Effects of soil conditions and drought on egg hatching and larval survival of the clover root weevil (Sitona lepidus) Scott N. Johnson a,c, *, Peter J. Gregory a,c , James W. McNicol b , Yasmina Oodally c,1 , Xiaoxian Zhang d , Philip J. Murray e a Scottish Crop Research Institute, Invergowrie, Dundee, Scotland DD2 5DA, United Kingdom b Biomathematics & Statistics Scotland, Scottish Crop Research Institute, Dundee DD2 5DA, United Kingdom c Department of Soil Science, University of Reading, Whiteknights, Reading RG6 6DW, United Kingdom d Department of Engineering, University of Liverpool, Brownlow Street, Liverpool L69 3GQ, United Kingdom e North Wyke Research, Okehampton, Devon EX20 2SB, United Kingdom 1. Introduction Soil-dwelling insect herbivores are increasingly recognised both as major crop pests and key drivers of terrestrial ecosystem processes (e.g. Villani and Wright, 1990; Johnson and Murray, 2008). Life-cycles vary depending on taxa, but most consist of an aboveground adult stage that lays eggs that hatch in the soil and give rise to root-feeding larvae (Brown and Gange, 1990). Because egg development and larval feeding takes place in the soil, it has been more challenging to observe these aspects of the lifecycle (Hunter, 2001; Johnson et al., 2007a). This has prompted researchers to develop a series of mathematical models to predict egg development patterns for some of the more economically important soil insects, including the vine weevil (Otiorhynchus sulcatus F.; Son and Lewis, 2005), the pea weevil (Sitona lineatus L.; Lerin, 2004) and the western corn rootworm (Diabrotica virgifera virgifera Le Conte; Schaafsma et al., 1993). More recently, such modelling approaches have been extended to include larval survival, in addition to egg development, for the clover root weevil (S. lepidus Gyllenhal; Johnson et al., 2007b). The benefits of forage legumes, such as white clover (Trifolium repens L.), in agricultural systems have long been known, primarily because of their capacity to fix atmospheric nitrogen and their high Applied Soil Ecology 44 (2010) 75–79 ARTICLE INFO Article history: Received 6 July 2009 Received in revised form 5 October 2009 Accepted 7 October 2009 Keywords: Belowground herbivore Insect cuticle Macro-fauna Soil pH Soil temperature Soil water ABSTRACT Soil-dwelling insect herbivores are significant pests in many managed ecosystems. Because eggs and larvae are difficult to observe, mathematical models have been developed to predict life-cycle events occurring in the soil. To date, these models have incorporated very little empirical information about how soil and drought conditions interact to shape these processes. This study investigated how soil temperature (10, 15, 20 and 25 8C), water content (0.02 (air dried), 0.10 and 0.25 g g À1 ) and pH (5, 7 and 9) interactively affected egg hatching and early larval lifespan of the clover root weevil (Sitona lepidus Gyllenhal, Coleoptera: Curculionidae). Eggs developed over 3.5 times faster at 25 8C compared with 10 8C (hatching after 40.1 and 11.5 days, respectively). The effect of drought on S. lepidus eggs was investigated by exposing eggs to drought conditions before wetting the soil (2–12 days later) at four temperatures. No eggs hatched in dry soil, suggesting that S. lepidus eggs require water to remain viable. Eggs hatched significantly sooner in slightly acidic soil (pH 5) compared with soils with higher pH values. There was also a significant interaction between soil temperature, pH and soil water content. Egg viability was significantly reduced by exposure to drought. When exposed to 2–6 days of drought, egg viability was 80–100% at all temperatures but fell to 50% after 12 days exposure at 10 8C and did not hatch at all at 20 8C and above. Drought exposure also increased hatching time of viable eggs. The effects of soil conditions on unfed larvae were less influential, except for soil temperature which significantly reduced larval longevity by 57% when reared at 25 8C compared with 10 8C (4.1 and 9.7 days, respectively). The effects of soil conditions on S. lepidus eggs and larvae are discussed in the context of global climate change and how such empirically based information could be useful for refining existing mathematical models of these processes. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author at: Scottish Crop Research Institute, Environment–Plant Interactions, Invergowrie, Dundee, Scotland DD2 5DA, United Kingdom. Tel.: +44 01382 560016; fax: +44 01382 568502. E-mail address: Scott.Johnson@scri.ac.uk (S.N. Johnson). 1 Current address: Regional Grants Coordination, International Fund for Agricultural Development, Rome, Italy. Contents lists available at ScienceDirect Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil 0929-1393/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2009.10.002