Quantifying Apple Maggot (Diptera: Tephritidae) Preference for Apples to Optimize the Distribution of Traps Among Trees B. C. MURPHY,' L. T. WILSON,2 AND ROBERT V. DOWELL' Environ. Entomol. 20(4): 981-987 (1991) ABSTRACT The spatial and temporal distribution pattern of apple maggot, Rhagoletis pomonella (Walsh), fly captures was monitored among trees within an unmanaged apple orchard. Each tree within the orchard was monitored weekly for the presence of flies using sticky traps. Fruit maturity was monitored weekly to determine percentage soluble solids. Significantly more apple maggot were captured on trees with mature fruit than on trees with immature fruit. A selective predation model was used to quantify the effect of fruit preference on apple maggot captures. Two hypotheses were evaluated. The first hypothesis was that fly capture among trees is a function of the relative sequence or phenology of fruit maturation (tree category hypothesis). The second hypothesis was that fly capture among trees is a function of apple maturity among trees, r6gardless of the phenology of fruit maturation (fruit maturity hypothesis). Both models explained the distribution of fly capture among trees early in the growing season, but the fruit maturity hypothesis best explained the entire season. The use of the model for predicting the distribution pattern of apple maggot captures and the optimum placement of traps for apple maggot detection are discussed. KEY WORDS Insecta, Rhagoletis pomonella, fruit preference, distribution WITH THE INTRODUCTION of the apple maggot, sequent elimination of apple maggot infestations, Rhagoletis pomonella (Walsh), to the western the state was advised that every host tree in the United States, increasing attention has been given infested region needed to be monitored with traps to the efficacy of detection procedures for deter- (Dowell 1985). The cost of labor and materials mining the presence and movement of apple mag- needed to monitor every host tree was prohibitive got populations. Apple maggot is detected through and became a contributing factor to ending the the use of various sticky traps which are placed eradication program (Dowell 1990). within host trees. These traps rely on visual and None of the traps used for detection of apple olfactory cues to attract flies (Still 1960, Prokopy maggot has been found to attract flies beyond the 1968, Kring 1970). Attempts to improve trap effi- canopy of the host tree in which it is placed (John- ciency in the western United States have concen- son 1983, Stanley et al. 1987). Therefore, fly cap- trated on evaluating the relative sensitivity of trap ture is dependent not only on the attractive char- types to the presence of flies within host trees (Davis acteristics of the trap but also on flies being within & Jones 1986, AliNiazee et al. 1987, Brunner 1987). the immediate vicinity of the trap. Studies exam- Although these comparisons were relevant for find- ining apple maggot dispersal have noted an asso- ing the most effective trap, they did not address ciation between the number of flies captured on the efficiency of areawide detection programs. Use sticky traps and the cultivar of the apple host (Max- of sticky traps for detecting and monitoring apple well & Parsons 1968, Neilson 1971, Dean & Chap- maggot populations over larger areas, such as large man 1973). Those cultivars with earlier ripening orchard blocks or apple-growing districts, is often fruit were found to capture a larger number of impractical because of the limited range of trap flies, and fruit were more susceptible to attack. The attraction to apple maggot adults, requiring a large present study examines the role of fruit maturity proportion of available host trees to be monitored in influencing the number of apple maggot flies to ensure fly detection (Johnson 1983, Stanley et caught on sticky traps, and addresses two questions al. 1987). The limited range of these traps became regarding apple maggot capture among trees: acutely apparent during California's eradication To what extent do relative differences in fruit program against the apple maggot, where the state maturity among trees determine the spatial pat- attempted to monitor fly numbers in an area of tern of fly capture within an apple orchard? 62,000 km2 in seven northern California counties How may information on fruit maturity be used (Dowell 1990). To ensure the detection and sub- to increase efficiency of fly detection among ap- ple hosts? ' Department of Entomology, University of California, Davis, In answering these questions, we examined the Calif. 