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Differences in egg morphology of the stored-grain pests Rhyzopertha dominica and Prostephanus truncatus (Coleoptera: Bostrichidae)
ARTICLE IN PRESS
Journal of Stored Products Research 44 (2008) 103–105 www.elsevier.com/locate/jspr
Short communication
Differences in egg morphology of the stored-grain pests Rhyzopertha dominica and Prostephanus truncatus (Coleoptera: Bostrichidae) Zuzana Kucˇerova´, Va´clav Stejskal Department of Stored Product Pest Control, Crop Research Institute, Drnovska´ 507, CZ-161 06 Prague 6, Czech Republic Accepted 14 June 2007
Abstract Differences in the egg morphology of the invasive stored-product pests Prostephanus truncatus and Rhyzopertha dominica (Coleoptera: Bostrichidae) based on optical and scanning electron microscope (SEM) micrographs are presented. Diagnostic characteristics and changes in external egg appearance during development of the embryo are described. Significant differences between the eggs of these two species were found in their shape, size, surface structures, colour changes during development, and the thickness and number of eggshell layers. r 2007 Elsevier Ltd. All rights reserved. Keywords: Egg morphology; Eggshell; SEM micrographs; Pest identification
1. Introduction Rhyzopertha dominica (F.) (the lesser grain borer) and Prostephanus truncatus (Horn) (the larger grain borer) (Coleoptera: Bostrichidae) are primary pests, of quarantine and economic importance, in stored cereals. Rhyzopertha dominica occurs commonly in subtropical and tropical grain stores (Stejskal et al., 2006), but is also able to survive in stores in the temperate zone (e.g. Hagstrum and Flinn, 1994; Stejskal et al., 1999, 2003). Prostephanus truncatus is a species originating from Central America. A few decades ago, this pest invaded Africa and Asia with dramatic consequences for human food security (Markham et al., 1991) and Contessi (1991) has indicated the risk of spread of this pest into Southern Europe. Exact identification of all developmental stages of pests is essential for quarantine standards to be maintained. Surprisingly, the diagnostic traits to distinguish between the eggs of these two pests are not available; we only traced a brief description of R. dominica eggs reported by Le Cato and Flaherty (1974).
Corresponding author. Tel.: +420 233 022 371; fax: +420 233 310 636.
E-mail address: [email protected] (Z. Kucˇerova´). 0022-474X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jspr.2007.06.005
The aim of this paper is to present the morphometry and external egg morphology of P. truncatus and R. dominica based on microscope techniques, to enable identification of both pest species. This work is a part of a comparative morphological study of eggs of stored-product arthropods (Kucˇerova´, 2002; Kucˇerova´ and Stejskal, 2002, 2003) useful for agricultural and food laboratories accredited for stored-product pests diagnostics. 2. Materials and methods Rhyzopertha dominica and P. truncatus were reared at 27 1C and 75% relative humidity on wheat and maize grains (respectively). Isolated females were allowed to oviposit for 2 days in small containers (4 cm deep 3 cm inner diameter) holding five grains. The eggs were collected with a brush, gently cleaned in a drop of distilled water and used for optical microscopic examination and observation of changes in egg appearance during their development (stereomicroscope Nicon SMZ 800, Axioscope Zeiss). The egg measurements included the following characters: length (L), width (W), L/W ratio and thickness of egg shell. Size measurements (n ¼ 30) were taken using a light microscope with an objective micrometre. The eggs and empty
ARTICLE IN PRESS Z. Kucˇerova´, V. Stejskal / Journal of Stored Products Research 44 (2008) 103–105
104 Table 1 Egg size measurements (mm) Pest species
Rhyzopertha dominica Prostephanus truncatus
n
30 30
Table 2 Morphological differences between eggs of R. dominica and P. truncatus Egg size (mean7S.D.) Length (L)
Width (W)
L/W ratio
551748 673732
192710 413721
2.870.3 1.670.1
Egg Shape Colour Distinct surface structures Eggshell thickness (mm) No. of layers in eggshell
R. dominica Nearly oblong White to pink Yes 42 2
P. truncatus Widely ellipsoid White to yellow No o1 1
Fig. 1. Egg of Rhyzopertha dominica (SEM micrographs): (a) overall appearance, (b) surface structures (middle of the egg), (c) detail of surface structure (network of struts with knobs), (d) eggshell after eclosion (two visible layers). Egg of Prostephanus truncatus (SEM micrographs): (e) overall appearance, (f) surface structure, (g) detail of surface structure, (h) eggshell rupture (detail).
