Acorn Weevil (Coleoptera: Curculionidae) Predation Dynamics in a Mississippi Bottomland Hardwood Forest
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CASTANEA 85(1): 159–168 April Copyright 2020 Southern Appalachian Botanical Society Acorn Weevil (Coleoptera: Curculionidae) Predation Dynamics in a Mississippi Bottomland Hardwood Forest Justin P. Williams1* and Tracy S. Hawkins2 1Biological Science Technician, USDA Forest Service State and Private Forestry, Forest Health Protection 4746 South 1900 East, Ogden, UT 84403 2Research Ecologist, USDA Forest Service Southern Research Station, Center for Bottomland Hardwoods Research 775 Stone Blvd. Mississippi State, MS 39762 ABSTRACT Curculio and Conotrachelus weevils (Coleoptera: Curculionidae) can render the majority of North American Quercus spp. acorn crops nonviable, thereby reducing food resources for wildlife and limiting opportunities for seedling establishment. Acorn predation by weevils at the individual tree level can be influenced by many factors, and research specifically investigating acorn predation by weevils in seasonally flooded bottomland oak forests is lacking. We placed cone emergence traps in a periodically flooded forest on the Sam D. Hamilton Noxubee National Wildlife Refuge near Brooksville, Mississippi to obtain weevil population parameter estimates, record emergence phenol- ogies, and identify variables that may aid in understanding tree-to-tree variability in acorn preda- tion rates. Forty-three Curculio weevils representing five species emerged from mid-August through early November, and 56% of those captured emerged over an 11-day period in mid-September. Sixty-four Conotrachelus weevils representing two species emerged from mid-August through late November and occurred at nearly twice the density of Curculio weevils. The exotic Asiatic oak weevil, Crytepistomus castaneus, a minor defoliator, emerged from August through early November. We also identified Quercus density, specifically the proportion of oaks that immediately neighbor the host tree, as a potentially important explanatory variable of acorn predation by weevils at the tree level in bottomland forest habitats. Key words: acorn, bottomland forest, Curculionidae, emergence, seed predation INTRODUCTION Curculio and Conotrachelus (Coleoptera: Curculionidae) weevil species can damage the majority, if not all, of an acorn crop of some North American Quercus spp. (Galford et al. 1991, Gribko 1995, Riccardi et al. 2004, Miller and Schlarbaum 2005). Curculio weevils are considered primary acorn predators due to their highly specialized rostrum that allows females to drill oviposition holes through the acorn pericarp (Gibson 1969). Conotrachelus weevils are considered secondary acorn predators because they cannot complete oviposition into sound acorns; therefore, acorns must be damaged, cracked, sprouted, or otherwise compromised (Gibson 1964). However, seed predation pressures on Quercus spp. are lessened through annual variability in acorn production. In this case, weevil populations are thought to decline after low crop years, thereby increasing opportunity for seedling establishment in subsequent high crop years via predator satiation (Kelly 1994; but see Bogdziewicz et al. 2018a). *email address: [email protected] Received 31 October 2019; Accepted 20 April 2020 159 160 Castanea, Vol. 85(1) 2020 The annual variability of seed production, synchronized across a population of plants, is referred to as mast seeding (Kelly 1994). Ultimately, advantages of mast seeding are predator satiation (Silvertown 1980), pollination efficiency (Kelly et al. 2001), and the timing of mast seeding events to environmental cues that offer a greater chance of reproductive success (e.g. herbaceous plant post- fire flowering) (Kelly 1994). Spatial synchrony of mast seeding has been well documented and may be highly correlated at the stand level (at distances <10 km) for Quercus populations (Liebhold et al. 2004). In North American conifers, masting synchrony may extend as far as 1,000 km (Koenig and Knops 1998). Research on spatial variability of acorn predation by weevils is somewhat limited and the results vary. Xiaodong et al. (2003), who examined spatial variability in insect-infested acorns in northern China, found that predation by insects varied due to site differences in aspect and topographic position. Govindan et al. (2012) and Govindan and Swihart (2015), who studied acorn weevil com- munity structure and host tree selection in Indiana, found weevil occupancy and survival rates increased with acorn production and overall neighborhood forest density (a remotely sensed variable). Xiao et al. (2017), who studied Curculio weevil predation on pre-dispersal Quercus serrata Murray acorns in southern China, noted a negative correlation with host tree density at the stand