The yellow pecan aphid, Monelliopsis pecanis Bissel, is a major pest on pecan nut trees in the eastern parts of South Africa. This aphid is indigenous to the USA and can cause reduced tree vitality and reduction of nut size and mass on infested trees.
The parasitoid, Trioxys pallidus (Halidae) (Hym.: Aphididae) has been introduced from California, USA, for the biological control of the yellow pecan aphid. During the summer of 1992/93, about 3000 parasitoids were released in five infested Pecan orchards. Small numbers of T. pallidus adults were collected the following summer but no further adults nor mummies were subsequently observed. Whether establishment took place is thus uncertain.
Aphelinus perpallidus Gahan (Hym.: Aphelinidae) was introduced from Byron (Georgia, USA) during the middle of 1995. Rearing is in progress and releases will probably be made shortly.
In South Africa there are about 2100 ha of pecans which produce approximately100 metric tons of shelled nuts.
The yellow pecan aphid, Monelliopsis pecanis Bissel, was first noticed in South Africa during 1976 and has been a major pest of pecans in the eastern parts of South Africa as from 1977 (Milne, 1977). M. pecanis is indigenous to the USA (Wood & Tedders, 1985) and can cause reduced tree vitality and nut size and mass on infested pecan trees (Tedders & Wood, 1984).
Monelliopsis pecanis typically occurs on the abaxial side of leaves. In South Africa, where the pecan tree sheds its leaves in winter, the yellow pecan aphid may be present on the leaves from October to April. This pest overwinters as eggs which are laid in cracks and crevices in pecan branches. These eggs hatch in spring to form winged females which give birth to nymphs. The nymphs also develop into winged females and the reproduction continues until late autumn when wingless females and winged males are produced. These mate and the females lay overwintering eggs (Tedders, 1978).
For many years (De Villiers, 1990) chemical control of the yellow pecan aphid was the only option in South Africa. However, the possibility of biological control of this pest has been apparent to us. During the latter part of 1991, the author had the opportunity to search for the natural enemies of this pest in California, USA.
Trioxys pallidus (Haliday) was introduced into California from France and Iran for the control of the walnut aphid, Chromaphis juglandicola (Kaltenbach) (Van den Bosch et al., 1962). According to Mizell & Tedders (1990), this is at present one of the major parasitoids of the yellow pecan aphid in the USA.
The aim of this study was to collect, import, rear and establish T. pallidus for the biological control of the yellow pecan aphid in South Africa.
Field collections of parasitoids made on pecan and hazelnut trees at Albany and Berkely (California, USA), were sent to South Africa and were placed in quarantine. Rearing of T. pallidus was done in wooden cages 350 x 350 mm x 470 mm high with glass on one side and organza on the other. The organza at the bottom of the cage was shaped like a funnel to allow the epigeal parts of a pecan seedling to be placed inside the cage. After about 3 months, progeny of the parasitoid were released from quarantine and brought to Nelspruit where rearing was continued.
To ensure the availability of leaves on pecan seedlings throughout the winter, 700seeds were planted at more or less monthly intervals and were kept in a glasshouse at 25±7 °C. In addition, 30 seedlings were defoliated once a month and were placed in the dark at 8 °C for 22 days (Neethling, 1993). They were then transferred to a room with a continuous light source at 28 °C.
Pecan aphids were reared on pecan seedlings without cages in an insectary at 25±2 °C with a 14 10 h light/dark cycle. Ten seedlings each with about 500 pecan aphids at all stages, were taken to a second insectary with the same temperature and light/dark cycles. The epigeal parts of these seedlings were placed into each of 10 wooden cages. Between 10 to 30 adult T. pallidus were placed in each cage. The parasitoids were allowed to parasitise the aphid nymphs. Adult parasitoids from successive generations were caught from these cages to reinfest new cages or to use for orchard releases.
Releases were made in five pecan orchards from December 1992 to April 1993.
One or two of the rearing cages containing parasitises nymphs, mummies and adult parasitoids, were opened in each of the orchards by removing the organza top and allowing adults to escape. In orchards where the brown house ant, Pheidole spp., was a problem, the ant bait, hydramethylnon, was used to reduce their numbers. The cages were left in the orchards for approximately 4 weeks.
