Gut content analysis of arthropod predators of codling moth in Washington apple orchards

Highlights

• Codling moth (CM) DNA was in 8.9% of the 2591 predators analyzed by PCR.

• Spiders, insect predators and harvestmen tested 8.2, 9.5 and 2.2% CM positive.

• Detection half-life for CM in earwigs was 3.6 d and 3.7 d using fecal pellets and adult bodies as templates for PCR.

• Homogenates of predators in a lysis buffer were used as DNA templates for Direct PCR.

Abstract

Codling moth, Cydia pomonella (L.), is the key pest of pome fruits in many temperate areas of North America, Eurasia, South Africa, South America and Australia. Many predatory arthropods species are found in organic apple orchards of central Washington; here we use PCR-based gut content analysis of arthropod predators to identify predators that attack codling moth. Predators were sampled from tree canopies, tree trunks and from the understory and were homogenized in a lysis buffer to provide a template for Direct PCR. PCR showed 8.9% of 2591 predators had preyed on C. pomonella. Spiders, including 25 genera from 15 families, two carabid beetle species (Pterostichus melanarius (Illiger) and Harpalus pennsylvanicus DeGeer) and the European earwig (Forficula auricularia [L.]) represented 87% of predator specimens analyzed and were 8.2%, 8.3% and 14.7% positive for C. pomonella. PCR products from 38% of predators that appeared positive for C. pomonella COI were sequenced; all showed 99% or more similarity to C. pomonella COI sequences in GenBank. Digestion rates of adult earwigs fed on mature codling moth larvae showed a detection half-life of 3.7 days; half-life from the fecal pellets from the same earwigs was 3.6 days. When fed mature codling moth larvae, the carabid P. melanarius showed a digestion half-life of 3.14 days. Identification of the key predators of C. pomonella can guide the use of selective insecticides and the conservation of these natural enemies, enhancing biological control and supporting stable IPM programs in pome fruit orchards in the western USA.

Introduction

Codling moth, Cydia pomonella (L.), (Lepidoptera: Tortricidae) (CM) is a key pest of apples (Malus domestica Borkh.), pears (Pyrus communis L.) and walnuts (Juglans regia L.) in the Western U.S. (Barnes, 1991). Traditionally, apples in central Washington were protected from CM and many secondary pests with applications of organophosphate pesticides as the basis of an Integrated Pest Management (IPM) program. The use of broad-spectrum insecticides did not allow for conservation biological control because of significant suppression of natural enemies, with the exception of predatory mites (Hoyt, 1969, Croft and Hoyt, 1983). In the last decade, most organophosphates have been removed from use in apple, and replaced by newer insecticides with alternative modes of action. Some of the newer insecticides will allow conservation of natural enemies, but their use will require revision of our IPM programs (Jones et al., 2009, Jones et al., 2010b).

Conservation biological control is a critical component of integrated pest management (IPM) and the key approach is to reduce use of disruptive insecticides (Stern et al., 1959, Kogan, 1998, Jones et al., 2009, Jones et al., 2010a, Jones et al., 2010b). When broad-spectrum pesticides are removed, natural enemies become more abundant and show greater diversity and evenness (Crowder et al., 2010). CM can be suppressed by new insecticides, but some of these materials can suppress natural enemies (Mills et al., 2016, Beers et al., 2016). Pheromone-based mating disruption (Vickers and Rothschild, 1991, Witzgall et al., 2008) and CM granulovirus (Lacey and Unruh, 2005, Lacey et al., 2008) are selective and are organic certified. In the last 20 years, organic apple production in Washington has increased 14-fold, due largely to mating disruption and granulovirus. Suppression of CM and leafrollers (Knight, 1994, Knight, 2008, Arthurs et al., 2007, Lacey et al., 2008, Monteiro et al., 2013) and the woolly apple aphid, Eriosoma lanigerum (Hausmann) (Nicholas et al., 2005, Gontijo et al., 2015) are due to the use of those selective products.

Multiple studies have demonstrated the importance of generalist predators in pest suppression in agro-ecosystems (Symondson et al., 2002). A great diversity of predatory arthropods has been collected in organic orchards in central Washington (Miliczky et al., 2000, Miliczky and Horton, 2005, Horton et al., 2012). A Leslie matrix model simulating larval survival of CM with 25% mortality from predation resulted in 68% reduction of female CM over the season (Jones et al., 2009). A question we ask is which predators would best help attain 25% or more predation of CM as well as secondary pests in apple orchards. Our goal was to identify predators collected from Washington apple orchards that showed evidence of feeding on CM based on molecular gut content analyses.

A recent study in France (Boreau de Roincé et al., 2012) analyzed gut contents to detect CM and Oriental fruit moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae) in predators collected from the understory of nine organically-managed apple orchards. In our study, we collected predators from abandoned, organic, research and conventionally-managed apple orchards for gut content analysis to detect the presence of CM. We also examined digestion patterns of CM in the gut contents of a carabid beetle and the European earwig. For the earwig, we compared the use of fecal pellets versus whole body extracts for PCR analysis to discover whether fecal pellets can be an effective, non-destructive, approach for detecting digestion rates and evaluating the inclusion of specific prey species within a predator’s diet. We discuss the influence of sampling methods on the probability of detection of feeding on CM for different predator taxa, and finish with a discussion of the need for mark-release-recapture methods to estimate predator abundance allowing for rational methods to identify the relative importance of predator species.