ResearcH Use of a New Immunomarking Method to Assess Movement by Generalist Predators BetweenDavid R. Horton,a Cover Vincent Crop P. Jones, and and Thomas Tree R. UnruhCanopy Abstract. in a Pear Orchard Poor understanding of natural enemy movement between cover crop and crop habitats has limited effective use of cover crops to enhance biological control of pest arthropods in agricultural systems. To address this problem, we used an egg-albumin immunomarker sprayed on a legume cover crop to monitor movement by generalist predators from the cover crop into the canopy of pear trees in a Central Washington State pear orchard. Generalist predators in four taxa (Heteroptera, Coccinellidae, Chrysopidae, Araneae) were collected from both cover crop and pear tree canopy, and tested using enzyme-linked immunonoassay (ELISA) methods to determine if the marker was present. In the three years of study, > 90% of arthropods collected from the cover crop had acquired the marker, indicating that we had excellent coverage and marking of arthropods with the solution. Between 17 and 29% (depending upon the year of study) of the approximately 2000 specimens collected from the tree canopy were also found to be marked, which is evidence that a percentage of the generalist predator community in the tree canopy had originated from or had visited the legume cover crop. These specimens included spiders and immature insects, which presumably colonized the tree either by walking up the tree trunk or (for spiders) by ballooning into the canopy. Some care must be taken in applying this technology or in interpreting results, due to unknown effects of the marker on arthropod behavior, and to questions of whether the marker might transfer from marked to unmarked insects as they interact in the field. Despite these caveats, the technology described here provides a new tool for assessing the effects of habitat management on movement of generalist predators in crop systems. onservation biological control can be defined as the use of and Waddington (1994), and includes studies in nut crops (e.g., Bugg C et al. 1991, Smith et al. 1996), stone fruits (e.g., Stephens et al. 1998), tactics that preserve natural enemies in crop systems (Barbosa 1998). These tactics include efforts to minimize the effects and pome fruits (e.g., Fye 1983, Haley and Hogue 1990). of pesticides on natural enemies, changes in cultivation or harvest Despite the extensive literature, there is a lack of consensus about methods, or manipulation of non-crop habitats to act as refuges for the actual value of using cover crops in orchards to enhance biological natural enemies within colonization distance of the crop. Habitat control. Numerous studies demonstrate that diversification of agri- manipulation to enhance biological control includes planting of cover cultural systems (as through cover cropping or intercropping) leads crops (Smith et al. 1996) and establishment of hedgerows or other to higher densities and diversities of generalist predators (reviews by refuges adjacent to the crop (Rieux et al. 1999). The term “ecological Sheehan 1986, Bugg and Waddington 1994, Landis et al. 2000, Langel- engineering” has been used to describe these tactics (Gurr et al. 2004). lotto and Denno 2004). However, it is often unclear whether higher There is interest in using habitat manipulation to enhance biological densities or diversities of natural enemies actually lead to improved control, as shown by the growth in numbers of books and reviews that pest control and protection of the crop (Bugg and Waddington 1994, address this issue (Barbosa 1998, Pickett and Bugg 1998, Gurr et al. Snyder et al. 2005). This uncertainty is in part due to methodological 2004, Snyder et al. 2005). problems in many studies, including lack of replication, insufficient Cover crops are used in fruit or nut orchards to reduce soil erosion, plot size, or lack of pest suppression data (Bugg and Waddington 1994, improve soil quality, and to help control weed and arthropod pests (Sus- StatementSnyder et al. of 2005). Research Question tainable Agriculture Network 1998). Interest in the use of cover crops in orchards to enhance biological control of arthropod pests is shown by the growing literature on this topic in technical journals or books In addition to the methodological shortcomings mentioned above, (Bugg and Waddington 1994, Pickett and Bugg 1998), in trade journals limitations in our understanding of natural enemy biology have slowed (Alway 1998), and on websites (U.C. Davis 2002). The intent is to use progress in using cover crops as a means to enhance biological control cover crop habitats to supplement natural enemies with resources not in orchards. In particular, we have a poor understanding of movement