Koffi Eric Kwadjo, Mamadou Doumbia, Tadashi Ishikawa, Yao Tano & Éric Haubruge

Morphometrical changes and description of eggs of Rhynocoris albopilosus Signoret (Heteroptera: Reduviidae) during their development

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Résumé

L’analyse morphométrique de 78 œufs de Rhynocoris albopilosus a été réalisée au laboratoire (température 28 ± 2°C, photopériode 12:12, humidité relative 65 ± 8%) à l’aide d’une loupe binoculaire avec graduation. Les mesures ont été effectuées chaque deux jours, de la ponte à l’émergence. Les résultats ont révélé que seules les largeurs de l’opercule et maximale connaissent un changement significatif lié au développement de l’embryon. Ces changements variaient entre 0,47 mm et 0,52 mm pour la largeur au niveau de l’opercule et entre 0,59 mm et 0,65 mm pour la largeur maximale. La description de l’embryon à travers le chorion transparent apparaît comme un élément important permettant de prévoir la période d’émergence des nymphes au cours de l’incubation.

Abstract

The morphometric analysis of 78 eggs of Rhynocoris albopilosus Signoret, 1858 was carried out at laboratory (temperature 28 ± 2°C, photoperiod 12:12, relative humidity 65 ± 8%) using a stereoscopic microscope with graduation. Measurements were taken every two days, from laying to emergence. The results revealed that only the operculum and maximal widths show a significant change related to the development of the embryo. These changes were 0.47 to 0.52 mm for the operculum width and 0.59 to 0.65 mm for the maximal width. Because the embryo can be seen and described through the transparent chorion, these results make it possible to predict when nymphs will emerge.

Table of content

1. Introduction

1The heteropteran family Reduviidae comprises essential predators on insect pests of crops, playing a significant role in keeping pest populations in check (George and Ambrose, 2001; Sahayaraj et al., 2003). The interest in Reduviidae as biological control agents has been highlighted by several authors (Odhiambo, 1959; Schaefer, 1988; George and Ambrose, 2001; Sahayaraj and Paulraj, 2001; Grundy and Maelzer, 2002; Claver and Ambrose, 2003; Claver et al., 2003; Sahayaraj et al., 2003; Sahayaraj and Raju, 2003). Since the beginning of the 20^th century mass production of natural enemies has been considered as a means of improving biological control programmes (Van Lenteren, 2008).

2Rhynocoris albopilosus Signoret (Heteroptera: Reduviidae: Harpactorinae) is known as the commune predator of Dysdercus in cotton fields in Centrafrica (Pierrard, 1972). James et al. (2003) have identified it among the major natural enemies of market gardening pests in Benin. In Ivory Coast, R. albopilosus has been observed to attack some insect pests of vegetable crops in crop fields and, therefore, may be considered as an important natural enemy against the pests in keeping their populations low. Accordingly we have tried to establish mass production of R. albopilosus toward their practical application as biological control agents.

3Odhiambo (1959), studying the life history of this species, reported that it deposits eggs in compact masses arranged in four or six long lines along the stem of the plant. No other information is available on the egg of R. albopilosus.

4The knowledge of the life cycle of R. albopilosus, particularly reproduction factors and immature stages description is important for establishing mass production of the predator. This work aims to describe the eggs during their development in order to recognize any morphometrical signs of the timing of their hatching.

5In this article, we examine whether there are morphometrical changes in R. albopilosus eggs from laying to hatching. Moreover, a description of various parts of the embryo, seen through the transparent eggshell, is given.

2. Materials and methods

6Females of R. albopilosus that emerged the same day have been maintained each one with a male in a Petri dish (140 mm of diameter). They were reared under laboratory conditions (temperature 28 ± 2°C, photoperiod 12:12 and relative humidity 65 ± 8%) on Tribolium castaneum (Herbst) larvae. Seventy-eight eggs were collected, separated, and vertically fixed on filter paper at the bottom of a Petri dish (90 mm in diameter) with adhesive tape, in order to maintain the same orientation as when they were oviposited. The eggs were then kept under the same conditions as the females. Every two days after egg-laying, the following measurements were made on each egg: whole length: maximum length from the bottom of the egg to the apex of the operculum; body length: maximum length from the base of the egg to the border of the egg body and operculum; maximal width: greatest width at the egg body level; and operculum width: greatest width at the border of the egg body and operculum. The position of the eyes in formation from the base of the egg was taken into account in measurements from the fourth day after egg-laying. Ventral and dorsal faces of the eggs were defined according to the position of the embryo during their development. The observations were carried out using a stereoscopic microscope provided with graduation (WILD M3). The eggs were studied with an enlargement of 160.

3. Results

7Nine days after laying, 74 eggs hatched. The examination of the four eggs that did not hatch revealed that they did not present any sign of an embryonic development. These eggs were probably not fertilized. Unlike the body length of the egg, which was almost constant, the total length and the opercula and maximal widths, as well as the position of the eyes in formation, varied significantly from the fourth day after the laying (Table 1).

