Delay of mating of codling moth in pheromone disrupted orchards

Alan L. Knight

• Introduction
• Materials and methods
• Results
• Discussion
• Reference

Key words - sex pheromone, mating disruption, reproductive behaviour, pome fruit orchard, codling moth, Cydia pomonella

Introduction
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Adoption of mating disruption for management of codling moth, Cydia pomonella  L., has proceeded at a rapid rate in several fruit growing regions of the United States following registration in 1991. Factors that affect the successful implementation of mating disruption (economics, moth population density, orchard topography) have been identified, and most successful uses of mating disruption have developed an intensive monitoring program and are integrating mating disruption with limited insecticide spray programs. Mating disruption for codling moth has been most successful in large contiguous orchards that have some degree of isolation.

The assumption of the various practioners has been that disruption of mating is the major mechanism affecting the success of their program. Since 1994 I have explored the potential of other factors associated with mating disruption-treated orchards that may contribute to grower's success in managing codling moth. For example, levels of egg predation were evaluated in eight orchards under mating disruption and conventional programs during 1995. These results found that egg mortality, primarily by earwigs, bugs, and spiders is ca. 20% higher in mating disruption orchards. Other studies have examined the impact of mating disruption on moth behavior, and moth distribution and movement within and between treated and untreated orchards. Using a passive interception trap (Weissling & Knight 1994) I found that 45% of the female codling moths were mated during the first adult flight within replicated pheromone-treated orchards in 1994 (unpubl. data). Herein, I report new findings on the mating status of female codling moths in mating disruption orchards during the first generation, and from experiments to determine whether mating is delayed in disrupted orchards and what impact a delay in mating could have on the reproductive potential of codling moth.

Materials and methods
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Laboratory experiments were conducted to evaluate the effect of female age on fecundity and fertility. Moths from a laboratory colony were maintained at 22 ± 2 °C, 50 to 65% relative humidity, and a 16:8 (L:D) photoperiod within 250-ml waxed paper cups. Adults were supplied with a cotton wick saturated with a 10% honey solution. Groups of 25 individual female moths <1 to 10 days old were placed in cups with two male moths 1 to 2 days old. Males were removed after one scotophase. Cups were checked daily for dead moths. Females were dissected to determine their mating status and the number of eggs; the number that hatched were counted two weeks after the female died.

Six solar-powered light traps and six groups of 18 interception traps were placed on April 27, 1995 within three orchards treated with Isomate-C+ dispensers (Shin-Etsu Chemical Co., Tokyo) at 1 000/ha on April 22 for codling moth. Light traps were run from 17:00 to 20:00. Traps were checked twice per week and interception traps were replaced weekly. All traps were removed on June 23.

Sterile codling moths were provided by Dr. Ken Bloem from the rearing facility in Osoyoos, British Columbia and transported chilled (0 to 2°C) to Yakima. Groups of 6 000 moths were released at ca. 15:00 on the first day of each experiment by sprinkling the chilled moths on the ground beneath trees in a 0.25-ha plot in the middle of four selected orchards. Two orchards were treated with Isomate-C+ (1 000 dispensers/ha applied in late April) and two were untreated but used a conventional spray program. Experiments were conducted from July 25 to August 28, 1996. Sixty interception traps (two per tree) were evenly spaced within the designated 0.25-ha plot. Traps were checked each day and panes were replaced every three days. Tests were run for six nights.

Results
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Under laboratory conditions female codling moth 1 to 2 days old laid an average of 135 eggs and 75% of these eggs hatched (Table 1). Both the number of eggs laid and fertility declined for moths when they were mated at an age older than 2 days. These two factors combined for >40% drop in the number of viable offspring produced by females >2 days old.

Interception and light traps caught predominately male moths (4:1 male/female ratio) during the first moth flight in three mating disruption-treated orchards in 1995. No females were caught during the first week of moth flight. The proportion of mated females captured was fairly consistent over six weeks and averaged 55%.
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In 1996, the number of moths recaptured on interception traps was slightly higher for males (54:46) and substantially lower for females (39:61) in the mating disruption vs the check orchards. Only 2.6 and 0.6% of the estimated 36 000 male and female moths were recaptured in these six releases, respectively. Recapture of moths dropped 98% from the first to the sixth night. The male/female ratio of captured sterile moths was 3.7 and 5.2 in the mating disruption and check orchards, respectively. Over 40% of the females recaptured on passive traps in the untreated plot were mated by the first night (Table 2).

Level of mating in the untreated orchards peaked after two nights and ranged from 77 to 100%. All females recaptured on the first night in the mating disruption orchards were virgin. On the second and third night ca. 20% of the females captured were mated. From the fourth to sixth night the level of mating was consistent and averaged 40% (Table 2).

Discussion
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Laboratory and field studies have demonstrated that delay in mating can occur and could have a significant affect on the population build-up of codling moth in mating disruption orchards. Field studies that could quantify this relationship are needed for both the spring and summer moth flights. The effect of moth density on mating success and delay in mating also needs to be investigated. Next season, I plan to compare the temporal patterns of mated female codling moth egg load within mating disruption and untreated orchards with high population densities. Field studies investigating mating success with low population densities are problematic, and may be best addressed experimentally.

Reference

Weissling TJ, Knight AL (1994) A passive trap for monitoring codling moth (Lepidoptera: Tortricidae) flight activity. J econ Entomol 83, 329-334