African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 4, Num. 3, 1996, pp. 345-350

F.K. Ewete and O.A. Joda

Department of Crop Protection and Environmental Biology, University of Ibadan, Ibadan, Nigeria

(Received 6 August, 1995; accepted 13 June, 1996)

Code Number: CS96074
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The development and damage caused by Riptorus dentipes Fabricius were studied on four varieties of soybean, TGX 536-02D, TGX 849-294D, TGX 996-28E and TGX 923-2E under ambient temperature (24.2-27.8oC) and relative humidity (88-92%). The pre-oviposition period of adult females fed on TGX 536-02D, TGX 996-28E and TGX 849-294D were 5.0, 7.6 and 8.2 days, respectively, while adults fed on TGX 923-2E died within a week. Oviposition period was relatively shorter on TGX 849-294D than on TGX 536-02D and TGX 996-28E. Significantly higher number of eggs, 381.9, was laid by mated females which fed on fresh maturing pods (R-7 stage) of TGX 536-02D compared to the lowest number of eggs, 142.1 and 162.8 on TGX 849-294D and TGX 996-28E, respectively. There was no development beyond the second instar nymph when reared on TGX 923-2E suggesting a high degree of antibiosis. A population of two R. dentipes per plant significantly caused higher seed damage, 66.3% and 82.5%, on TGX 849-294D and TGX 536-02D, respectively, compared to 29.9% on TGX 996-28E. A population of eight R. dentipes per plant caused 99-100% seed damage on TGX 536-02D and TGX 849-294D but 90.5% on TGX 996.28E.

Key Words: Damage, development, Glycine max, Riptorus dentipes

resume

Le developpement et degat causes par Riptorus dentipes Fabricus ont ete examines sur quatre varietes de graines de soja, TGX 536-02D, TGX 849-294D, TGX996-28E et TGX-2E sous la temperature (24.2-27.8¡C) et l'humidite relative (entre 88 et 92%). La periode de pre-oviposition les femelles adultes se nourrissent de TGX 923-2E. Le nombre sensiblement eleve des oeufs, 381.9, (ont ete) pondus par les femelles qui se nourrissent des gousses fraiches en maturite (R7T stade) du TGX BG-536, comparees en mombre limite du nombre inferieur, 1,421 et 162.8 sur TGX 996-28E respectivement. Il n'y a pas eu de developpement au delˆ du instar nymp quond TGX 849-294, TXG 993-6 suggrent un haut degre d'antibiosis. Une population de deux R. dentipes par plante qui causent sensiblement un grand degat de la semince, dont 66.3% et 82.5%, sur les TGX 849-294D et TGX 536-02D respectivement comparees ˆ 29.9% sur TGX 996-28E. Une population de 8 R. dentipes par plante a causee entre 99 et 100% de degats sur TGX 536-02D et TGX 849-294D mais 90.5% sur TGX 996.28E.

Mots Cles: Degat, developpement, Glycine max, Riptorus dentipes

INTRODUCTION

Soybean, Glycine max (L.) Merrill, is an important oil seed crop grown primarily in the warm temperate regions of the world. It is gradually assuming great importance in the tropics and sub-tropics where it has recently been introduced because of its high protein content and its utilisation to supplement the much needed dietary protein in human and animal feeds. The greatest constraints to its production is the hemipteran pod pests in two families, Pentatomidae and Alydidae which damage the seeds (Singh and Taylor, 1978; Singh and van Emden, 1979; Jackai and Singh, 1987; Jackai et al., 1990). The green stink bug, Nezara viridula L. (Hemiptera: Pentatomidae) is widespread on soybean in the areas where it is cultivated (Thomas et al., 1974; Todd and Turnipseed, 1974; Ezueh and Dina, 1978; Dumzo-Ajufo, 1984). Similarly, Robertson (1969) and Dumzo-Ajufo (1984) reported two alydids, Riptorus dentipes Fabricius and Mirperus jaculus Thunberg on soybean in Tanzania and Nigeria, respectively. Although the biology and pest status of N. viridula has been studied on soybean (Schalk and Ferry, 1982), such information is lacking on R. dentipes infesting soybean. This study reports the development and damage caused by R. dentipes on four soybean varieties.

MATERIAL AND METHODS

Four varieties of soybean; TGX 536-02D, TGX 849-294D, TGX 923-2E and TGX 996-28E were planted in each of 16 plots at the roof-top garden of the Department of Crop Protection and Environmental Biology, University of Ibadan, Nigeria. This was to provide a regular supply of fresh maturing pods for the developmental study of the insect. Stock cultures of field-collected R. dentipes adults were maintained in plastic cages measuring 14.0 x 9.0 x 6.0 cm in the laboratory under ambient temperature (24.2-27.8 C) and relative humidity (88-92%). The plastic cages had a hole bored on each of the four sides, three of which were covered with nylon mesh for aeration. A glass vial filled with water and stoppered with absorbent cotton wool was inserted through the fourth hole for supplying water to the insects.

Mating, oviposition and lifespan. A pair of teneral adults was placed in 10 replications in sealed plastic cages on each food source. The food sources were excised fresh maturing pods in the R-7 stage (Fehr and Caviness, 1977) of the four varieties of soybean. Daily observations were recorded on mating, pre-oviposition and ovipostion periods and number of eggs laid by mated females fed on each soybean variety. The insects that died were replaced with teneral adults of the appropriate sex in order to ensure pairing for mated sexes. Eggs collected daily from the cages were incubated on filter paper inside petri dishes. Observations were also recorded on the lifespan of 10 unmated males and females reared on the same food source.

