Integrating Biological Control into Cole Crop Production in the Pacific Northwest

Materials and Methods reprinted from: Prasad, RP and WE Snyder. 2006. Polyphagy complicates conservation biological control that targets generalist predators. Journal of Applied Ecology 43:343-352.

Several species of anthomyiid flies are major pests of vegetable crops in northwestern Washington, U.S.A., and southwestern British Columbia, Canada (Finch 1989; Howard et al. 1994). Female flies lay eggs at the base of host plants, and root feeding by the larvae results in cosmetic injury or death of seedlings. Carabid and staphylinid beetles are predators of the eggs of these pest flies (Finch 1989, 1996). In local vegetable fields, the most common species of epigeal, predatory beetles are several carabids (Coleoptera: Carabidae; Bradycellus congener LeConte, Bembidion tetracolum Say, Bembidion lampros Herbst, Amara littoralis Mannerheim, Amara apricaria Paykull and Pterostichus melanarius) and staphylinids (Coleoptera: Staphylinidae; Aleochara bilineata Gravenhorst, Philonthus politus L. and an Aleocharine morhpospecies) (Prasad & Snyder 2004).

ON-FARM BEETLE BANK PERFORMANCE

A beetle bank was established on each of three organic mixed-vegetable farms, located in Ladner, British Columbia, and in Mt. Vernon and Carnation, Washington. A fourth beetle bank was established in a radish Rhaphnus sativus L. field at Washington State University's research station in Mt. Vernon, WA, which was managed conventionally for nutrients and weeds, but received no insecticide applications. All four beetle banks were planted between April and June 2002, by broadcasting orchard grass Dactylis glomerata L. seed in strips 1.5 m wide and 30 to 60 m long. Two banks were raised approximately 50 cm, while the remaining two were level with the surrounding field; cooperating growers independently determined the dimensions of their beetle banks using previous designs as a guideline (Thomas et al. 1991; Frampton et al. 1995; Lee et al. 2001). In 2003, we surveyed the beetle fauna in the three organically managed beetle bank fields. In 2004, the beetle bank on one of the organic farms was tilled under, so we conducted our survey with the remaining two organic fields and the beetle bank field at the research station.
Concurrently, in both years we surveyed beetle densities and egg predation rates in three control fields that lacked a beetle bank, selected to minimize differences in field management between fields with and without beetle banks. In 2003 the control fields consisted of one mixed vegetable field, in Mt. Vernon WA, and two broccoli Brassica oleracea L. monocultures in Ladner, BC; all fields were managed organically. In 2004, control fields consisted of two organic mixed-vegetable fields, one each in Mt. Vernon, WA and Ladner, BC. The third control field was a cauliflower B. oleracea L. monoculture, located in Mt. Vernon, WA, under conventional weed and nutrient management, but no post-transplant insecticide applications.
Ground beetle activity was assessed using pitfall traps [design as in Prasad & Snyder (2004)] three times each year between May 20 - 25, August 8 - 12, and September 25 - 29. In fields containing beetle banks, traps were placed within 5 m of banks (and no less than 20 m from field margins), while in control fields traps were placed 20 m from field margins.
To determine whether beetle banks indirectly impacted predation of pest-fly eggs, our focal prey stage, we measured predation of sentinel Diptera eggs during the 2004 field season. Previous results indicated no preference among the commonly occurring carabid and staphylinid species for eggs of the housefly Musca domestica L. versus eggs of the economically important vegetable pest Delia radicum L. (Prasad & Snyder 2004); thus we used the easily propagated M. domestica eggs as a surrogate (Prasad & Snyder 2004). A group of five eggs (< 24 h old) were placed on 2-cm2 pieces of peat cut from transplant pots, placed at the base of a plant and then covered with a 0.5-cm layer of soil (Finch & Elliot 1994). We placed 5 egg cards per field, spaced 1 m apart and positioned either 5 m from the beetle bank or, in control fields, 20 m from the nearest field margin. Egg cards were placed in fields for 24 h concurrent with each pitfall survey (May, August and September).

