Effects of temperature on the establishment potential of the predatory mite Amblyseius californicus McGregor (Acari: Phytoseiidae) in the UK

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Abstract

Amblyseius californicus was introduced into the UK in the early 1990s as a biocontrol agent against glasshouse red spider mite Tetranychus urticae. This study investigated the effects of temperature on the establishment potential of A. californicus in the UK in the light of recent reports of their successful overwintering outside of glasshouse environments. The developmental thresholds were 9.9 and 8.6 °C respectively using simple and weighted linear regression. Using the day-degree requirement per generation calculated by weighted regression (143 day-degrees) in combination with climate data, it was estimated that up to seven generations would be possible annually outdoors in the UK. Non-diapausing adult females froze at −22 °C, with 100% mortality after reaching their freezing temperature. Up to 90% of mites died before freezing after short exposures to low temperatures. Significant acclimation responses occurred; 90% of acclimated individuals survived 26 days exposure at 0 °C and 11 days at −5 °C (acclimated mites were reared at 19 °C, 6L:18D followed by 1 week at 10 °C, 12L:12D). Non-diapausing adult females survived over 3 months outdoors in winter under sheltered conditions and oviposition was observed. The experimental protocol used in this study is discussed as a pre-release screen for the establishment potential of other Amblyseius species, and similar non-native biocontrol agents.

Introduction

Amblyseius californicus McGregor (Acari: Phytoseiidae), also known as Neoseiulus californicus, is a predatory mite originating from California and Florida, used extensively in biological control programs against red spider mite (Tetranychus urticae Koch, Tetranychidae) on a global scale. A. californicus is widely used in the Mediterranean region, particularly Italy and Spain where it is reported to occur naturally (Castagnoli and Simoni, 1991, Griffiths, 1999). Recently, A. californicus has been imported into the UK as a biocontrol agent and released in greenhouses in a series of ‘limited release’ trials. However, due to inevitable escapes from these environments, it is now reported to be established outdoors in several areas of southern Britain, and its distribution could potentially expand (Jolly, 2000).

Phytoseiid mites have long been used in the control of glasshouse pests including red spider mite and thrips (McMurtry and Croft, 1997), and their use is increasing owing to the pressure on growers to find alternatives to chemical pesticides (van Lenteren, 2000). Phytoseiulus persimilis was the first predatory mite to be used in the control of red spider mite in glasshouses in UK in the late 1960s (Hussey and Bravenboer, 1971). Since then, the use of this species has increased greatly and has become an example of successful biological control based on a thorough knowledge of the plant/pest/predator relationship (Caltagirone, 1981). The use of other predatory mites has been increasing since the 1980s, including species from the genus Amblyseius (van Lenteren and Woets, 1988, van Lenteren, 1995).

To assess the risk of establishment of a biocontrol agent such as A. californicus, various abiotic and biotic factors need to be considered (Bale and Walters, 2001, Hart et al., 2002). The most important abiotic determinant is temperature, which affects survival, reproduction and development (Bale and Walters, 2001). Successful establishment in a new habitat requires an adequate thermal budget (number of day-degrees above the developmental threshold) for development and reproduction, and the ability to survive harsh conditions that may be encountered during the winter season. In addition to the abiotic factors, availability of prey and in some cases, the ability to enter diapause, are also necessary considerations when assessing establishment potential. There is recent evidence that a naturalised strain of A. californicus has become established outdoors in some areas of the UK and possesses the ability to enter diapause in winter (Jolly, 2000). It is not known if commercial strains supplied by biocontrol companies can diapause immediately upon escape from release sites, but it is clear that the ability to diapause can be selected for relatively quickly, as all of the licensed releases of A. californicus in the UK have occurred within the last 10 years.

A relationship between diapause and cold hardiness ability has been shown in a number of arthropods, with the diapausing stage having an increased level of cold tolerance compared to non-diapausing counterparts (see Denlinger, 1991). Diapause can be induced in A. californicus and related species by exposing individuals throughout their juvenile development to short day lengths and temperatures below 20 °C (Fitzgerald and Solomon, 1991, Morewood and Gilkeson, 1991, Veerman, 1992, Morewood, 1993). Not all species, strains or populations are capable of entering diapause, and some predatory mites can overwinter successfully without diapausing (Morewood, 1993).

Although there are many studies on the biology and diapause of A. californicus and other phytoseiid mite species, few have investigated their cold hardiness (Morewood, 1992, Morewood, 1993), and, in general, there is insufficient information for a comprehensive risk assessment of the establishment potential of these species in new environments. Overwintering survival is an important factor often overlooked in the licensing system for the release of non-native biocontrol agents in the UK.

The aim of this study was to assess the overwintering ability and effects of temperature on a commercially available strain of A. californicus to assess the risk of establishment posed by individuals escaping from glasshouse environments.

Section snippets

Rearing of A. californicus

A strain of A. californicus, originally imported from USA, supplied by Syngenta Bioline (formerly Novartis BCM) was reared at 26 °C with a photoperiod of 18L:6D (Treatment 1) under quarantine conditions at the University of Birmingham. Populations to be used for experimentation were established on artificial arenas: black tiles (approximately 20×12cm2) ringed by moist filter paper and Oecotak® were placed on sponge blocks in containers half filled with water to act as a barrier to the mites. A.

Development of A. californicus

The development of A. californicus from egg lay to adult is shown in Fig. 1 with temperature plotted against rate of development (1/days to adult). Using simple linear regression the developmental threshold was estimated to be 9.9 °C with a day-degree requirement of 123.5 day-degrees per generation. When weighted linear regression was applied to the data, a lower estimated developmental threshold of 8.6 °C was obtained with an increased day-degree requirement of 142.9 day-degrees per generation.

Discussion

Development of A. californicus was approximately linear within the range of temperatures used in the experiment, with a development threshold of 9.9 °C, 1 °C higher than that estimated by Castagnoli and Simoni (1991) when using simple linear regression. Also, the strain used in this study developed from egg to adult at a slower rate (8.1 days at 25 °C) compared with that studied by Castagnoli and Simoni (5.8 days at 25 °C). The slower rate of development observed in this study may be

Acknowledgements

We are grateful to Syngenta Bioline (formerly Novartis BCM) for the supply of Amblyseius californicus and to Barbara Russell for technical help. This work was carried out as part of a project funded by the Department for Environment, Food and Rural Affairs (DEFRA).

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