Descriptions of the mature larva and pupa of the Scaly strawberry weevil, Sciaphilus asperatus (Bonsdorff, 1785) (Coleoptera, Curculionidae, Entiminae) and observations of its biology

Abstract The mature larva of Sciaphilus asperatus is redescribed and illustrated, and the pupa is described for the first time. Supplements to the identification keys for larvae and pupae of selected Palaearctic Entiminae genera and species are given. Data on the life history, especially oviposition capacity and voltinism, of S. asperatus are provided and discussed, and the number of the six larval instars was confirmed. The economic importance of S. asperatus is briefly highlighted.

Species of Sciaphilus form a rather uniform group characterized in the adult stage by: (1) small body size (< 6 mm); (2) short rostrum with acute carina close to apex; (3) flat eyes; (4) long, slender antennae; (5) rounded elytra densely covered with oblong, erect and spherical adherent scales, the latter forming a more or less contrasting pattern; (6) femora with a conspicuous tooth (Hoffmann 1950;Smreczyński 1966;Freude et al. 1981). Sciaphilus asperatus (Figs 1-3) is a wingless, parthenogenetic, triploid species (Suomalainen 1969;Morris 1997). The adult is a polyphagous feeder on leaves of many herbs, shrubs and trees, mainly in the herb or even in the lower shrub layer, producing more or less characteristic notches on the leaf edge (Fig. 4). In the larval stage it feeds on the roots of plants like strawberry (Fragaria L.), cinquefoil (Potentilla L.), raspberry, blackberry (both Rubus L.), hawthorn (Crataegus L., all Rosaceae), and primrose (Primula L., Primulaceae). In the Berggarten area of Hannover-Herrenhausen it was regularly found in beds of Astilbe Buch.-Ham. ex D. Don, Tiarella L. (both Saxifragaceae), Epimedium L. (Berberidaceae) and small Rhododendron L. species (Ericaceae) (Sprick and Stüben 2012). In Lublin adults of S. asperatus were observed feeding also on Weigela florida (Bunge) DC. (Caprifoliaceae). In the laboratory, adults readily fed on a great number of plant species from more than 15 families (Willis 1964;Dieckmann 1980;Burakowski et al. 1993;Sprick and Stüben 2012), many of which may also be host plants. It is a eurytopic species reported from a large variety of biotopes, preferring rather moist and shady places. It occurs mostly in forests, bushes, fallow grassland and on river banks, but also in cultivations, like tree nurseries, parks and gardens (Koch 1992;Burakowski et al. 1993;Morris 1997;Gosik 2007;Sprick and Stüben 2012).
Biology and life-cycle of Sciaphilus asperatus have been described by Willis (1964), Krause (1978), Dieckmann (1980), and Burakowski et al. (1993). Adults are observed on host plants from mid-April to the beginning of October (Dieckmann 1980). The oviposition period in the field extends from late April to the end of July. Eggs are laid in batches between adjoining surfaces which was stated by Marvaldi (1999a) to be a common oviposition type in many Entiminae genera. Willis (1964) reported egg masses of 6 to 157 eggs, while Dieckmann (1980) recorded ca. 80 eggs (several observations). In the laboratory, we observed masses of 21 and 95 eggs (Fig. 23), deposited in two or three rows and glued with a secretion between layers of filter paper, or between the paper or a leaf placed in the box as food supply and the substrate. In the field, eggs are glued between overlapping leaves, to leaf folds, leaf petioles and stems, usually close to the ground (Willis 1964). Dieckmann (1980) and Willis (1964) reported that per year a single specimen can produce up to 880 or to 1000 eggs, respectively.
The larva develops in spring and summer, and this species usually overwinters in the adult stage (Krause 1978). According to Willis (1964), a small number of newly emerged weevils lay eggs -after a rather long pre-oviposition period of 24-31 days (only 12 days in spring) -also between mid-August and the beginning of September,  which approximates the life-cycle of S. asperatus to that of many other soil-dwelling weevils by the presence of overwintering adults and larvae (see Gosik et al. 2016, Gosik et al. 2019. S. asperatus pupates between June and August; pupation in the field lasts between 14 and 21 days (Willis 1964).
The economic importance of Sciaphilus asperatus is usually low, compared to that of several Otiorhynchus species. Willis (1964) reported only two cases of severe damage in commercial strawberry cultivations in Northern Ireland and concluded: "From the limited data available it appears probable that severe damage to strawberry plants by larvae of S. asperatus occurs infrequently in Northern Ireland and tends to be associated with areas of light, well-drained soil." Alford (1999) also restricted the potential economic importance to strawberries: "Weevils from related genera [other than Otiorhynchus] (e.g. Exomias Bedel, 1883 and Sciaphilus) are also of pest status, e.g. on strawberry." Sprick and Stüben (2012), who studied the soil-dwelling weevil fauna of many tree nurseries, garden centres and parks in Germany, also ranked this species in the category of minor economic importance: "Species that usually rarely cause damage". In a study from North America, S. asperatus comprised nearly 10% of the total larvae in forest soils (ten sites) from the Great Lakes region in Michigan and Wisconsin (Pinski et al. 2005). S. asperatus sometimes forms rather large populations, which is possible, given its parthenogenesis and oviposition capacity, but Morris (1997) stated that this species rarely occurs in large numbers in Great Britain. In a small number of cultivations, mainly of Rosaceae herbs and preferably strawberries, it achieved large numbers and therefore pest status, too.
The morphology of immatures of Sciaphilus asperatus is still incompletely known. A piece of information on this topic is given by Emden (1950Emden ( , 1952: his paper contains descriptions of spiracles and the body shape of first larval instars, as well as diagnostic characters at genus and species level. Moreover, differences between first instar larva and the mature larva are provided. But only diagrams of labrum and epipharynx of the first instar larva are presented. On the other hand, the pupa remained still unknown. On the basis of head measurements, Willis (1964) reported the presence of six larval instars in S. asperatus.
All the larvae and pupae were collected in the field at a site where the life cycle had previously been studied using pitfall traps (see Sprick and Stüben 2012). The field work in the consecutive season was concentrated on obtaining materials for morphological study. The mature larva and pupa were described, whereas the first instar larva was used only for measurement purposes in order to ascertain the number of developmental stages (see for example Gosik 2014 or Gosik et al. 2019). Immature stages were preserved in 75% ethanol and used for measurements and morphological descriptions.
Slide preparation basically followed May (1994). The larvae selected for study under the microscope were cleared in 10% potassium hydroxide (KOH), then rinsed in distilled water and dissected. After clearing, head, mouthparts and body (thoracic and abdominal segments) were separated and mounted on permanent microscope slides in Faure-Berlese fluid (50 g Gum Arabic and 45 g chloral hydrate dissolved in 80 g of distilled water and 60 cm 3 of glycerol) (Hille Ris Lambers 1950). The specimens and slides are deposited in the collections of the Department of Zoology, Maria Curie-Skłodowska University (Lublin, Poland).

