Integrating Biological Control into Cole Crop Production in the Pacific Northwest

Final Report for SW03-101

Project Type: Research and Education
Funds awarded in 2003: $63,841.00
Projected End Date: 12/31/2006
Matching Non-Federal Funds: $6,386.00
Region: Western
State: Washington
Principal Investigator:
William Snyder
Washington State University
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Project Information


We examined the use of beetle banks, which are unplowed grassy strips within agricultural fields, for the conservation of predatory ground beetles. Beetle banks were located on the farms of several cooperative growers, and also on a university research farm. Beetle banks successfully conserved ground beetles, doubling their densities. However, control of root maggots, the target pest, was not always improved because ground beetles are generalists that feed on many different insect prey species.

Project Objectives:

There were 4 objectives:

1) Evaluate field margins and in-field refuges for predator conservation.

2) Document the seasonal abundance of different types of beneficial arthropods relative to the seasonal peaks in adult Delia spp. populations.

3) Evaluate the impact of the various natural enemies as biocontrol agents of root maggots.

4) Disseminate information to organic and conventional cole crop growers.


Cole crops are an important component of agricultural production in western Washington – 50% of the world's cabbage seed is produced in Skagit County alone (Jones and Foss 2000). Other cole crop seeds grown include rutabagas and radishes. Broccoli, cauliflower and other cole crops are also grown, both conventionally and organically, for fresh market and processing. Organic acreage in Washington State tripled to 40,000 acres from 1997 to 2001 (WSDA 2002). In western Washington, many organic farms are small family-run operations. Cole crops are an important component to these operations as they provide season-long income. Root maggots (Delia spp.) are one of the main pests for all cole crops in both organic and conventional production. Currently, conventional growers are completely reliant on organophosphate (OP) insecticides (Lorsban and Diazanon) for their maggot control. Although these insecticides are effective at preventing root maggot injury there are 3 issues that concern growers and precipitate their desire to develop less reliance on broad-spectrum OP insecticides. First, there is risk that OP usage will be limited or phased out as part of the Food Quality Protection Act (Epstein et al. 2000). Second, there are on-going public concerns about the impact of agrochemical run-off on salmon in Pacific Northwest waterways. Third, there is a potentially lucrative market for the production of organic cabbage (and other cole crop) seed as federal laws governing organic certification now require the use of organically produced seed. Thus, lowering reliance on OP insecticides could have both environmental and economic benefits for conventional growers in the region. For organic growers, improving root maggot control could increase farm income, especially from early season cole crops, e.g. radishes.

Crop protection that is less reliant on broad-spectrum insecticides can be achieved by using a variety of tactics, including biological control. In annual cropping systems the use of native beneficial organisms has been advocated (Weidenmann and Smith 1997). Conservation biological control involves the enhancement of endemic populations of beneficial fauna by way of various conservation strategies, including providing refuges, alternative sources of food, and reducing the use of broad-spectrum insecticides (Barbosa 1998). There are many natural enemies that attack root maggots at the egg, larval and pupal stages (Finch 1996, Finch 1989, Coaker and Williams 1963). One of the objectives of this project is to measure the effectiveness of these natural enemies, and to determine how their numbers can be enhanced in agricultural fields. Because many of these root maggot natural enemies are generalists they will likely consume a variety of pests, including aphids, cutworms, weevils and flea beetles. Thus, conservation of natural enemies primarily for root maggot control may also help reduce populations of other pests.