ARTICLE IN PRESS Z. Kucˇerova´, V. Stejskal / Journal of Stored Products Research 44 (2008) 103–105
eggshells after eclosion were used for scanning electron microscopy (n ¼ 30). They were placed on stubs covered with double-sided sticky tape and sputter-coated with platinum in a Sputter Coater (model SDC 050). The chorion surface was then studied with a JEOL JSM 6400 scanning electron microscope (SEM) at magnifications of 200 to 30,000 . Egg-shell thickness was measured directly from SEM micrographs. 3. Results and discussion 3.1. Descriptions The dimensions of eggs studied are recorded in Table 1 and morphological differences are summarized in Table 2. 3.2. Rhyzopertha dominica The egg is oblong with round ends. One of the ends is usually a little narrower than the other (Fig. 1a). The tip of anterior end is sometimes elongated into a short projection, but the egg shape is considerably variable. Fresh eggs are opaque, of waxy appearance, and alabaster white in colour; the tip of the anterior end is light brown in colour. The colour of the oldest eggs tends towards pink. The egg surface looks smooth (stereomicroscope), but SEM magnification reveals a distinct granulated microstructure (Fig. 1b and c). The dark rusty tips of the mandibles and the abdominal thorn of the larva are visible through the eggshell at the end of the development. The whole egg-shell has a thickness of about 2.7 mm. SEM micrographs of ruptured eggshell (Fig. 1d) reveal two layers. The outer layer can easily be stripped from the inner layer. Micropyles and aeropyles were not observed on the egg surface. 3.3. Prostephanus truncatus The egg is broadly ellipsoid (Fig. 1e). Fresh eggs are glossy and white. The colour of the oldest eggs tends towards yellow. The black tips of mandibles, the ochre spot on the head capsule and two short chitinized abdominal spines of the larva are visible through the chorion at the anterior end of the egg on completion of development. The chorion surface looks smooth (stereomicroscope). The indistinctive pattern of imprints of the follicular cells (Fig. 1f) and slight unevenness of the surface (Fig. 1g) are visible by SEM magnification, but there are no distinct microstructures as in R. dominica. The whole egg-shell has a thickness of about 0.55 mm (Fig. 1h) and seems to be compact. Neither micropyles nor aeropyles were observed on the surface of the chorion. Significant differences between the eggs of these two species were found in their shape, size, surface structures
105
and also in colour changes during development and in the thickness and number of layers in the egg shells. The openings facilitating sperm penetration (micropyles) and respiration (aeropyles), which are frequently present in the insect chorion (Chapman, 1998) and provide useful taxonomic characters, were not found in the eggs studied. The diagnostic characteristics shown here enable accurate identification of these two bostrichid stored-product pests on the basis of their eggs and also empty egg-shells. This is of importance both for phytoquarantine procedures and for tracing the history of infestation of commodity samples during local or international trade. Acknowledgements We thank to Mrs. J. Hroma´dkova´ (IMCH ASCR) for taking the SEM micrographs and Mrs. Sˇ. Tucˇkova´ for technical assistance. This project was supported by the Ministry of Agriculture of the Czech Republic (Project no. MZe 0002700603). References Chapman, R.F., 1998. The Insects: Structure and Function, fourth ed. Cambridge University Press, Cambridge, UK. Contessi, A., 1991. Prostephanus truncatus Horn, a potential new pest of stored foodstuffs. Informatore Fitopatologico 41, 21–24. Hagstrum, D.W., Flinn, P.W, 1994. Survival of Rhyzopertha dominica (Coleoptera: Bostrichidae) in stored wheat under fall and winter temperature conditions. Environmental Entomology 