Pyrrhocoris apterus, (Linnaeus, 1758) (Linnaeus, 1758)

Endrestøl, Anders & Roth, Steffen, 2020, The firebug Pyrrhocoris apterus (Linnaeus, 1758) (Hemiptera, Heteroptera) new to the Norwegian fauna - with an explosive expansion in Northern Europe, Norwegian Journal of Entomology 67, pp. 81-90 : 82-86

The records

The species was first observed by M. Grandahl in a garden in a residential area in Sarpsborg, Viken 18. April 2020. Photos where sent to the second author who identified them as P. apterus . Later, the first author went to the locality and confirmed the presence of the species.

Ø (Viken), Sarpsborg : Klokkerskogen (11. 18188, 59.25274), 18.IV.2020, 2 ex., leg. M. Grandahl, det. S. Roth; 23.IV.2020, 11♀♀ (brac.) 12♂♂ (brac.) 2♂♂ (macr.), leg./det. A. Endrestøl, coll. Norwegian institute for nature reseach ( NINA ) / University Museum of Bergen ( UMB); 25.IV.2020, 5 ex. leg./coll. T.J. Olsen; 27.IV.2020, 1♀ (brac.) 1♂ (brac.), leg. M. Grandahl, coll. NINA ; 30.IV.2020, 3♀♀ (brac.) 2♂♂ (brac.) 2♂♂ (macr.), leg. A. Endrestøl, coll. NINA .

From the 34 individuals investigated by the first author, 30 of the individuals were brachypter (brac. – short-winged) and four of the male specimens were macropter (macr. – long-winged). The body size of a pair in copula with an apparent size difference were measured to 7,5 mm (♂) and 10,3 mm (♀) ( Figure 1c ).

The species had never been observed in the area before, and there were several hundred individuals aggregated there at the time of the first observation. The species was observed several times the following days, and the number of individuals seemed to decrease (M. Grandahl pers. com). On the 23. April 2020 there were still probably several hundred individuals present, more scattered and in smaller aggregates ( Figure 1a,b ). The main population seemed to be distributed in an area of about 50 m 2, extending to 120 m 2 when counting single individuals. After some cold days at the end of April, most individuals seemed to have vanished at the 30. April (A. Endrestøl pers. obs.), and some where later seen burrowing themselves down into the sand (M. Grandahl pers. com.).

The population was centred around two trees ( Tilia cordata ) ( Figure 1d ), to a large part on the ground. The ground around the trees was slightly sloped towards southeast, consisting of a lawn with grasses, moss and non-vegetated spots of sand, and some debris of leaves and Tilia seeds. Several other garden plants and bushes were present around the locality, and a linden hedge ( Tilia ) (with hawthorn ( Crataegus ) in between) was surrounding the garden. The hedge was more than 100 m long, but no individuals of P. apterus seemed to be dwelling on or under this hedge. The hedge was presumably planted there in the mid 1970-ies (M. Grandahl pers. com). Several clusters of individuals were found under a bush of Philadelphus coronaries , a few meters from the two Tilia trees.

One record of P. apterus is published on Gbif. org (2020) from Norway (via naturgucker.de) from Innlandet county , Sør-Fron municipality, Gålå 12. July 2018. We expect that to be erroneous (probably Corizus hyoscyami (Linnaeus, 1758)) , as this is more than 900 m a.s.l., and no Tilia can be found in the area. This record is thus not considered. The closest known record from the new Norwegian population is in Munkedal, Sweden, some 85 km southeast of Klokkerskogen.

