SPATIAL AND SEASONAL DISTRIBUTION OF DAMAGE TO APPLES BY ARGYROTAENIA CITRANA (FERNALD)

AND PANDEMIS PYRUSANA KEARFOTTI

Frank Zalom and Carolyn Pickel' !PM Implementation Group

University of California Davis, CA 95616 USA

Abstract: Within-tree distribution and seasonal occurrence of fruit damage caused by Argyrotacnia citrana (Fernald) and Pandemis pyrusana Kearfott was studied in Watsonville, CA apple orchards in 1983 and 1985. There was no significant difference (P > 0.05) in the distribution of damage between 10 regions of the tree at bloom, but the proportion of fruit damaged at bloom which remained on the tree declined as the season progressed. 'Ibtal number of fruit damaged by both species increased as the season progressed, and stabilized by mid to late August. There was a significant difference (p < 0.05) in within-tree dislribution of damage by both species in both years. The proportion of total damage located in the lower regions of the tree was 0.32 in 1983 and 0.38 in 1985. The proportion of total damage in the upper regions of the tree was 0.68 in 1983 and 0.62 in 1985.

Key Words: Argyrotaenia citrona, Pandemis pyrosana, apples, distribution, sampling.

J. ACrie. Entomol. 5(1): 11-15 (January 1988)

California has five principal apple growing regions (Tyler et al. 1983): the central coast, the northern coast, the central valley, southern California, and the Sierra Nevada Mountain foothills. Pest complexes are different in each locality, but the principal insect pest in all localities is the codling moth, Cydia pomonella (L.). Two other tortricids, the orange tortrix, Argyrotaenia citrana (Fernald) and the apple pandemis, Pandemis pyrusana Kearfott also cause significant fruit damage in coastal apple orchards, especially those orchards not receiving insecticide treatment. Argyrotaenia citrana and P. pY11lsana which feed on fruit are known as 'apple skinworms' in this region because of their surface feeding habit which causes fruit scarring. Knowledge of the biology of these species on apple is limited, although Essig (1958) reported that both A. citrana and P. pyrusana reed on the leaves and fruit of apple.

Argyrotaenia citrana, the more damaging of these two species on apples in California, has a wide host range including orange, walnut, willow, Monterey pine, and numerous weeds including curley dock, filaree, mallow, mustard and various grasses (Lange 1936; Coquillet 1984). This insect has been a major pest of grapes in coastal regions since 1968, possibly due to changing orchard Ooor management from winter discing to using herbicides or cover crops (Kido et a1. 1981). Argyrotaenia citrana is found throughout coastal California including the interior valleys of the coast ranges. The warmer and drier climate inland limits its distribution (Basinger 1938). Pheromone lures are commercially available for A. citrana. However, their usefulness is limited because no numerical relationship has been established between the number of moths caught and the level of infestation of any crop (Kido et al. 1981). There are three generations per yeaI' in California.

LEPIDOPTERA: Tortricidac. Acccpl.ed for publiclltion 30 October 1987. ! Univcrsity of Califomia Coopcrnti\,c Extension, Watsonville. CA.

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12 J. Agric. Entomol. Vol. 5, No.1 (1988)

Here we describe damage to apples caused by A. citrana and P. pyrusana at bloom and after petal fall, including seasonal and within-tree distribution of damage. We also suggest how this knowledge may be applied in a monitoring program.

MATERIALS AND METHODS

The same ten 'Yellow Newtown Pippin' apple trees were examined for fruit damage every 2 to 3 weeks, beginning 8 weeks after petal fall in 1983, and 4 weeks after petal fall in 1985, until harvest in unsprayed Watsonville, CA orchards. The orchard used in 1983 was 15 years old with an average tree height of 4 m. The orchard used in 1985 was 10 years old with an average tree height of 3 m. Both orchards were densely planted in hedgerows, and were in an area of other apple orchards. Twenty randomly selected fruit were examined from the center .and each exterior quadrant delimitated by compass direction in both the upper and lower halves of the tree. Therefore, ten locations per tree canopy were sampled in this manner. Fruits were not removed. A total of 200 fruits were examined per tree on each date. The presence and cause of damage was identified in both years. In 1985, an effort was made to determine whether the damage was caused by A. citrana or P. pyrusana. Multivariate analysis was conducted from bloom damage, damage by each species, and total damage to determine distribution of damage in the tree and seasonal damage trends. Results of this analysis were used to develop sampling strategies. Binomial data were transformed using an arc sine conversion prior to analysis.

