Aspects of the ecology and management of apple leafcurling midge (Dasineura mali) (Diptera: Cecidomyiidae) on the Waimea Plains, Nelson, New Zealand

Abstract

Apple leafcurling midge (ALM) (Dasineura mali Kieffer) (DIPTERA: Cecidomyiidae) is an established pest of apple trees in New Zealand. Larval feeding causes rolled or twisted leaves, and contamination of fruit with pupal cocoons may occur when mature larvae seek pupation sites. The main aim of this study was to determine the phenology and population dynamics of ALM to aid the development of long term pest management programmes for ALM in New Zealand. Specific objectives were to determine appropriate sampling methods for studying and monitoring ALM, compare population phenology and abundance between apple cultivars and orchard properties, determine the level of mortality in some life stages and evaluate the effect of soil-applied diazinon and its importance in the management of ALM.

To achieve high precision (<20%), at low hypothetical ALM infestation levels, a sample size of in excess of 200 apple shoots was required. It was found that a sample size of 50 shoots, commonly used in recent phenological studies of ALM gave low precision (>20%) at hypothetical infestation levels of 60% or below. The evaluation of two emergence traps to determine adult emergence from soil found that design 1 caught significantly more ALM than trap design 2.

Emergence traps are unlikely to be used as monitoring tools in commercial orchards, because of the low number and variability of ALM caught, the requirement to relocate traps after each generation and the need to use a microscope to confirm the identity of ALM adults.

One block each of 'Braeburn' and 'Royal Gala' apple cultivars, on three conventionally managed orchards situated on the Waimea Plains, Nelson, were regularly monitored for ALM populations during the 1995/96 and 1996/97 growing seasons. Monitoring of egg laying on shoot tips indicated that five generations of ALM occurred in the 1995/96 season, whereas only four were likely to have occurred in the 1996/97 season. Peaks of egg laying occurred earlier on 'Royal Gala' and more shoot tips were infested with eggs than those of 'Braeburn'. Similarly, in the 1996/97 season, a higher average number of ALM adults were captured in emergence traps positioned under 'Royal Gala' compared with 'Braeburn'. This suggested that 'Royal Gala' may be more susceptible to ALM infestation than 'Braeburn '. Factors such as leaf volatiles and shoot type are suggested to be more important for ALM oviposition than tree shoot aspect.

'Royal Gala' had a higher percentage of shoots and leaves injured for both seasons compared with those of 'Braeburn '. This was likely to be related to the higher growth rates, and shoot length of 'Royal Gala'. All three properties showed similar levels of ALM injury in the 95/96 season. Fruit infestation by ALM was low (<2.5%) for both seasons at all properties, despite up to 50% shoot infestation occurring at harvest.

Life table analysis estimated that approximately 95% mortality of ALM occurred between the eggs to orange larval stage for most generations. Higher mortality occurred in the third and fourth generations of ALM on both cultivars, compared with mortality of earlier generations. This was attributed to the reduced availability of suitable oviposition sites and drier weather later in the season. It took 6-27 days for the transition from eggs to the end of the white larval stage (approximately second instar). Later generations of ALM developed more rapidly.

The emergence of ALM adults from three plots treated with a single diazinon application was compared with an untreated control to assess the efficacy of soil-applied diazinon. During the first adult emergence trapping period (25 Sep to 18 Nov), 2.6-fold more ALM adults were captured in the control and first diazinon application plots, compared with the later two diazinon applications. During the second trapping period (18 Nov to 15 Jan) no significant difference in the number of ALM adults captured between treatments occurred. This was probably due to the low level of diazinon residue (<0.45 ppm) in the soil and the high variability in the trapping data. The half life of diazinon in a Ranzau stoney clay loam varied between 12 and 28 days. Later spring ground applications of diazinon broke down more quickly. This was likely to be due to warmer soil temperatures and the effect of soil organisms on decay.

Published model formulae were used to describe insecticide-induced mortality as a function of the duration of exposure and residue concentration by a log tolerance distribution. Small changes in the decay rate of diazinon strongly influenced the estimated exposure time required to cause 50% mortality of a hypothetical population. Even under conditions where diazinon decay was slow (i.e., half life 27 days), 47 hours exposure was estimated to cause 50% mortality 14 days after application.

On conventionally-managed apple orchards the correct timing of insecticides would be essential for successful management of ALM. Spring soil applications of diazinon need to be applied as close to the start of adult emergence as possible; in Nelson this would be during the last week in September. How the adoption of an Integrated Fruit Production philosophy in the New Zealand apple industry, with its more target-specific insecticides and greater use of natural enemies will affect ALM management is discussed.