Description of the mature larva
All data in [mm], ( n : number of exemplars).
Body (Figs 5-9) slender, slightly curved, rounded in cross section. Prothorax slightly bigger than mesothorax; metathorax as wide as mesothorax. Abdominal segments 1-6 of almost equal length; segments 7-9 tapering gradually to the terminal parts of the body; segment 10 reduced to four anal lobes of unequal size: the biggest dorsal, the smallest ventral, both lateral equal in size. Spiracle of thorax bicameral, and of abdominal segments 1-8 annular. Chaetotaxy well developed, setae capilliform, variable in length, light yellow. Each side of prothorax with nine prns of unequal lengths: one long, three moderately long, five short or minute (seven of them placed on pronotal sclerite, next two close to spiracle); two ps (one long, one medium); and one very short eus. Meso-and metathorax (Fig. 5) on each side with one short prs, four pds, variable in length (pds 1 , pds 3 and pds 4 medium, pds 2 relatively short), one medium as, one medium and one minute ss, one medium eps, one medium ps, and one short eus. Each pedal area of thoracic segments with six pda, variable in length (seta "z" invisible). Abd. 1-7 (Figs 7-9) on each side with one short prs, five pds, variable in length (pds 1 , pds 3 , and pds 5 very long, pds 2 and pds 4 very short) and arranged along the posterior margin of each segment, one minute and one long ss, one minute and one long eps, one minute and one medium ps, one short lsts and two short eus. Abd. 8 (Figs 7-9) on each side with one short prs, three pds, variable in length (pds 1 and pds 3 very long, pds 2 very short) and arranged along the posterior margin of the segment, one minute ss, one minute and one long eps, one minute and one medium ps, one short lsts and two short eus. Abd. 9 (Figs 7-9) on each side with three ds (ds 1 and ds 3 very short, ds 2 long), all located close to the posterior margin of the segment, one short and one long ps and two short sts. Each lateral anal lobe (Abd. 10) with a pair of minute ts.