The species

Pyrrhocoris apterus is a very conspicuous species, strikingly colored in black and red (aposematism) ( Figure 1 ). The coloration is somewhat variable ( Soucha 1993), and several forms are described ( Kerzhner 2001). The species is given as 9,0 – 11,5 mm by Wachmann et al. (2007), and 6,5 –12 mm by Soucha (1993). In Norway it can only be confused with the widely distributed Corizus hyoscyami ( Rhopalidae ), but the markings are different and ocelli are lacking in Pyrrhocoridae ( P. apterus ). In southern Europe P. apterus could easily be confused with a very similar species, Scantius aegyptius (Linnaeus, 1758) ( Pyrrhocoridae ), most easily separated by the color of the abdominal sternites ( Mata et al. 2013). Other members of the genus Pyrrhocoris are in general colored in black or brown (Voigt 2004), except P. sibiricus Kuschakewitsch, 1866 , which is also colored in black and red, but much duller compared to P. apterus . Finally, several species of Lygaeidae from Central Europe are also conspicuously coloured in red and black, but they also have ocelli as opposed to Pyrrhocoridae .

Another strikingly feature with this species is that it aggregates in large groups as a result of aggregation pheromones and contact pheromones ( Wachmann et al. 2007). They also have alert pheromones that results in an alarm response that quickly can dissolve aggregations (Soucha 1993, Wachmann et al. 2007).

The species lives on various Malvaceae such as Tilia , Hibiscus , Malva , Alcea , Althaea and Lavatera (Voigt 2004) . They can also be found on and around other trees, eg. Robinia pseudacacia ( Fabaceae ) ( Wachmann et al. 2007) and even on spruces Picea (Spuris 1995) . In Europe, Tilia is the most common food source and P. apterus therefore tends to cluster under linden trees, which coincides with observations on the Norwegian locality reported here. They are adapted to an extremely dry diet of ripe seeds (Soucha 1993). To some extent they might also be zoophagous, necrophagous and cannibalistic (Soucha 1993, Wachmann et al. 2007). This form of polyphagy is according to Soucha (1993) one of the reasons the species is widespread and expanding its range.

According to Wachmann et al. (2007) the species overwinters as adults under moss, loose bark or leaves. That is in accordance with the observation of adults in mid-April in Norway. The females start to lay eggs in May, and can continue to lay eggs for a longer period (Voigt 2004, Wachmann et al. 2007). One brachypterous female lays about 383 eggs, but with substantial variation (± 208 eggs) ( Soucha 2013). The lifespans and total number of eggs laid by females of the two wing morphs do not differ significantly, even though such a difference is found in several other insect-species with wing dimorphism ( Soucha 2013 and references therein). Eggs are laid in the ground (Voigt 2004), and females makes small pits in the ground where she lays the eggs before covering them (Soucha 1993). The embryonic development lasts 10–14 days at 18–20°C, and the nymphs pass through 5 instars (Soucha 1993). Nymphs can appear in June, with adults appearing in August ( Wachmann et al. 2007). The life cycle in Central Europe takes about 2–3 months, and adults lifespan may vary from two months to a year depending on the conditions (Soucha 1993). In warm summers and mild winters, the species can have an uncompleted second generation with nymphal overwintering ( Wachmann et al. 2007). The chill tolerance for 50 % survival (LT 50) have been estimated to be -15°C/1–2 weeks ( Koštál & Šimek 2000).

P. apterus is a Holarctic species ( Kerzhner 2001). The natural habitat includes Europe, the Near East to Pakistan, in Russia it reaches Central Siberia and the southern states to Kazakhstan and Uzbekistan, as well as SW Mongolia and NW China (Voigt 2004). In Central Europe it can be found up till 1000 m a.s.l. ( Wachmann et al. 2007). It is reported from USA, Central America and India by Barber (1911) and Soucha (1993), but according to Kerzhner (2001) these records were based on occasional import or mislabelling. IThough it is published from Utah, USA ( Hodgson 2008) and Canada ( Oviedo Rojas & Jackson 2018) the last decade. Several observations from Melbourne, Australia (from 2018–2020) are found on Gbif.org (2020). It was published new to Malta in 2019 ( Cassar 2019). In the recent few years, the species has expanded explosively in Northern Europe ( Figure 2 and 3 ).

