein standard.

Results and discussion

A.
1. Suitability of apple foliage as Lacanobia food through the season.
Cumulative data over 2001-2003 indicates a consistent pattern of reduced performance of Lacanobia larvae on apple foliage as the season progresses.  From late May through August larvae fed apple leaves took longer to develop, suffered higher mortality rates, and grew to a smaller size before pupation, as the season progressed.
There is potential significance of these findings in the areas of biological control, on modeling based on degree days, and on larval damage to fruit and the economic injury levels of Lacanobia populations.  First, when larvae take longer to develop to pupation, they are exposed for a longer period of time in the field to predators and parasites, potentially increasing biological control.  Second, the differential rates of development on apple through the season were not temperature related (this was done under controlled conditions) and development models would have to take into consideration that development rates are dependent in part on food quality, in addition to temperature.  This relationship will affect the patterns of emergence of adults, and life stage phenology generally. Third, the deterioration in quality of apple foliage as larval food and increased length of time for larval development might increase risk to damage to apple fruit, as larvae search for higher quality food, and over a longer period of time than would occur on better foliage. This latter point also relates to economic injury levels of larval densities on apple trees.  At most larval densities encountered, the risk is fruit damage, not significant tree defoliation.  If that risk increases through the season, as from first generation larvae to second generation larvae, then the economic injury threshold of populations of Lacanobia may be lower in the second generation.  This remains however to be determined.

2. Effect of leaf position (as relative indicator of leaf age) on Lacanobia development.
The amount of weight gained by larvae when fed leaves of apple did not vary with the position of the leaf on the branch.   Leaf age decreases from the base of a branch to the tip, as the branch grows.  The hypothesis was that leaf quality as larval food  might deteriorate with leaf age, as constitutive defensive chemicals are accumulated over time in leaf tissue. Thus, older leaves at the base of a branch might have greater amounts of such chemicals, such as tannins, protease inhibitors, etc, compared to leaves at the growing tip.  In this experiment, we did not have information on the exact age of each leaf however, and we do not know how much leaf age varied with position on the branch.

3. Effect of methyl jasmonate treatment to apple leaves on Lacanobia development.
The amount of weight gained by larvae of Lacanobia when fed apple leaves treated with methyl jasmonate was significantly greater than weight gained by larvae fed untreated apple leaves. The starting hypothesis was that methyl jasmonate will induce the production of increased levels of defensive chemicals in the foliage that will decrease the quality of the leaves as food for larvae, resulting in a lowered weight gain. Clearly, the larvae did not suffer reduced weight gain on treated apple leaves. There is data in the literature indicating that increased levels of defensive chemicals can include an increase in leaf nitrogen through the production of certain proteins (see C below).  Perhaps increased nitrogen level in leaves improved leaf quality, while the larvae were not significantly affected by induced chemical defenses of apple (they are adapted). This experiment was repeated with Lacanobia on potato (after finding these larvae defoliating commercial potato fields).  Those larvae had a dramatic reduction in growth when fed potato foliage treated with methyl jasmonate, compared to untreated potato foliage.  These findings are interpreted to mean that Lacanobia larvae are not well adapted to potato defensive chemicals (such as alkaloids) but are well adapated to apple defensive chemicals (such as tannins and protease inhibitors).
These findings are interesting but do not help us interpret the reductions in leaf quality as larval food  through the season.  Another interesting aspect of these results relates to the use of chemicals commercially to trigger SAR, or systemic acquired resistance.  This phenomenon (SAR) involves much of the same induction of synthesis of defensive chemicals as methyl jasmonate treatments that we used here (methyl salicylate is another such compound).  Our results indicate that such treatments may result in improved susceptibility of apple to certain insects (such as Lacanobia), while the goal of such treatments is to improve plant resistance to insects. Again, this is somewhat speculative and field experiments with such SAR agents should be conducted with these findings in mind.
 

4.  Effect of apple variety on Lacanobia development.
Weights of pupae were smallest with Granny Smith foliage and greatest with Red Delicious foliage.  Development time was shortest with Golden and Red Delicious and longest with Granny Smith foliage. Survival to adult was highest with Red Delicious and lowest with Granny Smith. Other varieties were intermediate.  There have been stated observations or conclusions over the past 6 years of preference or attractiveness of particular varieties of apple to Lacanobia fruitworm, based possibly on the severity of infestations. Previous studies of the incidence of Lacanobia in commercial apple orchards did not reveal a pattern, but did not disprove any preference for a given variety.  These results do indicate the potential for faster growth, greater consumption of foliage, and higher survival of Lacanobia on certain varieties of apple compared to others, particularly Red Delicious.