23, 390–395. Kucˇerova´, Z., 2002. Stored product psocids (Insecta: Psocoptera): external morphology of eggs. European Journal of Entomology 99, 491–503. Kucˇerova´, Z., Stejskal, V., 2002. Comparative egg morphology of silvanid and laemophloeid beetles (Coleoptera) occurring in stored products. Journal of Stored Products Research 38, 219–227. Kucˇerova´, Z., Stejskal, V., 2003. External egg morphology of storedproduct and dust mites (Acarina). In: Credland, P.F., Armitage, D.M., Bell, C.H., Cogan, P.M., Highley, E. (Eds.), Advances in Stored Product Protection, Proceedings of the Eighth International Working Conference on Stored Product Protection, 22–26 July 2002, York, UK. CAB International, Wallingford, UK, pp. 356–359. Le Cato, G.L., Flaherty, B.R., 1974. Description of eggs of selected species of stored-product insects (Coleoptera and Lepidoptera). Journal of the Kansas Entomological Society 47, 308–317. Markham, R.H., Wright, V.F., Rios, I.R.M., 1991. A selective review of research on Prostephanus truncatus (Col.: Bostrichidae) with an annotated and updated bibliography. Ceiba 32, 1–90. Stejskal, V., Zuska, J., Werner, P., Kucˇerova´, Z., 1999. Survival over the winter of Rhyzopertha dominica in hot spots caused by improper grain storage technology: the first record in the Czech Republic. Plant Protection Science 35, 23–25. Stejskal, V., Hubert, J., Kucˇerova´, Z., Munzbergova´, Z., Luka´sˇ , J., Zˇd’a´rkova´, E., 2003. The influence of type of storage on pest infestation of stored grain in the Czech Republic. Plant Soil Environment 49, 55–62. Stejskal, V., Kosina, P., Kanyomeka, L., 2006. Arthropod pests and their natural enemies in stored-crops in northern Namibia. Journal of Pest Science 79, 51–55.
Journal of Stored Products Research 44 (2008) 103–105 www.elsevier.com/locate/jspr
Short communication
Differences in egg morphology of the stored-grain pests Rhyzopertha dominica and Prostephanus truncatus (Coleoptera: Bostrichidae) Zuzana Kucˇerova´, Va´clav Stejskal Department of Stored Product Pest Control, Crop Research Institute, Drnovska´ 507, CZ-161 06 Prague 6, Czech Republic Accepted 14 June 2007
Abstract Differences in the egg morphology of the invasive stored-product pests Prostephanus truncatus and Rhyzopertha dominica (Coleoptera: Bostrichidae) based on optical and scanning electron microscope (SEM) micrographs are presented. Diagnostic characteristics and changes in external egg appearance during development of the embryo are described. Significant differences between the eggs of these two species were found in their shape, size, surface structures, colour changes during development, and the thickness and number of eggshell layers. r 2007 Elsevier Ltd. All rights reserved. Keywords: Egg morphology; Eggshell; SEM micrographs; Pest identification
1. Introduction Rhyzopertha dominica (F.) (the lesser grain borer) and Prostephanus truncatus (Horn) (the larger grain borer) (Coleoptera: Bostrichidae) are primary pests, of quarantine and economic importance, in stored cereals. Rhyzopertha dominica occurs commonly in subtropical and tropical grain stores (Stejskal et al., 2006), but is also able to survive in stores in the temperate zone (e.g. Hagstrum and Flinn, 1994; Stejskal et al., 1999, 2003). Prostephanus truncatus is a species originating from Central America. A few decades ago, this pest invaded Africa and Asia with dramatic consequences for human food security (Markham et al., 1991) and Contessi (1991) has indicated the risk of spread of this pest into Southern Europe. Exact identification of all developmental stages of pests is essential for quarantine standards to be maintained. Surprisingly, the diagnostic traits to distinguish between the eggs of these two pests are not available; we only traced a brief description of R. dominica eggs reported by Le Cato and Flaherty (1974).