The name refers to the fact that most individuals of P. apterus are “apter”, wingless. According to Voith (2004), about 90 % of the individuals are apter, 5 % brachypter and 5 % macropter. There is a higher proportion of macropterous males than females ( Wachmann et al. 2007). Tolsgaard (2005) refers to a material collected by Lindgren from Lolland, where <4 % were macropterous. Jaus (1934) described different forms of wing polymorphism on P. apterus and found several different forms, including specimen with fully developed forewings and reduced hindwings, and specimens with asymmetrical wing development. The species thus has a high plasticity of wing morphism (Soucha 1993). In general, wing morphs are either macropterous (fully developed hind- and forewings) or brachypterous (reduced fore wing membrane and reduced hindwings). In macropters, the membrane tends to break off after the gonads have become active ( Honěk 1976). Long photoperiod and high temperature encourage production of macropters ( Honěk 1976). This might explain the recent expansion given that macropterous species really can fly (see discussion below)?

Range expansion

We have combined records from different databases (Gbif.org 2020, Fugleognatur 2020, ArtPortalen 2020, FinBif 2020, Dabas Dati 2020, eElurikkus 2020) to evaluate the increase in records of P. apterus and to do calculations of its expansion rate. All the records of the different databases where combined into a single shapefile and analyzed further in QGIS (v. 3.4.2-Madeira) in the following manner: 1) the vector layer was split by year, 2) the expansions distance in year X was calculated using the function “distance to nearest hub” to year X-1 (or year of previous record), 3) the vector layers X and X-1 was combined to Y, 4) expansion distances for layer in year X+1 were calculated using the nearest hub to layer Y, 5) this was done for the years with records in the period 1829–2020, a total of 50 layers. These layers were later combined again, and expansion distances per year were calculated based on the year of the nearest hub. Since the dataset was combined with several databases, the risk of duplicate records is high. We therefore only allowed for one record on each locality defined to records closer than 1,5 meters. So, if a locality was registered with the exact same coordinate several times through a season or during several years, we reduced this to one record. Records with zero hub distance were thus omitted, including records with hub distances differing less than 1,5 m compared to the next record. A few extreme distances within one year were removed: 1) the first Latvian record in the dataset, with nearest record in Estonia, 2) the first Kaliningrad record, with nearest record in Lithuania, and 3) the first Finnish record, with nearest record in Sweden. Based on the remaining 1929 records we calculated an average expansion rate for P. apterus to be 8397 m per year (±2994 95%CI).

We also compared this method to another method used to calculate expansion rates of alien species in Norway (see e.g. Saether et al. 2010, Sandvik et al. 2013). In this method, the expansion rate is estimated as the assumed invasion front starting from the position of the first observation. In the dataset presented here, that would be Denmark for Northern Europe. We used the same dataset as described above, with unique localities, in the program EXPANSION 1.4 (http://www. evol.no/hanno/12/expans.htm). The speed of the invasion front was obtained using linear regression under the assumption of sampling error and no process variance. With this method we calculated the expansion rate to be 8242 m per year (±1451 95%CI).

Figures

FIGURE 1. a,b. Aggregations of Pyrrhocoris apterus from Sarpsborg, Norway 23. April 2020. c. A pair of P. apterus in copula (♂:7,5 mm, ♀: 10,3 mm). d. The locality of P. apterus from Klokkerskogen, Sarpsborg, where the majority of individuals were observed under the two Tilia trees. e. P. apterus on the trunk of a Tilia tree. Photos: Anders Endrestøl, NINA.

FIGURE 2. Map of Pyrrhocoris apterus (Linnaeus, 1758) in Northern Europe from 1960 to 2020. Records prior to 1960 are not considered. Green dot is the record presented here. Source: Gbif.org (2020), Fugleognatur (2020), ArtPortalen (2020), FinBif (2020), Dabas Dati (2020), eElurikkus (2020).

FIGURE 3. Number of records (one record per locality) per year of Pyrrhocoris apterus (Linnaeus, 1758) in total in Norway, Sweden, Finland, Estonia, Latvia, Lithuania and Kaliningrad.