5. Effect of insect damage to apple leaves on Lacanobia development.
Mean weight gained by larvae fed undamaged apple leaves was 6.1 mg per larva, compared to 8.4 mg per larva fed caterpillar-damaged apple leaves.  These data were not significantly different by a t-test (p = 0.33, df = 8).  The trend however, of higher weight gain on damaged foliage, is in line with the results of feeding trials with apple foliage treated with methyl jasmonate, with greater larval weight gain on treated foliage.

6. Potato as food for Lacanobia.
Potato is the best host plant tested in series of comparisons over 4 years of crops and weeds as food for Lacanobia larvae.  In laboratory assays, larvae developed fastest, with the largest pupal weights, and highest survival rates to adult, compared to other plants tested, including apple. In the 7 day weight gain assay, Lacanobia larvae gained more weight in the 7 day period when fed potato (> 80 mg/larva) compared to larvae fed apple foliage (20 mg/larva).  Also reported here is the identification of Lacanobia larvae in collections of insects made from commercial potato fields defoliated by caterpillars.  Those collections were a mixture of Lacanobia, bertha armyworm, and variegated cutworm, but are of interest because of the superior performance of Lacanobia in the laboratory on potato foliage.
This information is of significance in relation to the regional population dynamics of Lacanobia fruitworm.  This insect is extremely abundant in pheromone, blacklight, and feeding attractant traps throughout the Yakima Valley and much of the Columbia Basin, in areas with Lacanobia problems in apple orchards and in areas without problems, in years where they are a problem, and in years when they do not appear to be a problem.  Lacanobia can feed on many types of plants, and may be reproducing on other irrigated crops in eastern Washington in addition to apple.  This is the first record of it’s pest status on potato, and suggests a possible role of potato in contributing to regionally high populations of Lacanobia. The conditions under which apple orchards are attacked are yet unknown. 
 

B.   Behavior of codling moth
1. Larval choices in arena assay.
In evaluating Golden Delicious fruit and foliage in winter, Pandemis leafroller larvae more often chose foliage to feed on (65%) instead of fruit (15%) or both foliage and fruit (20%).  Similarly, Lacanobia larvae fed primarily on foliage (90%), compared to fruit (10%) or both foliage and fruit (0%).  In evaluating Fuji fruit and foliage in early summer, Pandemis leafroller larvae again chose foliage most of the time (80%) , compared to fruit (10%), or both foliage and fruit (5%). Lacanobia larvae also chose Fuji foliage (75%), over Fuji fruit (5%) or both foliage and fruit (20% ) most of the time.
These results support the field observations that most food consumed by both species is foliage, not fruit, and indicate that even in close quarters with both fruit and foliage at hand, most larvae of both species feed on foliage. This experiment also demonstrates the usefulness of this arena assay design as a way to detect or document changes in choices made by larvae of both species with changes in the quality of apple leaves and changes in the maturity of apple fruit with the growing season.
2. Adult oviposition in response to pear in flight tunnel.
Codling moths in a container in the flight tunnel layed numerous eggs during the four hour assay period regardless of treatment, with similar numbers of eggs laid downwind of pear, infested pear, or no fruit.  Previous experiments demonstrated a consistently higher attraction response of codling moth to infested apple, compared to uninfested apple, but did not evaluate numbers of eggs laid under those circumstances.  It is generally considered that plant or host chemicals involved in host finding (attractants) might not be the same as plant chemicals that stimulate oviposition.

C. Chemical Defenses of Apple.
Apple foliage posseses terpenoid compounds and protease inhibitors, in addition to other chemicals that probably help defend against attack by insects.  We have repeatedly found strong increases in terpenoid compounds in fruit infested by codling moth, and to a lesser extent in fruit fed upon by leafroller larvae.
We were unable to correlate protease inhibitor activity in methyl jasmonate treated apple leaves with reduced performance (lowered weight gain, slower development) of Lacanobia larvae on those leaves.  However, methyl jasmonate treated leaves did have increased levels of protein content, which could explain the faster weight gain of larvae fed foliage that had been treated with methyl jasmonate.
It is not known if apple foliage responds the same to different reported inducers of SAR or chemical defenses, and we do not fully understand the chemical changes that take place or their effects on insect pests of apple.  Clearly though, the effects of these treatments are not very predictable and appropriate studies should be conducted with all pests of interest, when SAR treatments are to be considered as a means of protecting crops from diseases and insect attack.

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