Corresponding author. Tel.: +420 233 022 371; fax: +420 233 310 636.
E-mail address: [email protected] (Z. Kucˇerova´). 0022-474X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jspr.2007.06.005
The aim of this paper is to present the morphometry and external egg morphology of P. truncatus and R. dominica based on microscope techniques, to enable identification of both pest species. This work is a part of a comparative morphological study of eggs of stored-product arthropods (Kucˇerova´, 2002; Kucˇerova´ and Stejskal, 2002, 2003) useful for agricultural and food laboratories accredited for stored-product pests diagnostics. 2. Materials and methods Rhyzopertha dominica and P. truncatus were reared at 27 1C and 75% relative humidity on wheat and maize grains (respectively). Isolated females were allowed to oviposit for 2 days in small containers (4 cm deep 3 cm inner diameter) holding five grains. The eggs were collected with a brush, gently cleaned in a drop of distilled water and used for optical microscopic examination and observation of changes in egg appearance during their development (stereomicroscope Nicon SMZ 800, Axioscope Zeiss). The egg measurements included the following characters: length (L), width (W), L/W ratio and thickness of egg shell. Size measurements (n ¼ 30) were taken using a light microscope with an objective micrometre. The eggs and empty
ARTICLE IN PRESS Z. Kucˇerova´, V. Stejskal / Journal of Stored Products Research 44 (2008) 103–105
104 Table 1 Egg size measurements (mm) Pest species
Rhyzopertha dominica Prostephanus truncatus
n
30 30
Table 2 Morphological differences between eggs of R. dominica and P. truncatus Egg size (mean7S.D.) Length (L)
Width (W)
L/W ratio
551748 673732
192710 413721
2.870.3 1.670.1
Egg Shape Colour Distinct surface structures Eggshell thickness (mm) No. of layers in eggshell
R. dominica Nearly oblong White to pink Yes 42 2
P. truncatus Widely ellipsoid White to yellow No o1 1
Fig. 1. Egg of Rhyzopertha dominica (SEM micrographs): (a) overall appearance, (b) surface structures (middle of the egg), (c) detail of surface structure (network of struts with knobs), (d) eggshell after eclosion (two visible layers). Egg of Prostephanus truncatus (SEM micrographs): (e) overall appearance, (f) surface structure, (g) detail of surface structure, (h) eggshell rupture (detail).
ARTICLE IN PRESS Z. Kucˇerova´, V. Stejskal / Journal of Stored Products Research 44 (2008) 103–105
eggshells after eclosion were used for scanning electron microscopy (n ¼ 30). They were placed on stubs covered with double-sided sticky tape and sputter-coated with platinum in a Sputter Coater (model SDC 050). The chorion surface was then studied with a JEOL JSM 6400 scanning electron microscope (SEM) at magnifications of 200 to 30,000 . Egg-shell thickness was measured directly from SEM micrographs. 3. Results and discussion 3.1. Descriptions The dimensions of eggs studied are recorded in Table 1 and morphological differences are summarized in Table 2. 3.2. Rhyzopertha dominica The egg is oblong with round ends. One of the ends is usually a little narrower than the other (Fig. 1a). The tip of anterior end is sometimes elongated into a short projection, but the egg shape is considerably variable. Fresh eggs are opaque, of waxy appearance, and alabaster white in colour; the tip of the anterior end is light brown in colour. The colour of the oldest eggs tends towards pink. The egg surface looks smooth (stereomicroscope), but SEM magnification reveals a distinct granulated microstructure (Fig. 1b and c). The dark rusty tips of the mandibles and the abdominal thorn of the larva are visible through the eggshell at the end of the development. The whole egg-shell has a thickness of about 2.7 mm. SEM micrographs of ruptured eggshell (Fig. 1d) reveal two layers. The outer layer can easily be stripped from the inner layer. Micropyles and aeropyles were not observed on the egg surface. 