provided by the crop, with the expectation that the cover crop will then by natural enemies between the cover crop and trees. For example, if be a source of natural enemies moving into the tree. The literature on the cover crop is highly attractive to natural enemies, it may act to limit coverAmerican crops Entomologist and biological • Volume control 55, Numberin orchards 1 was reviewed in Bugg movement by natural enemies into the tree. Indeed, it is conceivable49 Table 1. Number of generalist predators collected in cover crop (by sweep net) and tree canopy (by beating tray), and apparent habitat that a cover crop could actually lead to a net loss of biological control preferences based upon numbers in each habitat if it is attractive enough to divert natural enemies away from the target No. collected pest (see Snyder et al. 2005). Many reviews of habitat manipulation ex- in plicitly state that poor understanding of movement by natural enemies Cover Tree CoverApparent Tree habitat between non-crop and crop habitats has hindered progress in using preference TAXON Generalist habitat manipulation to enhance biological control (Lavandero et al. crop crop 2004, Jervis et al. 2004, Snyder et al. 2005, Denno et al. 2005). Orius tristicolor X We have begun a project to assess whether a legume cover crop in GeocorisHETEROPTERA X Cacopsylla pyri- Nabis 662* 24*8 X pear orchards leads to increased biological control of a major insect pest cola Deraeocoris spp. brevis 329* 0 X in orchards of North America and Europe: pear psylla ( Anthocoris sp. tomentosus 99* X (Förster); Hemiptera: Psyllidae). Generalist predators, dominated 59* 1159* by the true bugs (Heteroptera), have long been implicated in pear psylla 10* 459* Chrysopa oculata X control (Madsen et al. 1963, McMullen and Jong 1967, Westigard et al. ChrysoperlaCHRYSOPIDAE plorabunda X 1968, Unruh et al.1995), and are key components of integrated pest Eremochrysa 194* 1113 X? management programs in North America and Europe. In the western Chrysopa coloradensis 132* 155* X? Deraeocoris brevis Anthocoris sp. U.S., these predators include mirid and anthocorid species such as Chrysopa nigricornis 33 X 13 5* (Uhler) and spp. (Westigard et al. 1968, 4 47* Unruh & Higbee 1994, Horton & Lewis 2000). Other generalist preda- Hippodamia convergens X tors, including lacewings (Neuroptera: Chrysopidae), ladybird beetles CoccinellaCOCCINELLIDAE septempunctata X 382* 15* (Coleoptera: Coccinellidae), and spiders (Araneae) may also be locally Coccinella transversoguttata X? Hyperaspis lateralis 127* 118*95 X important (Westigard et al. 1986, Miliczky and Calkins 2001, Horton et Harmonia axyridis 116* 36 X al. 2002). Unfortunately, we have little understanding about how cover 112 crops in orchards affect biology of these species, including whether 11* 159* *, asterisks indicate that the samples included immatures (identified to species by rear- these generalist predators move readily from cover crops into the tree ing in the laboratory, as necessary). canopy, where the target pest (pear psylla) could then be attacked. Inferences about whether a non-crop refuge acts as a source of a given predator species are often based upon abundances of the herbaceousAnthocoris vegetation tomentosus to arboreal and habitats Deraeocoris (Honěk brevis 1985), although our predator in the two habitats, where a significant presence in both is results suggested that this species was common in the pear tree canopy assumed to be evidence of movement between them (see Nguyen et (Table 1). appear to prefer al. 1984, Rieux et al. 1999, Scutareanu et al. 1999, Horton and Lewis shrubs and trees (Horton and Lewis 2000, ChrysopaMiliczky and oculata Horton 2005), 2000, Miliczky and Horton 2005 for examples in orchards). Table 1 but can also be collected from herbaceous vegetation (Tamaki and summarizes relative abundance data from 3 years of sampling for 3 Weeks 1968, Miliczky and Horton 2005). , which we taxa of generalist predators (Coccinellidae, Chrysopidae, Heteroptera) rarely collected in pear trees, has actually been discounted as a preda- collected from a legume cover crop (see below for details on cover tor of tree pestsChrysopa in orchards oculata just because of its apparent preference crop) and from the tree canopy at a pear orchard in Central Washing- for herbaceous vegetation (Putman 1932). However, others have sug- ton State. Arthropods were collected using sweep nets and beating gested that is an important predator of orchard pests trays. We list the five numerically mostOrius common tristicolor species in eachAnthocoris taxon, Hippodamia(including pear convergens psylla), and may exhibit high densities in pear trees tomentosusranked by numbers in theDeraeocoris cover crop brevis samples. The most abundant