95616. Current address: Department of Entomology, Texas A&M spatial and temporal distribution pattern of flies University, College Station, Tex. 77843. captured among orchard trees during the season 0046-225X/91/0981-0987$02.00/00 1991 Entomological Society of America 982 ENVIRONMENTAL ENTOMOLOGY Vol. 20, no. 4 and analyzed the association between these pat- sured twice for each apple sampled, once from the terns and the differences in the relative maturity blush side and once from the green side of the of fruit among the trees. Second, we quantified this fruit. The percentage BRIX values were averaged relationship by means of a fruit preference model among the fruit sampled and used as the estimate to explain and predict the distribution pattern of of fruit maturity for each tree for the sample date. fly captures among trees with different maturity The number of flies trapped and the average per- classes of fruit. centage BRIX of fruit for each tree were recorded weekly from 22 June through 5 October 1987. Distribution Pattern of Apple Maggot Captures. Materials and Methods The distribution of apple maggot captures among The study was conducted in an unmanaged ap- all trees during the season was tested against an ple orchard near Brookings, Oreg., 16 km north of expected uniform distribution, and the frequency the California border, during 1987. The orchard distribution of fly captures among trees was tested was infested with apple maggot and had not re- against expected random and negative binomial ceived insecticide treatments against the fly for the distributions using a xz goodness of fit test (Zar previous 2 yr. Of the 93 trees, 5 were nonapple 1984). We tested for temporal changes by com- species and 5 were apple trees that did not bear paring the frequency distribution of apple maggot fruit. The remaining 83 trees produced fruit and captures during each of the three 5-wk periods of consisted of the following cultivars: 14 ‘Red’ and the season against these same expected distribu- ‘Stripped Gravenstien’, 3 ‘Yellow Transpafent’, 7 tions, also using a x2 goodness of fit test. ‘Delicious’ (strain unknown), 4 ‘Johnathan’, and 11 The distribution of fly capture among trees whose ‘Winesap’. The cultivars of the remaining trees fruit matured at different times was compared by could not be identified with certainty and they grouping the trees into one of three categories based were classified according to the time to harvestable on the time fruit within each tree ripened to a maturity. stage of harvestable maturity (12% BRIX). The first Fly abundance was monitored using yellow sticky 28 trees to reach this stage were categorized as panels (Pherocon apple maggot trap, Zoecon, Palo early-maturing, the second group of 28 trees as Alto, Calif.) and 7.5-cm unbaited red sticky spheres. midseason-maturing, and the remaining 27 trees One trap of each type was hung toward the outside as late-maturing. Early-, mid-, and late-maturing of the tree canopy on a north-south axis 2.0-3.5 m trees had fruit which reached an average 12 per- from the ground. Compass direction of the trap cent BRIX by 3 August, 16 August, and 23 Sep- types was reversed for each tree trapped. The yel- tember, respectively. The seasonal differences in low sticky panels were replaced with new traps fruit maturation rates among the tree categories as every 2 wk and red spheres were replaced every defined by percentage BRIX were subjected to a 30 d. repeated measures analysis of variance. Linear con- The physiological maturity of fruit was moni- trasts giving the linear trend in percentage BRIX tored by measuring the total dissolved solids (per- for each tree were used for comparisons among the centage BRIX), using a hand-held refractometer tree categories (Gurevitch & Chester 1986). (American Optical, Buffalo, N.Y., Model 10430). We tested the relationship between the time of The percentage BRIX of the fruit is a measure of fruit maturity and the frequency of apple maggot maturity, where <IO% is considered unripe, and captures using two analyses. The first analysis test- a measure of 12% is considered ripe for harvest for ed for differences in apple maggot captures among many cultivars (California Food and Agriculture the three tree categories. The second analysis tested Code 1985). The degree of fruit maturity (per- for differences in the number of trees within each centage BRIX) within a tree was highly variable category which had one or more apple maggot and was influenced by factors such as fruit location captures. This analysis reduced any bias that might within the canopy, exposure to sun, tree health and have resulted from unusually high fly captures on vigor, and pest and disease pressures. At the be- a few trees within any one category. For both meth- ginning of the study, 40 fruit were sampled for ods, a two-factor analysis of variance was used, percentage BRIX from trees representing each of where tree category (corrected for unequal sample the known cultivars to determine sample locations size) was the first factor and sample date a blocking of the canopy that provided the maximum range factor. Fishers protected LSD test at P = 0.05 was of maturity. The ripest fruit were found on the used for pairwise comparisons between treatment south side of the tree toward the outside of the means.