insects Brief Report Cannibalism in the Brown Marmorated Stink Bug Halyomorpha halys (Stål) Giulia Papa and Ilaria Negri * Department of Sustainable Crop Production, Università Cattolica del Sacro Cuore Via Emilia Parmense 84, 29122 Piacenza, Italy; [email protected] * Correspondence: [email protected] Received: 7 August 2020; Accepted: 16 September 2020; Published: 19 September 2020 Simple Summary: Overwintering populations of the crop pest Halyomorpha halys exhibit cannibalistic behaviour towards conspecifics. Depletion of metabolic reserves and desiccation occurring in winter can be overcome by intraspecific predation. This behaviour may be facilitated by the aggregation of individuals and the suppression of species-specific signals that prevent predation upon conspecifics. Abstract: The phytophagous brown marmorated stink bug Halyomorpha halys (Stål) is known to exhibit cannibalistic behaviour towards eggs. Here, we provide evidence of cannibalism among overwintering H. halys adults. Since diapausing individuals have high physiological demands for surviving long periods under stressful conditions, including the risk of depletion of metabolic reserves and desiccation, we assumed that nutritional and water requirements can be met by intraspecific predation. The role of aggregative behaviour in promoting cannibalism is also discussed. Given its evolutionary advantage, this trait should be maintained over generations and may be more widespread than previously considered in species that display aggregative behaviour during adverse seasons. Keywords: cannibalism; Halyomorpha halys; Hemiptera; Pentatomidae; overwintering; phytophagous insect; diapause; aggregation 1. Introduction The brown marmorated stink bug Halyomorpha halys (Stål, 1855) (Hemiptera: Pentatomidae) is native to East Asia, but has been spreading in Europe since 2004, first in Switzerland and then in the rest of Europe [1,2]. This species is a significant invasive agricultural pest that damages fruits trees (e.g., apples, peaches, and pears), vegetables, row crops, and ornamental plants [3,4]. H. halys feeds by penetrating a stylet bundle into plant tissues and injecting digestive enzymes to allow sucking of plant fluids, resulting in feeding injuries in the plant tissues, such as deformities, scars, discolouration, and pitting [4]. H. halys overwinter as adults and exhibit aggregative behaviour in thermally buffered microhabitats [5]. Aggregated individuals then enter facultative diapause for several months until specific environmental cues in spring trigger their re-emergence. For example, in northern Italy, adults exit diapause when the maximum temperature exceeds 14 ◦C and the photoperiod has 13 h of light [6]. Adults may become active through winter in response to warm temperatures and a poor nutritional state [7,8]. The metabolic strategies and physiological mechanisms used by H. halys to survive during winter diapause are not fully known [7]. According to previous studies, depletion of energy reserves and desiccation are the main causes of mortality in the overwintering generation, and adequate nutrition and sequestration of energy reserves (e.g., lipids, glycogen, and sugars) in individuals before entering diapause majorly influence diapause success [7–9]. Insects 2020, 11, 643; doi:10.3390/insects11090643 www.mdpi.com/journal/insects Insects 2020, 11, x FOR PEER REVIEW 2 of 7 nutrition and sequestration of energy reserves (e.g., lipids, glycogen, and sugars) in individuals before entering diapause majorly influence diapause success [7–9]. Overwintering populations of herbivorous species displaying aggregative habits usually have few or no sources of food, other than dead conspecifics [10]. Conspecific necrophagy has been documented among overwintering adults, for example, in the boxelder bug Leptocoris trivittatus [10]. InsectsNecrophagy2020, 11, 643does not occur in H. halys, and freshly dead individuals can even repel conspecifics 2[11]. of 7 Here, we provide evidence that supports the hypothesis of cannibalism, i.e., feeding on live conspecifics, among overwintering adults of H. halys. Cannibalism is defined as intraspecific predationOverwintering and has been populations observed ofin herbivorousmany animals, species including displaying protozoa, aggregative insects, birds, habits and usually mammals have few[12]. orCannibalism no sources may of food, play otherdifferent than roles dead in conspecifics the ecology [10of ].a population, Conspecific such necrophagy as stabilising has been the documentedhost–plant/insect among relationship, overwintering regulating adults, population for example, density, in the boxelderand providing bug Leptocoris a selective trivittatus advantage[10]. Necrophagyby removing doesconspecifics not occur that in