8Description of eggs

9The embryo of Rhynocoris albopilosus was described during the egg development, through the transparent chorion (Figure 1).

10Development as a whole. Opercula white from oviposition to hatching. Egg yellowish to light brown at time of laying, turned into orange or reddish during development. Ventral face darker than dorsal one.

11Day of oviposition to two days after oviposition (Fig. 1A). Egg body yellowish to light brown, without particular markings.

12Three days after oviposition. Egg body ventrally with two small, spherical, symmetrical red spots; spots located at same level at the base of eggs.

13Four days after oviposition (Fig. 1B). Dorsum of egg body weakly but distinctly concave at both sides. In addition to two red spots, apex of chorion and two indistinct antennal articles at level of lower half of chorion redder.

14Five days after oviposition. Almost same as at fourth day after oviposition, but apex of chorion and antennal articles redder.

15Six days after oviposition (Fig. 1C). Red spots still spherical, but truncated at inner margin (toward operculum), with black triangular stain; this truncation giving slight concave to superior margin. One side of triangular stain confused with almost central third of truncated part of red spot; top opposed to that side of triangular stain included in red spot, close to lower edge. Antennae placed side by side in median part of ventral face, extending to base of chorion, then up laterally to almost two-thirds length of chorion; first articles contacted with each other at middle in ventral view, yellowish for the most part, with base and apex reddish; other segments slightly reddish and visible on ventral face in lower half of chorion.

16Seven days after oviposition. Black stain now spherical, occupying about two-thirds above red spots. All antennal articles yellowish with reddish base.

17Eight days after oviposition (Fig. 1D). Chorion with lacy structure at level of dorsal face; egg body dorsally with dark squirmed structure in basal half; squirmed structure with several lobes in lateral view; dorsal face somewhat invaginated, reducing maximal width for certain eggs. Red spots (the developing eyes) reduced to a marginal crescent in ventral view; black strain now round, occupying three-quarters of eye. Antennal articles and legs oxblood apically and yellowish medially, except fourth antennal article red with yellowish apex; fourth antennal articles close together, leaving lower quarter of ventral face of chorion and extend up laterally until a little more than three-quarters of chorion’s length, from base.

18Nine days after oviposition (Fig. 1E). Squirmed structure observed on previous day at dorsal face now darker; in ventral view, segmentation noticeably clear now; abdominal segments also distinctly visible, occupying almost lower half of chorion, with abdominal apex located at base of chorion. Second, third and base of fourth articles of antennae slightly dark; apex of fourth article reddish. Hatching begins at this stage.

4. Discussion

19The egg incubation period observed in Rhynocoris albopilosus was shorter than that obtained by Odhiambo (1959).  

20The change in the whole length of the egg during incubation cannot be accounted for by the change in the operculum height, because the body length remained constant. The deployment and the folding back of the veil present at the operculum level (Cobben and Henstra, 1968) might explain the total length changes. The maximal widths at the fourth day and the eighth day after egg-laying did not differ significantly, perhaps because of the light depression noted on the dorsal face of the eighth-day egg.

21During their work on eggs of Rhodnius proxilus (Reduviidae: Triatominae), Chaves et al. (2003) observed that only the maximal width of eggs varied significantly during their development. However, the eggs of Rhynocoris albopilosus varied not only in the maximal width but in the operculum width as well. The studies of Cobben (1968) showed the great homogeneity of Reduviidae during embryonic development. In general, the germinal band develops from the inferior pole towards the superior one, laterally on the surface of the vitella (yolk). Once the appendices start to develop, the embryo migrates (always on the surface of the vitella) so that the head is in the up position. In our study, the significant increase of the distance between the base of the egg and the position of the eyes in formation, from the fourth day to the eighth day could be explained by the migration of the embryo. Similarly, the head of the embryo moves more and more towards the operculum as the emergence period approaches. At hatching, the nymph pushes the operculum upward (Salkeld, 1972). These movements of the embryo could cause the change of the operculum width.

22There is no moment of rotation of the embryo in Reduvioidea (unlike in the Pentatomomorpha) (Moulet, 2002). This was checked in R. albopilosus; and indeed, the site of the embryo’s eyes on the dorsal face did not change, from their first appearance on the third day until hatching.

23According to Odhiambo (1959), the newly laid eggs of R. albopilosus appear light brown, but later become much darker. The change of the color of eggs during their development is probably related to the development of the embryo and the transparency of the shell. In most Reduviidae the chorion is unicolored and transparent, and through it the color of the vitelline mass can be seen; this may change frequently as maturation proceeds (Villiers, 1948).

24Acknowledgements

25We are grateful to the University Agency for Francophony through its mobility program which enables us to achieve this study. Our cordial thanks also go to Prof. Carl W. Schaefer (University of Connecticut, USA) for his critical reading of this manuscript and his valuable comments. This research was partially supported by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), 18405024, 2007, "Understanding the Interactions between Agricultural Organisms and its Application to Sustainable Pest Management Strategies in Africa".

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