RESULTS AND DISCUSSION

Damage by both species which occurs during bloom later appears as bronze colored, roughened scars on the fruit surface. Damage after petal fall results in shallow, irregular scars on the surface. Fruit packers in the Watsonville area have established quality thresholds for total fruit damaged in this manner: (7% for fresh market and 10% for processing market). Loads with proportion of damaged fruit exceeding these levels are not marketable.

Bloom damage There were no significant differences (P > 0.05) in the levels of damage that

occurred at bloom among the 10 locations sampled on the trees in 1983 (F = 1.62; 11 = 10) and 1985 (F = 0.693; n = 10), nor were there any interactions between location within the tree and the date on which the sample was taken in 1983 (F = 1.32; n = 50) and 1985 (F = 1.02; n = 70). The proportion of bloom damage located in the upper half of the tree was 0.53 in 1983 and 0.45 in 1985.

In both years the proportion of fruit that remained on the tree after being damaged at bloom declined as the season progressed. In 1983, 0.074 of the fruit on the tree on the initial sampling date (23 June) had been damaged at the time of bloom. Some fruit damaged during bloom remained on the tree through harvest, but there was no, significant difference (F = 1.42; P > 0.05; n = 5) between the sampling dates for these fruits. Fruit damage at bloom was lower in 1985, and only 0.001 of these fruits were present on the trees on the 25 June sampling date. No fruit damaged at bloom remained on the b'ee on the 12 July sampling date. There was a significant difference (F = 6.316; P < 0.01; n = 7) in fruit damaged at bloom that remained on the trees for the sampling dates in 1985 (Table 1).

13 ZALOM and PICKEL: Damage by A. citrana and P. pyrnsana on Apples

Table 1. Mean and standard deviation proportion of fruit (n = 200 per tree) damaged at bloom remaining on each tree (n = 10) on each sampling date in 1985.

Date x± SD

5/24 0.14 ± 0.49a· 6/7 0.15 ± 0.43a 6/25 0.09 ± 0.32a 7/12 om ± O.lOb 7/30 0.00 ± O.OOb 8/14 0.00 ± O.OOb 8/20 0.00 + O.OOb

... Means followed by different lellers nrc significantly different by Duncan's (1951) multiple range lest.

Total damage Total damage resulting from the feeding of A. citrana and P. pyrusana increased

significantly (P < 0.05) as tbe season progressed in both 1983 (F = 6.32; n ~ 5) and 1985 (F = 6.27; n = 7) (Table 2). In both years, the greatest increase in damage occurred after the late June sampling date. Total damage did not increase significantly after mid to late August.

Table 2. Mean and standard deviation number of total fruit damaged (200 fruit per tree from 10 trees) by A. citrana and P. pyrusana on each sampling date in 1983 and 1985.

1983 1985

Date x± SD Date x ± SD

5/24 0.2 ± l.4a· 6/7 0.5 ± 2.2a

6/23 7.4 ± 7.0a· 6/25 0.9 ± 2.9ab 7/12 12.4 ± 11.2b 7/12 2.5 ± 5.6bc 7/28 13.2 ± 11.0b 7/30 3.9 ± 7.9c 8/16 10.5 ± 1O.Ob 8/14 6.3 ± 10.2d 9/13 13.2 + 13.2b 8/20 5.7 + 9.4d .. Means foHowed by different letters in each year are significantly different by Duncnn's (1951) multiple

range test.

No effort was made to· distinguish the damage of the two species in 1983. In 1985, we distinguished between damage of each species on each sampling date. There were significant differences (P < 0.01) in damage increments between sampling dates for both A. citrana (F = 19.51; n = 7) and P. pyrusana (F = 5.88; n = 7). Some new damage by A. citrana was observed on all but the last sampling date. New damage by P. pyrusana was only found for the three sampling dates from mid July to mid August. Overall, most of the damage was caused by A. citrana (Fig. 1).

