Environmental Entomology, 45(5), 2016, 1294–1299 doi: 10.1093/ee/nvw091 Advance Access Publication Date: 6 August 2016 Physiological Ecology Research Supercooling Points of Murgantia histrionica (Hemiptera: Pentatomidae) and Field Mortality in the Mid-Atlantic United States Following Lethal Low Temperatures Anthony S. DiMeglio,1,2 Anna K. Wallingford,3 Donald C. Weber,4 Thomas P. Kuhar,1 and Donald Mullins1 1Department of Entomology, Virginia Tech, 170 Drillfield Drive, Blacksburg, VA 24061 ([email protected]; [email protected]; [email protected]), 2Corresponding author, e-mail: [email protected], 3Department of Entomology, Cornell University, 630 W. North St., Geneva, NY 14456 ([email protected]), and 4Invasive Insect Biocontrol and Behavior Lab, USDA Agricultural Research Service, BARC-West 007, Beltsville, MD 20705 ([email protected]) Received 24 April 2016; Accepted 28 June 2016 Abstract The harlequin bug, Murgantia histrionica (Hahn), is a serious pest of brassicaceous vegetables in southern North America. While this insect is limited in its northern range of North America, presumably by severe cold winter temperatures, specific information on its cold hardiness remains unknown. We determined the super- cooling points (SCPs) for Maryland and Virginia adult populations and found no significant difference among these populations. SCPs were similar for adults (X ¼10.35 C; rX ¼ 2.54) and early and late instar (X ¼11.00 C; rX ¼ 4.92) and between adult males and females. However, SCPs for first instars (X ¼21.56 C; rX ¼1.47) and eggs (X ¼23.24 C; rX ¼1.00) were significantly lower. We also evaluated field survival of overwintering harlequin bug adults during extreme cold episodes of January 2014 and January 2015, which produced widespread air temperatures lower than À15 C and subfreezing soil temperatures in the Mid-Atlantic Region. After the 48-h episode in 2014, bug mortality in exposed field sites averaged 88%, com- pared to <5% mortality of bugs sheltered in an unheated greenhouse (recorded minimum temperature 4.4 C). After the 2015 episode, 80% of adults that were established in the field the previous November and then shel- tered in an unheated garage during the episode, died, in contrast to 96% mortality in exposed field sites. Our re- sults provide new information on M. histrionica overwintering biology, and thermal limitations to its distribu- tion, which leads to improved predictive capabilities to forecast pest severity. Key words: supercooling point, lethal low temperature, field survival, polar vortex Murgantia histrionica (Hahn), native to Central America (Paddock abundance and distributions (Bale and Hayward 2010). A record- 1918), has expanded widely into the United States since it was first breaking cold air mass blanketed much of the United States with a collected in Texas in 1864 (Paddock 1915). With two to three gener- high pressure system propagating from the Arctic in mid-February ations per year (Wallingford et al. 2011), this key pest of Brassica 1899 (Kocin et al. 1988). Sanderson (1908) reported significant re- spp. can quickly infest farms, damaging vegetables such as cabbage, ductions in M. histrionica populations following this bitterly cold broccoli, collards, radishes, and kale. Occasional extreme winter Arctic air mass, when minimum temperatures reached À28.9 Cin events have historically suppressed its distribution in northern lati- Ohio and À26.1 C in Washington D.C. (Sanderson 1908), and tudes just south of the Mason-Dixon Line (Walker and Anderson À15 C in the Carolinas (Kocin et al. 1988). Although field mortal- 1933). A combination of physiological and behavioral adaptations ity of M. histrionica was not formally measured before and after this allow for overwintering insects, like M. histrionica, to survive a winter event, the 1899 North American winter serves as anecdotal range of winter extremes common in temperate regions (Duman evidence of the negative impact of extreme low temperatures on M. et al. 1991). As Venette et al. (2010) explained, understanding impli- histrionica abundance. cations of insect cold-hardiness can help us better understand pest Events of prolonged subfreezing temperatures may occur more severity and outbreaks and forecast geographic distribution and po- frequently in the southeastern United States than in the past decades tential range expansions. due to altered atmospheric thermal gradients between northern and Extreme winter weather events, such as sudden shifts in Arctic mid-latitudes (Frauenfeld and Davis 2003, Limpasuvan et al. 2004, air currents, can have significant effects on temperate insect pest Francis and Vavurs 2012). Therefore, it is important to understand VC The Authors 2016. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For Permissions, please email: [email protected] 1294 Environmental Entomology, 2016, Vol. 45, No. 5 1295 how freeze-avoiding insects are affected by potentially lethal lower instruments (Decagon Devices, Pullman, WA) positioned within temperatures (Elsey 1993), especially for temperate insects in a overwintering habitats. For both 2014 and 2015, microclimate sen- changing climate (Bradshaw 2010). sors were installed to record temperature at 1 cm soil depth, and air Saulich and Musolin (2012) reviewed literature and experimen- within the plant canopy (0.05 m) and above (1 m) the M. histrionica tal data on diapause in 43 Pentatomidae species overwintering in overwintering habitat. After “polar vortex” episodes, both control temperate climates, although did not include M. histrionica, and and exposed bugs were monitored for 3 d at ambient temperature concluded that the majority of species undergo facultative imaginal (20–25 C), to visually characterize external ice formation on insect diapause—that is, extrinsic cues such as temperature and photope- cuticle, and to record activity and mortality. riod trigger arrested development in adults. Murgantia histrionica undergo homodynamic development, and therefore activity during winter months is primarily dictated by temperature; in mild winters Insects they are active (Thomas 1915, Paddock 1915, 1918) with feeding, SCP Determination copulation, and oviposition possible (Sullivan and Brett 1974). M. histrionica adults were collected in early September 2013 in 0 00 0 00 Diapause and categories of cold-hardiness occur conjointly. Salt Beltsville, MD (39 1 37.37 N, 76 56 0.96 W; elevation ¼ 38 m), (1961) and Lee (1991) divided cold-hardiness of insects into three and were used to start a colony, which was maintained on potted groups: chilling intolerant, freezing tolerant, and freeze avoidant. collards (Brassica oleracea group acephala cv. ‘Champion’ and According to Sinclair et al. (2003), most temperate insects in the ‘VATES’) at 25 C and a photoperiod of 16:8 (L:D) h. Only bugs Northern Hemisphere are freeze intolerant. Internal ice formation is from generations F1–F5 were used in these experiments. Bugs were lethal to this group of insects; thus, they are often classified as freeze carefully isolated from colonies in ventilated plastic containers with avoidant (Lee 1991). Hemolymph will instantaneously crystallize an excised collard leaf, and held at ambient temperature (20À25 C) within a range of temperatures; the temperature at which this occurs up to 5 h prior to SCP determination. Eggs laid in these colonies is referred to as the supercooling point (Baust and Rojas 1985). within the previous 24 h were shipped overnight to Geneva, NY 0 00 0 00 Supercooling points vary in insects according to body size, water (42 52 38.40 N, 77 00 26.14 W; elevation ¼ 188 m) where egg content, dissolved solutes such as sugars and ions, and composition clusters and first instars that hatched after arrival were assayed for of amino acid and proteins in the hemolymph (Lee 2010). The su- SCPs. An additional field-collected population in early April 2014 0 00 0 00 percooling point is conventionally measured thermoelectrically to from Painter, VA (37 35 04.61 N, 75 49 15.64 W; elevation ¼ 9 detect the exotherm resulting from the latent heat of fusion. m), was tested with individuals acclimated (25 C and a photoperiod Measuring supercooling points for freeze avoiding insects yields a of 16:8 [L:D] h) for at least 14 d following the same procedures. concrete value for beginning to understand cold hardiness (Carrillo et al. 2004). 2014 Field Survival Elsey (1993), without providing the recorded range, approxi- In late September 2013, wild M. histrionica adults collected on sum- mated a supercooling point of Nezara viridula (L.) (Hemiptera: mer cultivated collards were isolated in aluminum mesh cages used Pentatomidae) at À11 C with thermocouples in direct contact with by Cabrera Walsh et al. (2016) over patches of volunteer cultivated their abdomen, insulated by a polystyrene box and held in a À20 C Brassica species. The habitat also included wild grasses and weeds, reach-in freezer. There was no difference of supercooling points be- which provided overwintering refuges. Bug densities per cage did tween males and females, nor between diapausing and nondiapaus- not exceed 500 individuals per square meter. Three days prior to the ing adults. However, the author did note that N. viridula had a extreme cold episode, caged bugs were recovered and held at much higher SCP than two other pentatomids, M. histrionica at 10 C in an unheated greenhouse for 24 h to segregate active ver- À14.7 C and Euschistus servus (Say) at