Recapturing T. pallidus was done from the middle to the end of December 1993 by searching for T. pallidus adults and mummies in the same orchards where releases were made.
Aphelinus perpallidus Gahan (Hym.: Aphelinidae) was introduced from Byron (Georgia, USA) during the middle of 1995. Rearing is in progress in quarantine in Pretoria (South Africa).
The progeny of T. pallidus collected at Albany and Berkely (California) were successfully reared both in quarantine in Pretoria and also at Nelspruit. At a temperature of 25±2 °C the life cycle of T. pallidus took about 2 weeks. Under these conditions the parasitoids and their progeny were able to remain in the same cage for about a month to 6 weeks before the aphids became too few for maximum breeding. A total of 450 pecan seedlings were used for rearing an estimated 30 000 aphids which produced about 5000 parasitoids. Of these a total of about 3000 were released.
During December 1993, small numbers of T. pallidus adults were collected in two of the orchards where releases were made but no aphid mummies were found. No T. pallidus adults or aphid mummies were found the following season.
Aphelinus perpallidus is being reared in quarantine. This parasitoid will soon be taken to Nelspruit where rearing will continue. It will probably be released near the end of the year.
Trioxys pallidus was released in five orchards during the summer and autumn of 1992/93 and was collected again in two of the orchards during December 1993.
As no T. pallidus or aphid mummies were observed the following year, it is uncertain whether establishment took place.
The author is indebted to Mr J. Andrews and Dr. W.L. Tedders for collecting and sending T. pallidus and A. perpallidus to South Africa. To the ITSC, Mr E.W. Driemeyer, Mr H. Blight, Mr J. Lubbe and H.L. Hall & Sons for providing orchards where releases could be made. Also to Mrs M. Maritz and Mrs C. Neethling for rearing T. pallidus and growing pecan trees.
de Villiers, E.A. 1990. The yellow pecan aphid and how to control it. Subtropica 11 (12) : 20 - 22.
Milne, L. 1977. Yellow pecan aphid appears again. ( In Afrikaans). Citrus and Subtropical Fruit Research Institute Information Bulletin 56 : 12 - 13.
Mizell, R.F. & Tedders, W.L. 1990. Pests of fruits and nuts. In: Classical biological control in the Southern United States. D.H. Habeck, F.D. Bennett & J.H. Frank (Eds.). USA Southern Cooperative Series Bulletin 355 : 111 -112.
Neethling Charmaine 1993. Manipulating to force pecan seedlings out of their resting period for the rearing of yellow pecan aphids. (In Afrikaans with English abstract). Subtropica 14 (4) : 16 - 18.
Tedders, W.L. 1977. Trioxys pallidus and Trioxys complanatus as parasites of Monellia costalis, Monelliopsis nigropuncta and Tinocallis caryaefoliae. Annuals of the Entomological Society of America 70 : 687 - 690.
Tedders, W.L. 1978. Important biological and morphological characteristics of the foliar feeding aphids on pecan. USA Department of Agriculture, Technical Bulletin 1579 : 29 pp.
Tedders, W.L. & Wood, B.W. 1984. The effect of yellow aphids on the fruit and foliage of mature "Stuart" pecans. Proceedings of the 77th annual convention of the Southeastern Pecan Growers Association. Charleston, South Carolina. 11 -13 March, p. 127 - 135.
van den Bosch, R., Schlinger, E.I. & Hagen, K.S. 1962 Initial field observations in California on Trioxys pallidus (Haliday) a recently introduced parasite of the walnut aphid. Journal of Economic Entomology 55 : 857 -862.
Wood, B.W. & Tedders, W.L. 1985. Estimates of energy drain by three pecan aphid species and their influence on leaf net photosynthesis. In: Aphids and phylloxeras of pecan. W.W. Neel, W.L. Tedders & J.D. Dutcher (Eds.). Georgia Agricultural Experiment Station College of Agriculture, Special publication 38 : 45 -50.