Description of the pupa
All data in [mm].
Chaetotaxy well developed, setae of various lengths and shapes: on head (except vs), rostrum and mandibular thecae, capilliform, straight; on dorsal parts of thoracic (except ls) and abdominal segments, thorn-like. Setae yellowish to brownish, usually located on visible protuberances. Head capsule and rostrum with one pair of vs, two pairs of sos, os, pas, three pairs of rs, and two pairs of es. Vs thorn-like, medium-sized; all sos, os, pas and rs medium long, straight, equal in length; es and mts straight, very short (Fig. 20). Pronotum with two pairs of as, ls, ds, pls, and three pairs of sls. Only ls and sls 3 thin, capilliform, remaining setae thorn-like, placed on distinct protuberances. Sls 2 and sls 3 growing together on a single protuberance (Fig. 22).
Meso-and metathorax each with five pairs of rather small setae forming a line medially. Abdominal segments 1-7 each with 4 pairs of thorn-like ds (placed along posterior margin), and 2 minute, capilliform ls. Dorsal setae on abdominal segments 1 and 2 small, equal in length, on next segments increasing gradually in size; segment 8 with two pairs of minute, capilliform ls, two minute, capilliform vs, and three pairs of ds: first and second thorn-like, third capilliform, ds 2 and ds 3 growing together on a single protuberance; segment 9 with two pairs of minute, capilliform vs, next two with minute setae on each urogomphus (Fig. 22). Apex of femora with 2 fes; fes 1 long, straight, fes 2 short, thorn-like, both placed on protuberances (Figs 20-22).

Discussion
Sciaphilus asperatus is a common species. Biology and life cycle are in general well known. However, some special aspects of development, such as number of larval instars, voltinism and oviposition capacity have to be discussed herein. Some differences in chaetotaxy between S. asperatus and selected genera from Entiminae are also discussed. Finally, larva and pupa are integrated in current determination keys.  -9, ur -urogomphus, setae: as -apical, d -dorsal, ds -discal, es -epistomal, fes -femoral, l, ls -lateral, mts -mandibular theca, os -orbital, pas -postantennal, pls -posterolateral, rs -rostral, sls -superlateral, sos -superorbital, v -ventral, vs -vertical. Larval instar determination Willis (1964) reported six larval instars, but the diagram on which the results of his measurements are based shows only five. He provides measurement data for 377 larvae. We checked this using the method of Sprick and Gosik (2014) or Gosik et al. (2019): see Tables 1, 2. The data listed in Table 1 show that the mean values are very close for L 1 larvae: 0.277 mm (our data) and 0.233 mm according to Emden (1952). In mature larvae the range is a little larger but also quite close: 1.117 mm (our data) and 1.215 mm (Emden 1952). The HW of six measured pupae lies within this range. These are the best pre-conditions for larval instar determination ( Table 2). The tested Growth Factor (GF) values are around 1.40, as in some other species (see for example Gosik 2014 or Gosik et al. 2019).
From Table 2 it can be inferred that larval growth is rather slow: the best approximation in both cases is achieved with GF values < 1.4: 1.37 -1.38 (1.375) from our own data and 1.39 -1.40 (1.391) from the data of Emden (1952). These values are   (Willis 1964). Hence, these two larvae are not considered typical of this species and will be excluded from the instar determination. Willis' data for mature larvae range between ca. 0.88 mm and 1.35 mm with a maximum at 1.12 mm (71 larvae) according to his Figure 45 and between 1.06 mm and 1.30 mm according to the text (page 97). Italics: calculated values; bold: measurements (except head line). -1) : own data; 2) data of Emden (1952) much smaller than in Tanymecus (Gosik et al. 2019), where GF values ranged between 1.44 and 1.45. Furthermore, it is immediately obvious that Sciaphilus asperatus has six larval instars and that the indefinite larvae of Emden (1952) (Table 1) belong to the 4 th larval instar.