Nymphal development. Sixty neonate first instar nymphs in four replications of 15 each were reared singly on excised fresh maturing pods (R-7 stage) of each of the four varietes of soybean. The main treatments were the four food sources arranged in a randomised complete block design. Daily records were taken of moulting, nymphal survival and developmental periods until the adults emerged. The cages were regularly cleaned and replenished with the appropriate food source. The data obtained were subjected to analysis of variance and the treatment means compared by the Least Significant Difference test.

Damage caused by the insect. The three soybean varieties on which the insect completed its development were each grown in 12 plastic pots at the roof-top garden of the Department of Crop Protection and Environmental Biology, University of Ibadan. A completely randomised design experiment was set up for each variety of soybean with four population levels: 0 (control) 2, 4 and 8 adults of R. dentipes in four replications. This population was based on a previous study with another pod sucking bug, Clavigralla (synonym, Acanthomia) tomentosicollis Stal on cowpea (Jackai et al., 1989). The insects were caged at the R-5 stage (beginning seed) of soybean development (Fehr et al., 1971; Fehr and Caviness, 1977). A fine netting muslin cloth was used for the 45 x 14 cm sleeve cages. The loose ends were properly tied with twine to the base of the stem. The plants remained infested until R-8 stage (full maturity). The dead insects were replaced with those of the same sex and age. The pods were harvested, sun-dired and assessed for insect damage. The indices of damage were unfilled pods and wrinkled seeds. The data were subjected to analysis of variance and the treatment means compared by the Least Significant Difference test.

RESULTS

Oviposition and lifespan. The pre-oviposition periods were 5.0, 7.6 and 8.2 days on soybean varieties TGX 536-02D, TGX 996-28E and TGX 849-294D, respectively (Table 1). A longer oviposition period was recorded and a significantly higher number of eggs (F=110.3; P < 0.05) was laid by mated females when fed on soybean variety TGX 536-02D than on the other two varieties (Table 1). Mated females fed on TGX 923-2E did not oviposit and later died. There was no significant difference in the lifespan of mated and unmated females of R. dentipes fed on the three soybean varieties but the mated males had significantly shorter lifespan (F=85.8; P < 0.05) than unmated males (Table 2).

Table 1

Table 2

Table 3

Table 4

Table 5

Damage caused by the insect. Artificial infestation of soybean with different populations of R. dentipes caused varying degrees of shrivelling of soybean seed in relation to insect density. While two bugs per plant significantly caused 82.5% and 66.3% seed damage (F=83.22;P<0.05 and F=93.63; P<0.05) on TGX 536-02D and TGN 849-294D, respectively, the same bug population caused no significant damage on TGX 996-28E (Table 5). Significant seed damage (F=17.82; P<0.05) was recorded at four and eight bugs per plant in this latter variety. Generally, a population of four R. dentipes per plant significantly caused between 80.0% and 98.5% seed damage across the three soybean varieties (Table 5).

DISCUSSION

Iin this study, the longer oviposition period and the significantly higher number of eggs laid by R. dentipes when fed on TGX 536-02D soybean suggest that it is the most suitable and potentially the most susceptible of the four varieties tested. Riptortus dentupes successfully completed its development (nymph to adult) on fresh maturing pods (R-7 stage) of the three soybean varieties viz: TGX 536-02D, TGX 849-294D and TGX 996-28E but the variety TGX 923-2E did not support its develpment beyond the second instar. This is an indication of some degree of antibiosis probably due to some chemicals in the seeds of TGX 923-2E. Akob (1990) reported similar observation on the pentatomid bug, Aspavia armigera Fabricius, when fed on the same variety of soybean. Slansky and Panizzi (1987) had reported that differences in the suitability of the pods and seeds of different plant species to support growth, development and reproduction of seed-sucking insects could be associated with difference in nutrients, allelo-chemicals and physical factors. The presence of various allelochemical such as alkaloids, saponins and protease inhibitors in Leguminoseae has been reported by Weder (1981). These chemicals generally form the basis of resistance or susceptibility of crop varieties to insect pests (Painter, 1951; Beck, 1965; NAS, 1969).

The significantly high nymphal survival of the bug on TGX 536-02D further indicates that it is the most suitable of the three food sources and potentially the most susceptible. The varying degrees of nymphal survival on TGX 996-28E and TGX 849-294D suggest different levels of antibiosis.

Our study also shows that the severity of seed damage is closely related to insect density. Similar reports have been given on another sucking bug, N. viridula, which was artificially caged on soybean at varying infestation levels (Todd and Turnipseed, 1974; Yeargan, 1977; Mcpherson et al., 1979). A population of two to eight bugs per plant caused significant damage on two soybean varieties TGX 536-02D and TGX 849-294D, when compared to the non-infested check. However, there was no significant seed damage between the non-infested check and the lowest bug density in TGX 996-28E whereas four and eight R. dentipes caused significant seed damage. This clearly suggests a varietal response to seed damage at varying densities of the bug and hence a justification for economic thresholds to be determined for each variety and the applications of chemical control measures accordingly.

ACKNOWLEDGMENT

The authors are grateful to the International Institute of Tropical Agriculture (IITA), Ibadan, for the supply of soybean seed.

REFERENCES

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Copyright 1996 The African Crop Science Society

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