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Beneficial Insect Attraction to Milkweeds

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Beneficial Insect Attraction to Milkweeds insects Article Beneficial Insect Attraction to Milkweeds (Asclepias speciosa, Asclepias fascicularis) in Washington State, USA David G. James *, Lorraine Seymour, Gerry Lauby and Katie Buckley Department of Entomology, Washington State University, Prosser, WA 99350, USA; [email protected] (L.S.); [email protected] (G.L.); [email protected] (K.B.) * Correspondence: [email protected]; Tel.: +1-509-786-9280 Academic Editors: Andrew G. S. Cuthbertson and Eric W. Riddick Received: 14 April 2016; Accepted: 17 June 2016; Published: 29 June 2016 Abstract: Native plant and beneficial insect associations are relatively unstudied yet are important in native habitat restoration programs for improving and sustaining conservation biological control of arthropod pests in agricultural crops. Milkweeds (Asclepias spp.) are currently the focus of restoration programs in the USA aimed at reversing a decline in populations of the milkweed-dependent monarch butterfly (Danaus plexippus); however, little is known of the benefits of these plants to other beneficial insects. Beneficial insects (predators, parasitoids, pollinators) attracted to two milkweed species (Asclepias speciosa, Asclepias fascicularis) in central Washington State, WA, USA were identified and counted on transparent sticky traps attached to blooms over five seasons. Combining all categories of beneficial insects, means of 128 and 126 insects per trap were recorded for A. speciosa and A. fascicularis, respectively. Predatory and parasitic flies dominated trap catches for A. speciosa while parasitic wasps were the most commonly trapped beneficial insects on A. fascicularis. Bees were trapped commonly on both species, especially A. speciosa with native bees trapped in significantly greater numbers than honey bees. Beneficial insect attraction to A. speciosa and A. fascicularis was substantial. Therefore, these plants are ideal candidates for habitat restoration, intended to enhance conservation biological control, and for pollinator conservation. In central Washington, milkweed restoration programs for enhancement of D. plexippus populations should also provide benefits for pest suppression and pollinator conservation. Keywords: Milkweed; Asclepias; beneficial insects; conservation biological control; pollinators 1. Introduction Restoring native plants and habitats is increasingly seen as a critical part of enhancing and sustaining conservation biological control of insects and mites in agricultural crop pest management [1–3]. Native natural enemies co-evolved with native plants long before agriculture fragmented the landscape disrupting natural ecosystem services like the suppression of herbivores. Greater access to native plant resources should have a positive impact on the persistence and function of native beneficial insects like predators, parasitoids and pollinators in crop ecosystems. Beneficial insect and native plant associations are poorly studied in many regions; the identity of the most valuable native plants in terms of the beneficial insects they harbor and sustain is thus frequently unknown. Fortunately, this situation is improving with a number of recent studies showing the benefit of local native plants in enhancing attraction and sustenance of beneficial insects [4–10]. In Washington State, James et al. [11,12] reported substantial attraction of beneficial insects to flowering native buckwheats (Eriogonum spp.) and stinging nettles (Urtica dioica L.). Asclepias L. is a genus in the Apocynaceae containing at least 76 species of perennial herbaceous plants known as “milkweeds” that occur throughout the United States into southern Canada [13]. Insects 2016, 7, 30; doi:10.3390/insects7030030 www.mdpi.com/journal/insects Insects 2016, 7, 30 2 of 11 Milkweeds are best known as the larval hosts of monarch butterflies (Danaus plexippus L.) and two other danaid species in North America [14]. An apparent decline in milkweed populations throughout the United States in recent years [15,16] has been blamed for a similar decline in populations of D. plexippus since the late 1990s [16]. Consequently, milkweed restoration efforts have been initiated by numerous private and public enterprises [17] as part of a federal campaign to help reverse the decline in monarch butterfly populations [18]. Organizations like the Xerces Society for Invertebrate Conservation have also promoted the cultivation and benefits of milkweeds for monarchs and pollinators generally [19]. The attraction of pollinators (bees, butterflies, moths, flies, beetles) to Asclepias spp. has been recognized for some time [20–24], but the value of Asclepias spp. to other beneficial insects like predators and parasitoids has not received the same attention. To