Egg predation is probably the most effective way to reduce plant damage from this pest, since it is the larval or maggot stage that causes the damage. Research that we conducted in 2002 has demonstrated that natural enemies are important predators of the egg stage. In preliminary field cage experiments, we found that significantly more fly eggs were eaten in treatments that included predators than in treatments that had reduced predator populations. We found that of the 12 most commonly occurring species of ground dwelling beetles, six were very effective predators of Delia spp. eggs. In an effort to understand how populations of natural enemies might be enhanced in fields, we conducted a survey of organic and conventional fields with various qualities of field margins. A good quality margin, for example, is composed primarily of perennial grasses, has good ground cover e.g. >80%, especially in winter and is infrequently disturbed. These types of habitats have been shown to be preferred by natural enemies, especially for over-wintering (Dennis and Fry 1992, Thomas et al. 1992). During the early part of the season, we found that the in-field activity density (measured 20m from the field margin) of natural enemies was higher in organic fields with good quality margins. Indeed, the richness of the six important Delia spp. egg predators was also higher in these fields, at the same time. Later in the season the in-field activity of natural enemies was similar in organic and conventional fields, regardless of the quality of arthropod habitat provided in the margin. Our results suggest that good quality margins, i.e. those that provide over-wintering habitat for arthropods, may enhance biological control for early season organic cole crops. More research is needed, however, to determine how the margin may contribute to natural enemy populations in conventional fields and in cabbage seed fields, which are planted in September.

In our WSARE-funded work we continued our research to better understand how biological control by endemic natural enemies could be integrated into cole crop production. Endemic arthropods are often overlooked as a natural resource, but they have the potential to reduce pest damage and reliance on insecticides. Management practices that minimize harm to natural enemies and maximize their conservation are important aspects of good agricultural stewardship. Thus, our research directly addressed the Western SARE goal of “…optimizing the use of on-farm resources and integrating, where appropriate, biological cycles and controls.” Our project also indirectly addressed two other WSARE goals: increasing wildlife habitat and increasing the possibility of innovative marketing opportunities. Field margins and in-field refuges, in addition to conserving beneficial arthropods, also act as habitat for small mammals and birds (Burel 1996, Osborne 1984, Middleton and Merriam 1981). Finally, effective biological control for root maggots can bring us a step closer toward the development of organic cole crop seed production.


Barbosa, P. 1998. Conservation Biological Control. Academic Press, Newbury, UK.

Burel, F. 1996. Hedgerows and their role in agricultural landscapes. Critical Reviews in Plant Sciences 15(2):169-190.

Coaker, T.H. and Williams, D.A. 1963. The importance of some carabidae and staphylinidae as predators of the cabbage root fly, Erioschia brassicae (Bouche). Entomologia Experimentalis et Applicata. 6:156-164.

Dennis, P. and Fry, G.L.A. 1992. Field margins: can they enhance natural enemy population densities and general arthropod diversity on farmland? Agriculture, Ecosystems and Environment. 40:95-115.

Epstein, D.L., Zack, R.S., Brunner, J.F., Gut, L. and Brown,J.J. 2000. Effects of broad-spectrum insecticides on epigeal arthropod diversity in Pacific Northwest apple orchards. Environmental Entomology. 29:340-348.

Finch, S. 1996. Effect of beetle size on predation of cabbage root fly eggs by ground beetles. Entomologica Experimentalis et Applicata. 81:199-206.

Finch, S. 1989. Ecological considerations in the management of Delia pests species in vegetable crops. Annual Review of Entomology. 34:117-137.

Jones, L.J., and Foss, C.R. 2000. Crop profiles for cabbage seed in Washington.

Lee, J.C., Menalled, F.D. and Landis, D.A. 2001. Refuge habitats modify impact of insecticide disturbance on carabid beetle communities. Journal of Applied Ecology. 38:472-483.

Middleton, J. and Merriam, G. 1981. Woodland mice in a farmland mosaic. Journal of Applied Ecology. 18:703-710.

Osborne, P. 1984. Bird numbers and habitat characteristics in farmland hedgerows. Journal of Applied Ecology. 21:63-82.

Thomas, M.B., Wratten, S.D. and Sotherton, N.W. 1991. Creation of ‘island’ habitats in farmland to manipulate populations of beneficial arthropods: predator densities and emigration. Journal of Applied Ecology. 28:906-917.

