Laboratory Rearing of the Cocoa Aphid Toxoptera Aurantii (Boy) and Screening Cocoa Genotypes for their Resistance to the Aphid

Abstract

A study was conducted to develop a quick and efficient laboratory mass-rearing method for the cocoa aphid Toxoptera aurantii (Boy) and to screen cocoa genotypes for their resistance to the aphid. In a similar experiment, two cocoa genotypes were screened to determine their level of attractiveness to the cocoa capsid Sahlbergella singularis Hagl and the results compared with those from the aphid study. The study formed part of an on-going programme being conducted by the Cocoa Research Institute of Ghana (CRIG) to develop more vigorous, high-yielding cocoa genotypes that are also resistant to diseases and insect pests. To raise adequate numbers of experimental aphids, a number of leguminous and vegetable plants as well as tree crops, including cocoa (Y44), were evaluated for their suitability as rearing materials. Two aphid susceptibility evaluation methods were developed, one involving the use of very young seedlings of open pollinated Amazon cocoa (T85/799) and Trinitario (Y44) origins, and the other involving older seedlings of 25 clonal materials and 25 pair-wise crosses. Four cocoa progenies (Na32 x T60/887, P30 x P 30, ICS39 x Y44 and T79/501 x Pa 150) of different genetic sources were also screened to determine their levels of resistance to T. aurantii. In addition, tender shoots of the T85/799 and Y44 were screened to determine their level of attractiveness to the cocoa capsid Sahlbergella singularis Hagl. using a laboratory “microtest” method and the results compared with findings from the aphid study. None of the leguminous and vegetable plants tested were suitable for rearing T. aurantii. Of the tree crops tested, cocoa (Y44) emerged as the best rearing material (producing 107 aphids/seedling in two weeks), followed by the Citrus varieties Mediterranean sweet (50 aphids/seedling) and Late Valencia (24 aphids/seedling), and Coffea canephora (18 aphids/seedling). Cola nitida was found unsuitable. For both the Y44 and T85/799 young seedlings, aphid multiplication rate was highest on seedlings infested one week after cotyledon opening (179 & 91 aphids/ seedling, respectively) and lowest on those infested three weeks after cotyledon opening (38 & 9 aphids/seedling, respectively). There was positive correlation between the aphid infestation levels and the number of crinkled leaves produced (r =0.94 for Y44; r = 0.93 for T85/799). Thus, the number of crinkled leaves produced after eight weeks were highest on seedlings infested one week after cotyledon opening (4 for Y 44 & 2 for T85/799) and lowest on seedlings infested three weeks after cotyledon opening (one for Y44 and zero for T85/799). The number o f aphids produced after one week and the number of crinkled leaves produced after eight weeks were scaled to provide criteria for determining aphid resistance. For the older cocoa seedlings, aphid multiplication rates differed significantly (P<0.01), with the clonal materials T85/799 x Pound 25 (58 aphids/seedlings) and T60/887 x Pound 7 (52 aphids/seedlings) emerging as the most susceptible whilst P30 (5 aphids/seedling) and T63/971 (7 aphids/seedling) were the least susceptible. With the pairwise crosses, (T85/799 x Pound 7) x (T60/887 x Pound 25) (62 aphids/ seedling) and IMC 85 x IMC 47 (57 aphids/seedling) were the most susceptible while Pound 10 x Pound 15 (4 aphids/seedling) and T60/887 x T63/971 (10 aphids/ seedling) were the least susceptible to aphid infestation. Results from the two evaluation methods (using very young and older seedlings) established differences in susceptibility levels among the four cocoa progenies screened. The Trinitario progeny Y44 x ICS39 was the most susceptible, followed by the Amazon progenies T79/502 x P a l50, Na32 x T60/887 and the Amelonado progeny, P30x P30, in decreasing order. It was concluded that young cocoa seedlings of Trinitario Y 4 4 , not more than one week after cotyledon opening, were the best of all the materials tested for rearing T. aurantii under insectary conditions at 24 } 30C and relative humidity range of 72-85%. The comparable results obtained on the multiplication rate/crinkled leaf formation from the aphid study and the number of capsid lesions recorded in the laboratory microtest screening, strengthen the view by earlier workers that level of aphid resistance/ preference in cocoa types can be used as an index for determining their resistance to or preference by capsids and other sucking insects. It is anticipated that, at least, a few more of the cocoa types screened in the present study for aphid resistance/preference will be used in future studies to further confirm the view that cocoa types susceptible/resistant to 71 aurantii are also susceptible/resistant to capsids