date, the tachinid fly parasitoid of stink bugs, Trichopoda pennipes (F.), is the only natural enemy of a herbivore demonstrated to be attracted to a milkweed species (Asclepias curassavica L.) [25]. Here, we report the results of a field study on the attraction of predators, parasitoids and pollinators to the two endemic species of Asclepias, A. speciosa Torr. and A. fascicularis Decne occurring in an agriculturally intensive area of central Washington. Some horticultural industries in this region, for example wine grapes, have low inputs of pesticides and depend upon conservation biological control for much of their arthropod pest management [26,27]. Restoration of native flora and habitats is being pursued as a method of enhancing and sustaining the ecosystem services provided by natural enemies of pests as well aiding the conservation of threatened invertebrate fauna like butterflies [28]. 2. Materials and Methods 2.1. Sites This study was conducted over four seasons (2010–2014) in central Washington by counting and identifying beneficial insects attracted to blooming showy milkweed (Asclepias speciosa Torr.) and narrow-leaved milkweed (Asclepias fascicularis Decne) using transparent sticky traps. Milkweed plants were located growing in riparian or natural areas at six locations (Wishram (45.40˝ N, 120.46˝ W), Satus pass (45.60˝ N, 120.38˝ W), Snow Mountain (46.39˝ N, 120.46˝ W), Yakima (46.32˝ N, 120.90˝ W), Prosser (46.14˝ N, 119.42˝ W), and Horn rapids (46.22˝ N, 119.26˝ W) (Figure1). Asclepias speciosa was sampled at all sites except Wishram. Asclepias fascicularis was sampled at Wishram, Satus pass and Snow Mountain only. Asclepias speciosa and A. fascicularis are the only milkweed species present in Washington and the sampling sites used in this study for these plants are representative of the low-rainfall eastern areas of the state in which these milkweeds occur. 2.2. Traps Transparent sticky traps (WindowBugCatcher, large 40.6 cm ˆ 12.1 cm, Alpha Scents Inc., Portland, OR, USA) were used, avoiding trap color as a potential influence on insect attraction. Transparent sticky traps were used in earlier studies in Washington on predatory and parasitic insects attracted to native plants and were successful in trapping a wide diversity of these insects. Traps were attached to plants as soon as blooming commenced. At each site and on each occasion trapping was conducted, three traps were placed on three Asclepias plants. Plants with traps were at least 5 m from other plants/traps at each site and traps were attached to plants using flexible wires and positioned to provide a sticky surface immediately above or adjacent to the flowers. Traps were left in place for 12–14 days before removal and were replaced if blooming continued. In a few instances, follow-up trapping occurred on the same plants (when plant numbers were limited) but usually different plants were chosen. Traps collected from the field were transported to the laboratory and stored at ´30 ˝C until examined under a stereomicroscope. Insects 2016, 7, 30 3 of 11 Insects 2016, 7, 30 3 of 11 Figure 1. Washington State, WA, USA showing locations of Asclepias speciosa and Asclepias fascicularis Figure 1. Washingtonsampled for beneficial State, WA, insect USA attraction. showing locations of Asclepias speciosa and Asclepias fascicularis sampled for beneficial insect attraction. 2.3. Trap Processing 2.3. Trap ProcessingAll insects were identified to family or species and counted. The incidence and abundance of 34 species, genera or groups of winged beneficial insects were recorded (Table 1). Numbers of All insectsleafhopper were (Erythroneura identified spp.) to and family lygus orbug species(Lygus spp.) and pests counted. were also recorded. The incidence and abundance of 34 species, genera or groups of winged beneficial insects were recorded (Table1). Numbers of Table 1. Categories of beneficial insects identified and recorded in this study, along with species, leafhopper (Erythroneuragenera and familiesspp.) within and each lygus category. bug (Lygus spp.) pests were also recorded. Beneficial insect categories Species, genera or family included Table 1. Categories of beneficial insects identified and recordedChrysoperla in this plorabunda study, along (Fitch) with species, genera and families within each category. Chrysopa nigricornis Burmeister Neuroptera (Lacewings) Chrysopa coloradensis Banks Beneficial Insect Categories Species, GeneraChrysopa or Family oculataIncluded Say Harmonia axyridis (Pallas) ChrysoperlaCoccinella plorabunda septempunctata(Fitch) L. Chrysopa nigricornis Burmeister Neuroptera (Lacewings) Coccinella transversogutatta Mulsant Coccinellidae (Ladybeetles) Chrysopa coloradensis Banks Hippodamia convergens (Guerin-Meneville) Chrysopa oculata Say Stethorus picipes Casey HarmoniaStethorus axyridis punctillum(Pallas)
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