FIELD CAGE EXPERIMENTS

Field cage experiments were conducted in a 0.81 ha radish field at the Washington State University farm in Mt. Vernon. Experimental units were 2 x 2 x 2-m field cages, covered on all sides but the bottom with a fine mesh screen and positioned to contain three parallel rows of plants [a full description of cages is provided in Prasad & Snyder (2004)]. Between experiments cages were moved to a new section of the field and reassembled.
Experiment 1 was designed to isolate the impact of the intraguild predator P. melanarius on fly egg consumption by smaller beetles. The experiment had an additive design (sensu Goldberg & Scheiner 2001); in addition to 28 small beetles, cages received either no P. melanarius (0X), 7 P. melanarius (1X), or 28 P. melanarius (4X). Ratios of P. melanarius to small beetles, and overall beetle densities, approximated the range observed at different times of the season in our beetle bank demonstration fields (see Results). We also included two control treatments, all species trapped-out but not replaced (Removal), and un-caged and un-manipulated 4-m2 reference plots (Open). The experiment was conducted in 2003, as two replicated blocks separated in time, an appropriate approach when a limited number of replicates can be conducted simultaneously (Gotelli & Ellison 2004). The first block began on July 25, and the second August 13, with five replicates of each treatment per block (total N = 50).
In the second experiment our objective was to examine whether, in the absence of the putative intraguild predator P. melanarius, fly egg consumption would increase with increasing densities of smaller beetles. This replicated the early-season rarity of P. melanarius and higher small beetle densities in the presence of beetle banks, observed in our open-field data (see Results). Cages received either 7 (1X) or 28 small beetles (4X); Removal and Open treatments were again included as controls. This experiment was also conducted in two temporal blocks, during 2003, beginning on 10 June in the first block and on 1 July in the second block, with five replicates of each treatment within each block (total N = 40).
As results from one block of Experiment 2 suggested that aphid alternate prey could disrupt egg predation by small beetles (see Results), a third experiment was conducted to examine the effect of aphids on egg predation by small beetles. In this experiment 28 small beetles were added to all cages, along with no aphids (No), 20 aphids added to a single radish plant per cage (Low), or 20 aphids added to 10 plants per cage (High). Removal and Open treatments were again included, and this experiment was conducted once starting on 19 May 2004. All treatments were replicated five times (total N = 25).
Experiment 1 was conducted as a pulse experiment (Gotelli & Ellison 2004), with beetles added to cages only on day 1 [Table 1; for a similar experiment run as a press see Prasad & Snyder (2004)]. In contrast, a press design (Gotelli & Ellison 2004) was appropriate for Experiments 2 and 3 because for these experiments, our goal was to mimic predation rates in a field adjacent to a beetle bank with immigrant predators arriving regularly (Table 1; Snyder & Wise 1999). Ambient beetle densities (1X treatments) reflected typical densities in conventionally managed vegetable fields (Prasad & Snyder 2004). Beetles were added to cages as an assortment of the seven species or morphospecies of common smaller beetles: B. congener, B. tetracolum, B. lampros, Amara spp., A. bilineata, P. politus and an Aleocharine morphospecies (Prasad & Snyder 2004), with the restriction that at least one individual from each taxon was added to each cage on each release date. Beetles were field-collected < 24 h prior to being released into cages. We used pitfall traps to lower predator densities in the Removal treatments, and to assess predator activity-density in all other treatments (Table 1).
Predation rates in field cages were measured using 20 sentinel M. domestica eggs, as described previously, but with two cards of ten eggs placed at the base of two haphazardly selected plants per plot (Table 1). Full pitfall trapping and predation measurement protocols are described in detail in Prasad & Snyder (2004). In Experiment 2, aphids were counted on 5 randomly selected plants per plot, at the conclusion of each block. Aphids were not manipulated experimentally in Experiment 2. For Experiment 3 green peach aphids Myzus persicae Sulzer were collected from the field and reared on radish plants in a greenhouse, under natural photoperiod at 21 ± 2oC. At the end of the experiment, one randomly selected leaf from each plant per replicate, was removed and the number of aphids was counted.

STATISTICS
For our on-farm data, beetle activity-densities (“activity-densities” because pitfall trapping measures a mix of both the number of beetles present, and their movement) were analyzed using repeated measures MANOVA (von Ende 1993), following square root transformation to homogenize variances. Relationships between beetle activity-densities and sentinel egg predation rates were examined using multiple regression, with the critical P-value adjusted to reflect multiple comparisons (Jones 1984).
For the field cage experiments, predation of fly eggs in all experiments, and predator activity-densities in Experiments 2 and 3, were first analyzed using repeated-measures MANOVA. However, treatment x time interactions were never significant in a way that clearly was ecologically significant, and so for the sake of brevity only statistical outputs for the main treatment, block, and treatment x block effects are presented and discussed herein; the full time series data, and associated repeated measures analysis, are presented in Prasad (2005). We first tested for cage effects, by comparing Open plots with the appropriate caged treatment (the treatment designed to most closely match open-field conditions). Next, we tested for the efficacy of beetle manipulations by comparing the Removal cage with the appropriate addition treatment(s). Finally, for analysis of main treatment effects, only the beetle addition treatments were compared to one another. We analyzed initial and final beetle activity-densities in Experiment 1 using two-way ANOVA, and final aphid densities in Experiment 3 using one-way ANOVA. When appropriate, post-hoc tests were performed using a Tukey-Kramer HSD test (α = 0.05).
All analyses were conducted in SYSTAT version 9.0 (SPSS, Chicago, Illinois).