Distribution of damage There was a significant difference (P < 0.05) in the distribution (Table 3) of

total fruit damage that was recorded for the 10 locations sampled on the trees in

14 J. Agric. Entomol. Vol. 5, No.1 (1988)

both 1983 (F = 12.21; n = 10) and 1985 (F = 1.899; n = 10). The most consistent difference was between damage in the lower five locations sampled and the upper five locations sampled. The upper region of the tree had the greater proportion of damaged fruit on every sampling date in both years. The average proportion of total damage located in the upper regions of the tree was 0.68 in 1983 and 0.62 in

Fig. 1. Proportion of all fruit sampled that had heen damaged since the previous sample by A. citrana and P pyrusana. On each sampling date in 1985, 200 fruits were examined on each of 10 trees.

Monitoring Management of A. citrana and P. pyrusana can he improved by assessing the

severity of fruit damage at regular intervals, then making a control action decision based on' observation of total damage and increase in damage from the previous sampling date.

As there is no significant difference in the distribution of damaged fruit between the center of the tree and the four tree quadrants, samples of equal size taken from both the lower and upper regions of the tree could be used to represent accurately the amount of damage present on the tree. However, if an individual samples fruit exclusively from the lower region of the tree, as would be most convenient in a monitoring program, the damage estimate must be weighted to reflect the differences in distribution of damage by the two species between the

1985.

0 0.06 W <9 e:t ~ 0.05 e:t 0

I­0.04 ::::> 0:: u.. u.. 0.03 0 Z 0- 0.02 I­0:: 0

0.01a.. 0 0:: a..

0.00

-.-Argyroloenio citrano - -0- - Pondemis pyrusona

, , , ~, '0.

,,~ --- .... --<l.. , .......

MAY JUNE JULY AUGUST

SAMPLING DATE

15 ZALOM and PICKE L: Damage by A. citrana and P. pyrusana on Apples

lower and upper tree regions. \Veighting can be achieved by increasing the damage estimate of the sample by a factor of 1.41 (the ratio of vertical distribution of damage on the tree if damage in both tree regions were equivalent [0.500] to the average proportion of actual damage from the lower portion of the tree [0.354]). A control action should be initiated during the season if the estimated amount of damage (actual damage in lower portion of tree X 1.41) approaches the level of damage at which packers or processors would reject the fruit. Similarly, the grower should sort damaged fruit from undamaged fruit in the harvesting process if the estimated amount of damage in a pre-harvest sample exceeded the level of damage acceptable to the packer or processor at harvest.

Table 3. Mean, standard deviation, and proportion fruit damage (20 fruit per location) for the 10 tree locations sampled.

Fruit Damage

Location on tree x ± SD Proportion

Lower/North 1.14 ± 1.23.' 5.7 Lower/South 1.36 ± 1.66. 6.8 Lower/East 1.70 ± 1.73a 8.5 Lower/West 1.28 ± 1.23. 6.4 Lower/Center 1.58 ± 1.95. 7.9 Upper/North 3.10 ± 2.42bc 15.5 Upper/South 3.14 ± 2.04bc 15.7 Upper/East 3.24 ± 2.84bc 16.2 Upper/West 2.46 ± 2.04b 12.3 Upper/Center 3.64 + 2.69c 18.2 • Means followed by different letters are significantly different by Duncan's (1951) multiple range

test.

REFERENCES CITED

Basinger, A. J. 1938. The orange tortrix Argyrotaenia cilra/la. Hilgardia 11: 635-669. Coquillet, D. W. 1884. Report on some of the injurious insects of California. The orange

leafroller (7brtn'x citrana Fernald), p. 24. In: C. V. Riley, Reports of observations and experiments in the practical work of the division. U. S. Dept. Agric. Di\!. Entomol. Bull. 32, 1-59.

Duncan, E. O. 1951. A significance test for difference between ranked treatments in an analysis of variance. Virginia J. Sci. 2: 171-189.

Essig, E. O. 1958. Insects and mites of western North America. MacMillan Pub!. Co. New York. 1050 pp.

Kido, H., D. L. Flaherty, C. E. Kennett, N. F. McCalley, and D. F. Bosch. 1981. Seeking the reasons for differences in orange tortrix infestalions. Calif. Agric. 27-28.

Lange. W. H. 1936. The biology of the orange tortrix, Eulia (Argyrotaenia) cilmna Fern. Bull. Calif. Dept. Agric. 17: 282~285.

Tyler, R. H., W. C. Micke, D. S. Brown, and F. G. Mitchell. 1983. Commercial Apple Growing in California. Univ. Calif. Div. Agric. Sci. Pub!. 2456. 20 pp.