Tilth Producers. 2001. Washington Tilth Producers Directory. 38pp. Washington Tilth Association. Seattle WA.

Washington State Department of Agriculture. 2002. Organic Producer Survey. 52 pp.

Wiedenmann, R.N. and Smith, J.W. 1997. Attributes of natural enemies in ephemeral crop habitats. Biological Control. 10:16-22.


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  • Renee Prasad
  • John Stark


Materials and methods:

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).


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 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.

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).

Research results and discussion:

All of the results of our WSARE-funded project have now been published in peer-reviewed journals. Below we provide the abstracts of these papers that summarize key results, along with references to the complete papers.

1. We examined whether predator interference could prevent effective conservation biological control of Delia spp. flies, important pests of cole crops, by an assemblage of carabid and staphylinid beetles. In laboratory feeding trials we found that the smaller (< 1cm) beetle species common at our site readily ate dipteran eggs, while the most common large carabid species, Pterostichus melanarius, rarely did. However, P. melanarius did eat several of the smaller beetle species. We conducted two field experiments where we manipulated immigration rates of the ground predator guild and then measured predation on fly eggs. Predation rates were consistently higher in cages where predators were added at ambient densities, compared to cages where ground predators were removed. However, in the second field experiment, when we quadrupled predator immigration rates neither beetle activity-density nor predation rate increased. High immigration rate plots had a higher proportion of P. melanarius in the predator community, compared to plots with beetles added at ambient densities, suggesting that P. melanarius was reducing activity-densities of the smaller beetles, perhaps through intraguild predation. Thus, tactics to improve the biological control of Delia spp. by conserving generalist predators, such as providing in- or extra-field refuges, could be thwarted if the primary predators of fly eggs, small carabids and staphylinids, are the targets of intraguild predation by also-conserved larger predators.

Complete manuscript:Prasad, RP and WE Snyder. 2004. Predator interference limits fly egg biological control by a guild of ground-active beetles. Biological Control 31:428-437.

2. Whilst evidence suggests that undisturbed refuges within agricultural fields conserve natural enemies, few studies examine whether pest control actually improves following predator conservation. When the targets of conservation are generalists, polyphagy may complicate the impact of the conserved predators on agricultural pests. We examined the use of beetle banks to conserve predatory beetles for the control of pest Diptera. Locally, the community of predatory beetles includes several species of small carabid and staphylinid beetles ( 1.5 cm), Pterostichus melanarius, that rarely eats fly eggs but does eat smaller beetles. Predator beetle activity-densities, but not rates of fly egg predation, increased in fields including beetle banks. A series of field experiments was conducted to examine whether two types of polyphagy, intraguild predation and feeding on non-target prey, could be preventing increased egg predation following successful predator conservation. The putative intraguild predator P. melanarius reduced activity-densities of smaller beetles, and thus weakened fly egg predation. The strength of fly suppression increased with increasing densities of small beetles in the absence of P. melanarius, but not when aphid alternative prey were readily available. In the presence of abundant aphids, egg predation rates did not increase at higher small beetle densities. Synthesis and applications. Overall, our results suggest that both intraguild predation and the presence of alternative prey could limit conservation biological control that targeted generalist predators. Thus, higher predator densities will not necessarily lead to improved pest control. Ecologists must consider the impact of predator manipulations at multiple trophic levels when assessing the success or failure of conservation biological control.

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

3. Trait-mediated interactions (TMII) can alter the outcome or magnitude of species interactions. We examined how the interaction between a guild of ground and rove beetles and their fly egg prey was altered by a larger predator, the ground beetle Pterostichus melanarius, and an additional prey, aphids. In field and laboratory experiments we manipulated the presence or absence of P. melanarius and aphids and recorded the impact of these manipulations on beetle activity and fly egg predation. Individually, aphids, by serving as preferred prey, and P. melanarius, by reducing focal beetle activity, weakened egg predation. However, egg predation was restored when both aphids and P. melanarius were present together, because aphids triggered greater foraging activity, and thus increased incidental predation of fly eggs, by P. melanarius. Thus, TMII among subsets of the community that were disruptive to predation on fly eggs could not be summed to predict the dominant, positive TMII within a more diverse community. Future TMII studies should include more realistic representations of species diversity, and should not ignore the influence of prey on predator behavior.