level during a high seed year. Finally, Bogdziewicz et al. (2018b) reported more effective acorn weevil satiation at isolated trees versus dense patches of Quercus ilex L. in central Spain. These reports illustrate how a multitude of factors may influence spatial variability of acorn predation by weevils, and suggest that the relative importance of each factor may vary by location. Research documenting acorn weevil species and acorn predation in the contiguous United States has been somewhat limited to mesic deciduous forests (Miller and Schlarbaum 2005, Kellner et al. 2014). However, nine economically and ecologically important Quercus species are commonly found in the canopy of bottomland forests in the southeastern United States (Gardiner 2001), and knowledge is lacking regarding acorn weevil presence and associated acorn predation in seasonally flooded bottomland habitats. In light of this, our objectives were: 1) to assemble baseline data on presence and abundance of acorn predating weevil species; 2) to construct an emergence phenology for identified insect genera; and, 3) to investigate the relationships between acorn production and predation, more specifically inter-tree variability in acorn predation by weevils, in a bottomland forest stand. MATERIALS AND METHODS Study Site Our study was conducted in a bottomland hardwood forest on the Sam D. Hamilton Noxubee National Wildlife Refuge, Noxubee County, Mississippi. The site, which was located on the Noxubee River floodplain (N33.258593, W88.725206), floods periodically in winter and spring. Quercus pagoda Raf., Quercus nigra L., and Liquidambar styraciflua L. are dominant canopy trees. Canopy species of lesser importance are Fraxinus pennsylvanica Marshall, Quercus lyrata Walter, Quercus michauxii Nutt., and Quercus phellos L. (Meadows and Skojack 2012). Emergence Trap Design and Placement Cone-shaped emergence traps (0.5-m2 ground coverage), modeled after Raney and Eikenbary (1969), were constructed from polyethylene tubing and aluminum insect screen. The Raney and Eikenbary (1969) design was further modified by installing a boll weevil (Anthonomus grandis Boheman) trap (Great Lakes IPM, Inc., Vestaburg, Michigan) on top and using three sections of Sturdy Stake ® (Midwest Air Technologies, Inc., Long Grove, Illinois) to stabilize the cone. We also added a 10 cm screen skirt that was pressed down into the soil to prevent weevil escape and non-emergence cap- tures. Two traps were secured to the ground midway between the bole and canopy dripline for each of nine Quercus trees (two Q. michauxii (Nutt.) and seven Q. pagoda (Raf.); DBH = 90.0 ± 13.4 cm) in a 6,000 m2 (1.5 acre) plot of bottomland forest. Trees selected were evaluated to be of similar canopy position, crown exposure, and vigor (Schomaker et al. 2007). Williams and Hawkins, Acorn Weevil Predation Dynamics in a Mississippi Bottomland Hardwood Forest 161 Insect Abundance and Emergence Emergence traps were installed on 17 August 2017, and emptied with an aspirator at approximately two week intervals (13.7±2.3 days) from 29 August through 18 December 2017. The sample period was determined to be the optimal time to trap emerging Curculio weevils during a trap design testing phase in 2016. This yielded nine independent samples for each of nine replicate collection periods. Aspirator contents were emptied into labeled polyethylene bags and stored at -20°C until identifi- cation. Trapped weevils were identified to the genus level by the senior author using taxonomic keys from Anderson (2002). Whitehead et al. (2018) and Gibson (1969) were referenced for species level identification of Curculio, and Schoof (1942) was referenced for species level identification of Conotrachelus. Species were cross referenced with online identification databases (Ratnasingham and Hebert 2007, Franz et al. 2012). Insect data were summarized by collection date and tree. Voucher specimens are housed at the Mississippi State University Entomology Museum in Starkville, Mississippi. Acorn Production and Predation Acorn production was estimated using two methods. Prior to acorn dispersal, crop size was estimated using the visual survey developed by Koenig (1994). The intent of the visual survey was to validate total acorn counts from quadrat collections. Two sections of each tree crown were surveyed using binoculars; the surveyor counted as many acorns as possible within 15 seconds. The two counts were combined and log transformed (lnx+1). The second method involved collecting all fallen acorns within three, 0.5-m2 circular plots under each tree (Govindan et al. 2012). Quadrat plots were established by placing three 0.5-m2 rings at random aspects midway between the bole and the dripline of the crown. These plots were flagged and resampled biweekly from November 2017 through January 2018. Collected acorns