Anthonomus santacruzi, Hustache, 1924

Gosik, Rafał, Sasa, Archbold & Witkowski, Ed. T. F., 2017, Description of the mature larva and pupa of Anthonomus santacruzi Hustache (Coleoptera, Curculionidae), a biological control agent of Solanum mauritianum Scop. (Solanaceae), and remarks about its biology, Zootaxa 4294 (5), pp. 545-558 : 547-555

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Anthonomus santacruzi


Description of the mature larva of Anthonomus santacruzi

General. Body elongated, slightly curved ( Fig. 2 View FIGURES 1 ‒ 3 ), rounded in cross section.

Colouration. Pale to dark brown head ( Figs 2 View FIGURES 1 ‒ 3 , 10 View FIGURES 10, 11 ). Thorax and abdomen white to whitish yellow ( Fig. 2 View FIGURES 1 ‒ 3 ).

Vestiture. Setae on body thin, yellow or brown, of distinctly different lengths (minute to short or relatively elongate).

Body shape ( Fig. 2 View FIGURES 1 ‒ 3 ). Prothorax smaller than meso- and metathorax (both with similar size). Spiracle bicameral, located on prothorax. Abdominal segments ĪV subequal in length; VĪVIII with length decreasing gradually posteriorly; IX almost hemispherical; X reduced to four anal lobes; dorsal anal lobe narrow; laterals lobes distinct; and ventral lobe very small. Anus terminal. Abdominal ambulatory ampullae bilobate to circular. Abdominal spiracles I–VIII bicameral, placed mediolaterally.

Chaetotaxy. (Number of setae is given for only one side of the body.) Prothorax ( Figs 4, 6, 8 View FIGURES 4 ‒ 9 ) with 9 long and 2 medium prns 1‾11 on a weakly pigmented dorsal sclerite; 2 ps 1-2, and 1 minute eus. Mesothorax ( Figs 4, 6, 8 View FIGURES 4 ‒ 9 ) with 3 long and 2 medium pds 1‾5 (1, 3, and 5 long, 2 and 4 medial); 1 short as; 1 medium eps; 1 long ps and 1 minute eus. Metathorax ( Figs 4, 6, 8 View FIGURES 4 ‒ 9 ) similar to mesothorax. Each thoracic pedal area well separated, and with 2 long pda 1‾2. Abdominal segments ĪVIII ( Figs 4–9 View FIGURES 4 ‒ 9 ) with 1 short prs; 3 long and 2 medium pds 1‾5 (1, 3, and 5 long, 2 and 4 short); 1 long sps; 1 long and 1 minute eps 1‾2; 1 medium ps; and 2 minute eus. Abdominal segment IX ( Figs 4 View FIGURES 4 ‒ 9 ̄9) with 5 ds (1 minute, 2 and 3 medium, 4 and 5 minute), 2 ps 1‾2 (1 long, and 1 minute) and 1 very short sts. Abdominal segment X ( Figs 4 View FIGURES 4 ‒ 9 ̄9) without setae.

Head capsule ( Fig. 10 View FIGURES 10, 11 ). Head hypognathous, suboval; endocarinal line present and reaching behind middle of frons. Frontal sutures distinct, extending to antennae. One stemma (st) present and close to antenna. Dorsal setae: des 1‾3 and des 5 elongate; des 4 and des 6 minute. des 1 and des 2 located near to middle part of epicranium; des 3 and des 4 close to frontal suture, or within it; des 5 anterolaterally; des 6 near to side of epicranium ( Fig. 10 View FIGURES 10, 11 ). Frontal setae: fs 1 and fs 3 minute; fs 2 absent; fs 4 long, located anterolaterally; fs 5 long, located laterally, close to border of frons ( Fig. 10 View FIGURES 10, 11 ). Lateral setae: les 1 very short, les 2 as long as des 5. Vertical seta (ves) very short. Epicranial area with 4 postepicranial setae (pes 1-4) and 3 pores. Antenna with a membranous and slightly convex basal article bearing one arrow-like sensorium; basal membranous article with 7 sensilla of different types: 2 apullacea (sa), 3 styloconium (ss), and 2 basiconium (sb) ( Fig. 11 View FIGURES 10, 11 ).