Complete manuscript: Prasad, RP and WE Snyder. 2006. Diverse trait-mediated indirect interactions in a multi-predator, multi-prey community. Ecology 87:1131-1137.

Research conclusions:

In a long series of open-field and cage experiments, we found that beetle banks are unlikely, by themselves, to allow a dramatic improvement in root maggot control. Because our experimental results showed that beetle banks were not particularly effective as a control option for our target pest, adoption (of course) has not been widespread.

The most promising future direction would be to develop refuge strips that are not targeted to just one natural enemy, or for the control of one particular pest. For example, refuge strips that incorporated flowering plants might conserve ground beetles AND many other beneficial insects (parasitoids, ladybird beetles, predatory bugs, etc). In this way many natural enemies, each contributing to control of many pests, could be conserved simultaneously.

Nonetheless, through our many extension presentations and our long-running participation in field days, we have increased grower awareness of the natural enemies present in their fields.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

Extension presentations:


Bug-scaping Faire, Oregon State University, Corvallis, OR. Informal presentations in which growers learn about various farmscaping strategies in a market or “faire” format.

Farm Walks for Growers, Washington State University. A series of farm walks conducted over the course of the summer, in which growers and WSU extension agents and researchers discussed problems associated with organic production on small farms (at least 3 presentations each year of project).

2005. Gardening and Entomological Odyssey. Talk presented to Orcas Island Master Gardeners. January 17, Orcas Island, WA.

2005. The challenges of successful conservation biocontrol in vegetable fields. Talk presented at Western Washington Horticultural Association Meeting. January 11, Sea-Tac, WA.

2004. Insects and Humans: Beyond Honey and Diazinon. Informal brown-bag seminar at Northwest Agricultural Research and Extension Center. June 6, Mt. Vernon, WA.

2004. Identification and biology of common vegetable pests and natural enemies. Coquitlam Organic Growers Association Meeting. May 31, Coquitlam, BC.

2003. Natural enemies of root maggots (Delia spp.). Talk presented at Pacific Northwest Vegetable Association Annual Meeting. November 19, Pasco, WA.

2003. Research up-date and informal presentation at annual Puget Sound Seed Growers' Executive Meeting and tour of research plots as part of Vegetable Seed Growers' Field Day. July 29, Mt. Vernon, WA.

2003. Insect pests of (cole crop) seed: Identification and Control. Talk presented to Puget Sound Seed Growers' Association Meeting. February 27, Mt. Vernon, WA.

2003. Conservation biological control for management of cole crop pests. Talk presented at Western Washington Horticultural Association Meeting. January 9, Sea-Tac, WA.

2002. Evaluating arthropod density in organic and conventional fields: The role of field margins. Western Sustainable Agriculture Conference. November 8, Yakima, WA.


Prasad, RP and WE Snyder. 2006. Diverse trait-mediated indirect interactions in a multi-predator, multi-prey community. Ecology 87:1131-1137.

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

Prasad, RP and WE Snyder. 2004. Predator interference limits fly egg biological control by a guild of ground-active beetles. Biological Control 31:428-437.

Education and Outreach Outcomes

Recommendations for education and outreach:

Areas needing additional study

The most promising future direction would be to develop refuge strips that are not targeted to just one natural enemy, or for the control of one particular pest. For example, refuge strips that incorporated flowering plants might conserve ground beetles AND many other beneficial insects (parasitoids, ladybird beetles, predatory bugs, etc). In this way many natural enemies, each contributing to control of many pests, could be conserved simultaneously.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.