Oviposition capacity
Some data are available regarding the egg-laying capacities of Sciaphilus asperatus. According to Emden (1952), the volume of a female's abdomen is 277 times that of a single egg. In actual fact, however, the available space must be less because of the space requirements of the digestive system, the ovipositor, viscose fluids, bordering structures and others. The highest recorded egg mass was 157 eggs per oviposition event (Willis 1964). Willis (1964) and Dieckmann (1980) respectively reported ca. 880 and 1000 eggs laid by a single female during one season. But these data are from (or probably from) weevils maintained in the laboratory. The data relating to weevils maintained under outdoor conditions by Willis (1964) resulted in lower values of 450 to 700 eggs per female. If the pre-oviposition period lasts around 10 days (see Willis 1964), there could be 12 egg deposition events between mid-April and the end of July. If this is right, 38 to 60 eggs could be laid per oviposition event under outdoor conditions.

Generations and voltinism
According to the data presented by Krause (1978) and Dieckmann (1980), Sciaphilus asperatus should be a univoltine species: overwintered adults produce eggs, larvae hatch in spring and summer, pupation takes place in summer, and adult weevils of the new generation emerge also in summer. But Burakowski et al. (1993) reported that a small part of the new generation lays eggs in August, producing larvae that overwinter. Moreover Willis (1964) presumed that usually all larvae overwinter. This hypothesis has still to be checked. It appears equally possible that larvae from eggs laid early in the season develop in the same year as is true for many other soil-dwelling weevils (see for example Gosik et al. 2016, Gosik and. A species that develops within one season is univoltine, whereas a species that needs longer than one year for its development is semivoltine. Neither definition fits S. asperatus or many other soil-dwelling weevils. Apparently, there is a mix of univoltine summer development, and univoltine or semivoltine (if development of the overwintering larvae should last longer than one year) autumn/spring development; in winter there is not usually any development. It seems these definitions are hard to apply to soil-dwelling weevils, as they do not fit the facts very well.

Remarks on chaetotaxy
There are only several small discrepancies between the description of the mature larva given by Emden (1952) and those presently described: e.g. Emden reported two mandibular setae (one prominent and next very small), whereas we observed only a single seta. It is possible that the second (very small) mds visible on the first instar larva was torn off during intensive feeding of the mature larva. Emden (1952) reported on the presence of seven setae on the pedal area of the prothorax, the seventh a minute seta ("z"), and three further minute setae of each lateral anal lobe. We noticed only six setae on each pedal lobe and only two pairs of minute terminal setae on the lateral lobes of the tenth segment.
It is worth stressing that the presently described mature larva of S. asperatus possesses exactly all essential characters listed by Marvaldi (1998) for Entiminae larvae, Type "A", namely: single as; setae mes 1 close together whereas setae mes 2 placed far one another; mes 2 placed close to ams; sensilla cluster placed between mes 2 ; labral rods curved outwards; premental sclerite trident-shaped, with posterior extension truncate, expanded at apex.

Supplement to the key to selected genera and tribes of Palaearctic Entiminae larvae
Based on Gosik et al. (2016), , , and Gosik et al. (2019): in Graptus, Peritelus, and Sciaphilus the key is based on one species each (G. triguttatus triguttatus, P. sphaeroides, and S. asperatus).
(Previous step as in Gosik  Taking into consideration the shape, number, and distribution of setae, and the general body shape, the pupae of Sciaphilus asperatus and of Exomias pellucidus (Boheman, 1834) are very similar (see Gosik and Sprick 2013). Especially due to hair-like setae on head and rostrum and to thorn-like setae on pronotum and abdomen, which are observed on both species as well as the presence of paired sls growing on single protuberances and the absence of ventral setae on abdominal segments 1-7. This morphological information is coherent with the close systematic position of both species in the tribe Sciaphilini.