Mouthparts. Clypeus ( Fig. 12 View FIGURES 12, 13 a) 3.3 times wider than long with 2 very short posterolateral cls, and 1 sensillum (clss) between the cls; anterior margin of clypeus curved. Labrum ( Figs 12 View FIGURES 12, 13 āc) 1.7 times wider than long, with 3 pairs of piliform lms 1‾3; lms 1 and lms 2 with similar length and distinctly exceeds anterior margin of labrum, lms 3 very short. Anterior margin of labrum double sinuate. Epipharynx ( Figs 12 View FIGURES 12, 13 b, c) with 3 finger-like als 1‾3, a pair of pilifrom and relatively elongated ams, 2 relatively short and finger-like mes 1‾2, a pair of sensory pores (snp) outside of labral rods, and a pair of epipharyngeal sensillum clusters (esc). Labral rods (lr) elongate, slightly converging posteriorly, apical part more sclerotized, and basal part slightly enlarged. Mandible ( Fig. 13 View FIGURES 12, 13 ) distinctly broad, bifid, with curved teeth of subequal height; mds piliform, mds 2 two times longer than mds 1 and located laterally. Maxillae ( Figs 14 View FIGURES 14 – 16 ̄16); stipes with 1 stps, 2 pfs, and 1 mbs, with one sensillum (sn) close to mbs; stps and pfs 1–2 relatively long, mbs short; mala with 7 dms 1‾7, all dms piliform, almost equal in length; 3 medium to short vms 1‾3; vms distinctly shorter than dms ( Figs 15 View FIGURES 14 – 16 a, b). Maxillary palpi with two palpomeres: I with 1 short mxps and two sensilla; length ratio of I and II: 1:0.7; II with one sensillum and a group of conical, cuticular apical processes. Prelabium ( Fig. 16 View FIGURES 14 – 16 ) heart-shaped, with 1 relatively long prms; ligula with sinuate margin and 2 short ligs 1-2; premental sclerite well visible, Q-shaped, with distinctly elongated basal parts; labial palpi one segmented, palpomere with one sensillum and short, cuticular apical processes. Postlabium ( Fig. 16 View FIGURES 14 – 16 ) with 3 pms 1‾3; pms 1 located medially, and pms 2 and pms 3 laterally; pms 1 very short, almost conical; pms 3 medium, and pms 2 elongate, distinctly longer than others.

Description of pupae

Measurements (in mm). Body length 3.10–4.50; widest part (at the level of the middle legs) 1.30–2.50 (see Table 1 View TABLE 1 ).

Colouration. Body white or light yellow. Cuticle smooth.

Body shape ( Figs 3 View FIGURES 1 ‒ 3 , 17–19 View FIGURES 17 ‒ 19 ). Body moderately elongate and rather slender. Rostrum very long (5 times longer than wide). Antennae relatively long and slender. Pronotum 1.7 times wider than long. Mesonotum almost as long as metanotum. Abdominal segments ĪVII of equal length; VIII narrow, IX distinctly smaller than others. Urogomphus elongate; straight; with sclerotized apex.

Male and female rostrum of similar length. The shape of the gonothecae is typical for weevil pupae: divided in females ( Fig. 23 View FIGURES 20 ‒ 23 a), undivided in males ( Fig. 23 View FIGURES 20 ‒ 23 b).

Chaetotaxy ( Figs 20 View FIGURES 20 ‒ 23 ̄23). Setae yellow to dark brown; very long to minute; hair-like, slightly curved. Long setae located on conical protuberances; medium and minute setae without protuberances. Head capsule ( Figs 20, 21 View FIGURES 20 ‒ 23 ) with 1 vs, 1 os, and 1 pas. Rostrum with 2 pairs of rs; rs 1 medium, rs 2 short. pas as long as rs 2. Pronotum ( Figs 20, 21 View FIGURES 20 ‒ 23 ) with 2 as 1‾2, 1 sls, 2 ds 1‾2, and 3 pls 1‾3. Meso- and metanotum ( Fig. 22 View FIGURES 20 ‒ 23 ) with 3 long setae placed medially. Femoral setae (fes) absent. Abdominal tergites ĪVIII with 3 very long setae (d 1‾3) and a single lateral seta (l). All abdominal setae located on small conical protuberances. Abdomianl sternites ĪVIII without setae. Abdominal segment IX with 3 pairs of dorsal setae (d); urogomphus with 2 pairs of ventral setae (v).

Addendum to key to pupae of selected Anthonomus species presented by Burke (1968)

The species A. monostigma (described by Chacón-Madrigal et al. 2012), A. vis (by Bená & Vanin 2013), and A. santacruzi (described in the present work) are herein added to the key of Burke (1968).