3.3. Prostephanus truncatus The egg is broadly ellipsoid (Fig. 1e). Fresh eggs are glossy and white. The colour of the oldest eggs tends towards yellow. The black tips of mandibles, the ochre spot on the head capsule and two short chitinized abdominal spines of the larva are visible through the chorion at the anterior end of the egg on completion of development. The chorion surface looks smooth (stereomicroscope). The indistinctive pattern of imprints of the follicular cells (Fig. 1f) and slight unevenness of the surface (Fig. 1g) are visible by SEM magnification, but there are no distinct microstructures as in R. dominica. The whole egg-shell has a thickness of about 0.55 mm (Fig. 1h) and seems to be compact. Neither micropyles nor aeropyles were observed on the surface of the chorion. Significant differences between the eggs of these two species were found in their shape, size, surface structures
105
and also in colour changes during development and in the thickness and number of layers in the egg shells. The openings facilitating sperm penetration (micropyles) and respiration (aeropyles), which are frequently present in the insect chorion (Chapman, 1998) and provide useful taxonomic characters, were not found in the eggs studied. The diagnostic characteristics shown here enable accurate identification of these two bostrichid stored-product pests on the basis of their eggs and also empty egg-shells. This is of importance both for phytoquarantine procedures and for tracing the history of infestation of commodity samples during local or international trade. Acknowledgements We thank to Mrs. J. Hroma´dkova´ (IMCH ASCR) for taking the SEM micrographs and Mrs. Sˇ. Tucˇkova´ for technical assistance. This project was supported by the Ministry of Agriculture of the Czech Republic (Project no. MZe 0002700603). References Chapman, R.F., 1998. The Insects: Structure and Function, fourth ed. Cambridge University Press, Cambridge, UK. Contessi, A., 1991. Prostephanus truncatus Horn, a potential new pest of stored foodstuffs. Informatore Fitopatologico 41, 21–24. Hagstrum, D.W., Flinn, P.W, 1994. Survival of Rhyzopertha dominica (Coleoptera: Bostrichidae) in stored wheat under fall and winter temperature conditions. Environmental Entomology 23, 390–395. Kucˇerova´, Z., 2002. Stored product psocids (Insecta: Psocoptera): external morphology of eggs. European Journal of Entomology 99, 491–503. Kucˇerova´, Z., Stejskal, V., 2002. Comparative egg morphology of silvanid and laemophloeid beetles (Coleoptera) occurring in stored products. Journal of Stored Products Research 38, 219–227. Kucˇerova´, Z., Stejskal, V., 2003. External egg morphology of storedproduct and dust mites (Acarina). In: Credland, P.F., Armitage, D.M., Bell, C.H., Cogan, P.M., Highley, E. (Eds.), Advances in Stored Product Protection, Proceedings of the Eighth International Working Conference on Stored Product Protection, 22–26 July 2002, York, UK. CAB International, Wallingford, UK, pp. 356–359. Le Cato, G.L., Flaherty, B.R., 1974. Description of eggs of selected species of stored-product insects (Coleoptera and Lepidoptera). Journal of the Kansas Entomological Society 47, 308–317. Markham, R.H., Wright, V.F., Rios, I.R.M., 1991. A selective review of research on Prostephanus truncatus (Col.: Bostrichidae) with an annotated and updated bibliography. Ceiba 32, 1–90. Stejskal, V., Zuska, J., Werner, P., Kucˇerova´, Z., 1999. Survival over the winter of Rhyzopertha dominica in hot spots caused by improper grain storage technology: the first record in the Czech Republic. Plant Protection Science 35, 23–25. Stejskal, V., Hubert, J., Kucˇerova´, Z., Munzbergova´, Z., Luka´sˇ , J., Zˇd’a´rkova´, E., 2003. The influence of type of storage on pest infestation of stored grain in the Czech Republic. Plant Soil Environment 49, 55–62. Stejskal, V., Kosina, P., Kanyomeka, L., 2006. Arthropod pests and their natural enemies in stored-crops in northern Namibia. Journal of Pest Science 79, 51–55.