H.are halys infected, and with freshly pathogens, dead individuals which reduces can even the repel rates conspecifics of parasitism [11 in]. subsequentHere, we generations provide evidence[13]. Cann thatibalistic supports behaviour the hypothesis may be influe of cannibalism,nced by both i.e., density-dependent feeding on live conspecifics,and -independent among factors; overwintering for example, adults developmenta of H. halys. Cannibalisml asynchrony is with defined the ashost intraspecific plant, sex or predation genetic andrelatedness has been of the observed participants, in many and animals, limitations including in the amount protozoa, or quality insects, of birds, food andand mammalsenvironmental [12]. Cannibalismfactors, such as may humidity play diandfferent high rolestemperature in the [13]. ecology of a population, such as stabilising the host–plantAlthough/insect cannibalism relationship, has regulatingbeen observed population in predatory density, insects, and providing it has also a been selective documented advantage in bymany removing phytophagous conspecifics species, that areincluding infected member with pathogens,s of Lepidoptera, which reduces Coleoptera, the rates and of parasitism Hemiptera in subsequent[12,13]. In phytophagous generations [13 hemipter]. Cannibalisticans, cannibalism behaviour has may been be influencedreported in byadults both and density-dependent nymphs which andpredate -independent on eggs, factors;e.g., Parastrachia for example, japonensis developmental, Nezara asynchrony viridula, withand theH. hosthalys plant, [14–17], sex orwhereas genetic relatednesscannibalism ofamong the participants, adults has only and been limitations suggested in the but amount never documented or quality of [16]. food and environmental factors, such as humidity and high temperature [13]. 2. MaterialsAlthough and cannibalism Methods has been observed in predatory insects, it has also been documented in many phytophagous species, including members of Lepidoptera, Coleoptera, and Hemiptera [12,13]. During November 2017, about 400 H. halys adults in diapause at aggregation sites were In phytophagous hemipterans, cannibalism has been reported in adults and nymphs which predate on collected from barns in the Province of Cremona (northern Italy, 45°08′17.3′′ N, 10°26′10.9′′ E). After eggs, e.g., Parastrachia japonensis, Nezara viridula, and H. halys [14–17], whereas cannibalism among removing dead/damaged individuals, 215 live H. halys adults were selected and placed into a single adults has only been suggested but never documented [16]. cage (ca. 40 cm high × 20 cm diameter) filled with cardboard panels. The cage was kept under 2.outdoor Materials conditions, and Methods following the protocol adopted by Costi et al. [6]. The individuals were maintained in diapause without any food or water for about 14 weeks. In mid-February 2018, diapauseDuring was November terminated 2017, by about exposure 400 H. halysto 25adults °C/L16:D8 in diapause to start at aggregation laboratory sitescolonies. were collectedAmong fromindividuals barns in that the had Province just exited of Cremona diapause, (northern we ob Italy,served 45◦ 08one017.3 living00 N, 10adult◦26 010.9being00 E).cannibalised After removing by a deadconspecific/damaged (Figure individuals, 1A; Video 215S1). liveThisH. observation halys adults was were shortly selected followed and by placed two other into occurrences a single cage of (ca. 40 cm high 20 cm diameter) filled with cardboard panels. The cage was kept under outdoor this behaviour (Videos× S2 and S3). conditions,Dead specimens following left the in protocol the overwintering adopted by cage Costi were et al.counted [6]. The and individuals the bodies inspected were maintained using a instereomicroscope diapause without to check any food for the or presence water for of about predation 14 weeks. marks. In Scanning mid-February electron 2018, microscopy diapause (SEM; was terminatedZeiss Gemini by exposureSEM 500) to 25was◦C /usedL16:D8 to to confirm start laboratory the occurrence colonies. Amongof predation individuals marks. that For had SEM just exitedobservations, diapause, naturally we observed dehydrated one living specimens adult being were cannibalised gold-coated by and a conspecific mounted onto (Figure a carbon-coated1A; Video S1). Thisstub. observation was shortly followed by two other occurrences of this behaviour (Videos S2 and S3). Figure 1. Prey and predators of Halyomorpha halys.(. (A) Frame Frame of Video Video S1 showing cannibalism in the metathoracic ventral part of prey. (B) PicturePicture showingshowing predation in dorsaldorsal partpart ofof prey.prey. MovementMovement responses ofof preyprey proddedprodded withwith aa brushbrush demonstrateddemonstrated thatthat