(previous steps as in Burke’s [1968] original key)

9. Pronotal setae long and very slender...................................................................... 9a

- Pronotal setae short and stout...........................................................................9b

9a. Urogomphus forked at apex................................................... Pseudoanthonomus validus Dietz

- Urogomphus peaked at apex................................................... Anthonomus santacruzi Hustache

9b. Three pairs of basirostral (pas) setae present; urogomphus with slightly bifurcate apex...... Anthonomus albopilosus Dietz

- Basirostral (pas) setae absent; urogomphus with strongly bifurcate apex...................... A. monostigma Champion

(next steps as in original key)

27. sos present; rs absent......................................................................... A. tenuis Fall

- sos absent; two pairs of rs present............................................................... A. vis Clark


Pupae of A. vis (described by Bená & Vanin 2013) are similar to those of A. cycliferus Fall, 1913 , and the larvae of both species are inquilines in galls ( Burke 1968). Chacón-Madrigal et al. (2012) remarked that despite the broad range of plant associations seen among Anthonomini, most known Anthonomus are monophagous species, and host preferences of the larvae are always more restricted than those observed for adults. It is worth stressing that weevil species regarded as polyphagous often establish strictly monophagous local populations even though other potential host plants were reachable (Gosik 2009).

On the other hand, food preferences of adult weevils examined in the laboratory can be quite surprising, e.g., Bagous binodulus Herbst 1795 is regarded as monophagous on Stratiotes aloides L. but preferred Trifolium repens L. and Plantago major L. under laboratory conditions ( Dieckmann 1964).

The number of larval instars and the average length of head capsules estimated by Chacón-Madrigal et al. (2012) on A. monostigma larvae were similar to those observed on A. santacruzi . The total number of three larval instars (three) and the head widths of mature larvae (~0.50 mm) corresponded with the results of our observations. Also, the setal index of both the larvae and the pupae of A. santacruzi agree with those given by May (1993, 1994) as typical for Curculionidae immatures, and to previously described Anthonomus species as well ( Loicáno et al. 2004; Chacón-Madrigal et al. 2012; Bená & Vanin 2013). Based on larval characters of previously described Anthonomus species, A. santacruzi is close to A. rubicosus (especially with respect to head width, length of fs 1 and fs 3, the one-segmented labial palpus, and length of pms).

Remarks on the development of Anthonomus santacruzi

Field observations showed that A. santacruzi has a high fecundity, short generation time, and overlapping generations that facilitate rapid population increases. Similar facts were noted by Olckers (2003), who did the initial biological studies of this weevil. The destructive nature of the larval feeding (within flower buds: Figs 24 View FIGURES 24 ‒ 27 ̄27) prevents fruit set, and can thus reduce S. mauritianum ’s extensive seed production ( Florentine et al. 2003; Olckers 2003; Witkowski & Garner 2008).

The knowledge about an insects biology is fundamental not only for their protection ( Trnka et al. 2015; Shuhrovec & Bogusch 2016) but also for their use in biological control of weeds ( Gosik & Skuhrovec 2011; Münzbergová & Skuhrovec 2013). Potential uses of an insect as a control agent depends on its larval biology ( Skuhrovec et al. 2008; Stejskal et al. 2014; Abela-Hofbauerova et al. 2011; Münzbergová et al. 2015). Effective control agents should cause a reduction in the shoot biomass of infected plants or else inhibit their generative abilities. Hence weevil species with larvae that feed on roots or that develop on flower buds are the most effective control agents ( Rowe & Kok 1985; Steinger & Müller-Schärer 1992; Corn et al. 2006; Koprdova et al. 2015). On the other hand, species with larvae that feed on leaves (for example) usually only cause minor damage to plants ( Stejskal et al. 2014). Moreover, weevils living ectophytically are relatively more vulnerable to predators ( Skuhrovec et al. 2017) and thus less effective as control agents. Moreover, some species (potential biocontrol agents) are in fact less effective control agents because of their high rates of infestation by parasites and parasitoids ( Koprdova & Skuhrovec 2007). However, parasites or parasitoids of the immature stages of A. santacruzi were rarely observed. Less than 5% of A. santacruzi larvae collected during our observations were associated with immatures of an unidentified parasitoid wasp Pteromalidae ( Hymenoptera ). Hence, A. santacruzi appears to be a good example of a species that is well adapted to the role of a biocontrol agent.