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Bull. ent. Res. 73, 137-151 137 Published 1983 l'TIte assessment of mealybugs (Pseudococcidae) and other Homoptera on mame cocoa trees in Ghana / C. A. M. CAMPBELL * Cocoa Research Institute, P.O. Box 8, New Tafo, Ghqna Abstract The numbers of mealybugs and other Homoptera on 96 coco a trees in Ghana were assessed by examining four branches from each tree. The trees were then felled and examined in detail. Ten species of mealybugs, eight other coccoids, an aphid and a psyIlid were recorded. Ninety-five of the trees were infested with mealybugs, 66, 90 and 69 trees, respectively, with Planococcoides njalensis (Laing), Planococcus citri (Risso) and Phenacoccus htJrgreavesi (Laing). Stictococcus sjostedti CkIl., a species of Gascardia near G. zonata (Newst.) and Mesohomotoma tessmanni (Aulm.) were found on 70, 68 and 48 trees, respectively. The remaining 14 species were found on fewer than 48 trees. All species oíher than Pseudococcus calceolariae (Mask.) and Ferrisia virgata (Ckll.) were most prevalent in the canopy. In the field, mealybug densities recorded on shoots underesti­ mated true leveIs by an average of 95%. Branches were as effective as whole trees for detecting mealybugs, but less effective for other Homoptera. Branches were suitable sample units for estimating population densities of mealybugs, stictococcids and psyIlids. All species of mealybug other than F. virgata infested vegetative buds. An average of 21'7% of buds on the youngest shoots were infested. Samples of buds can be used for detecting mealybug infestations. Introduction Mealybugs (Pseudococcidae) of cocoa are important pests in West Africa because most species are vectors of cocoa swollen shoot virus (CSSV). In the Eastern Region of Ghana, cocoa production has been devastated by swollen shoot disease, and elsewhere large sums have also been spent in eradication measures (Legg, 1979). Studies of factors affecting the abundance of mealybugs on mature cocoa have be en hampered by the lack of an efficient technique for estimating population densities. The first of the two main techniques used on mature cocoa involves examining trees after felling (Strickland, 1951.a; Hanna et al., 1952; Cornwell, 1955, 1957; Entwistle, 1959; Bigger, 1981a). The main limitations are that it precIudes re-sampling, sorne trees of interest may not be available for felling, the environment is altered progressively, and felling is costly in time and labour. These drawbacks were avoided by the visual inspection of trees to either eye or hand height (Donald, 1955; SutherIand, 1955; Entwistle, 1958). However, on comparing estimates of mealybug abundance from felling with those from visual inspection, it was found that most of the mealybug population was in the • Present address: East MaIling Research Station, East Mailing, Maidstone, Kent, UK. 138 C. A. M. CAMPBELL canopy, and because preferences between the main trunk and canopy varíed among species, sorne species were virtualIy undetected by visual inspection (Donald, 1955). Entwistle (1958) also recognised the inadequacy of counts to hand height. Attempts have been made to improve sampling techniques using simple branch sampling or other procedures (Anon., 1950; CornwelI, 1955), but detailed results were not reported. The primary aim of the present study was to improve techniques for estimating densities of the several species of mealybugs on cocoa. CornwelI (1958) demonstrated dispersal of nymphs of Planococcoides njalensis (Laing) between the interlocking canopies of adjacent trees, which probably explains the local spread of swolIen shoot disease around initial foei of virus infection. Strick­ land (1951a) recorded almost 90% of P. njalensis in the canopy, and similarly high per­ centages were found by Donald (1955) and CampbelI (1975). However, Bigger (1972) observed that the proportion in the canopy varied in response to environmental fac­ torso Over the last 30 years at Tafo, the predominance of P. njalensis over other species of mealybugs has declined (Bigger, 1972, 1981h). The limited data available at the start of this study suggested that most of the other species which are now important are also most numerous in the canopy (Donald, 1955). The abundance of P. nj.alensis and sorne other mealybugs is inter-related with that of stictococeid and coccid scales which are also tended by coccidophilic ants for honeydew (Strickland, 1951h; Bigger, 1981a). From 1973 to 1976 Bigger (unpub­ lished) recorded the changing abundance of ants and Coccoidea in a plot of 866 trees. He found that as the boundaries of neighbouring territories of dominant ants altered (Leston, 1970), corresponding changes occurred among honeydew-producing Homop­ tera. Leston (l973) and later Majer (1975) argued that manipulating the ant mosaic to encourage so-caHed beneficial species of ants, notably Oecophylla longinoda (Latreille) and Tetramorium aculeatum (Mayr), neither of which habituaHy tend mealybugs, would result in partial protection from swoHen shoot disease and other maladies. Sampling techniques were needed, therefore, not only for mealybugs, but also for stictococcid and coccid scales and possibly other species of Homoptera of unknown importance in the dynamics of cocoa pests (Entwistle, 1972). Methods Between 1973 and 1977, four trees at each of 24 sites on the Cocoa Research In­ stitute, Tafo, were investigated. Sites 1-4 were in al-ha plot of Amelonado cocoa planted in 1952 and grown under dense mixed forest-tree shade. The remaining si tes were in a 13-ha 3 x 33 shade-fertilizer experiment planted in 1959 with cocoa of open­ pollinated Upper-Amazon parentage. At all sites, trees were being destroyed in attempts to control outbreaks of coco a swoIlen shoot virus disease. The four trees at each site were selected at random from both visibly infected and neighbouring trees. Four fan branches from three heights were cut from each tree using a long-handled pruner. One branch (upper) was emergent aboye the main canopy, two branches (middle) were from opposite sides of the tree at canopy level, and one branch (lower) protruded below the main canopy. The branches were about 25 mm in diameter where severed. Sub-samples were cut from each branch. Up to four growth flushes were cut at the morphological discontinuities left by earlier abscissing terminal bud stipules. Each flush was examined separately, but together they formed the shoot's sub-sample. Flush 1, when present consisted of flaccid distal growth; flush 2 was either hardened distal growth, or, if flush 1 was present, the previous growth increment; and flushes 3 and 4 were successive earlier growth increments. The remainder of the branch formed another sub-sample (the framework), which was cut into approximately 150-mm iengths for handling. AH examinations were made in the laboratory using stereo-microscopes. Records were made of the identity, feeding position and numbers HOMOPTERA OF COCOA IN GHANA 139 of all Homoptera. Additional records were made of the numbers of leaves, shoots, vegetative buds and ftower cushions (inc1uding cherelles) examined. Trees visibly infected with cocoa swollen shoot virus were felled below ground level, and contact trees were coppiced at 0'5 m. The girth of all trees at 0'3 m was recorded. Representative specimens of all ants present on the tree were collected for later identi­ fication. The main branches were then severed from the trunk and separate fractions made of framework and shoots from ftushes 1-4. The fractions were examined in the field using x 10 hand lenses. Records were made of the identity, number and location of all Homoptera, other than aphids and psyllids, which were not counted. Additional records were made of the numbers of shoots examined and the numbers of pods on the trunk and branches. Not more than two trees were examined per day. Results Environmental conditions at the 24 sites were diverse. The four Amelonado sites were aH similar to weIl-maintained farmers' plantations. Ten of the sites planted with Upper-Amazon hybrids were unshaded, eight and two sites, respectively, were under light and heavy shade of regularly spaced trees (Termina/ia ivorensis). Nine of the 14 ant species or genera listed by Leston (1973) as dominants were represented. O. longi­ noda was present at 13 sites, Tetramorium aculeatum at 10, Pheidole spp. and Cremato­ gaster castane,a F. Smith each at nine, C. africana Mayr at four, C. kneri Mayr (representing the C. wellmani ForeI aggregate) and C. stadelmanni Mayr each at two, while Acantholepis capensis Mayr and C. clariventris Mayr were each present at one site. The length of this list indicates that sorne species were at times co-dominant or sub-dominant. C. castanea was found at eight of its sites, and C. kneri at both sites, with O. longinod.a. T. aculeatum and Pheidole spp. were variously found with O. longinoda, C. clariventris and C. africana. Twenty-three trees showed cocoa-swollen shoot virus symptoms. Ants, shade, fertilizers, cocoa variety, season and virus infec­ tion, all inftuence the abundance of mealybugs and possibly other Homoptera (Strick­ land, 1951b; CornweH, 1956; Bigger, 1975, 1981a; Campbell, unpublished). In the present study, these factors were not separable because the investigation was dependent upon the sporadic discovery of virus outbreaks. Twenty species of Homoptera were recorded from the 96 trees examined (Table 1). Ninety-five of the 96 trees examined were infested with mealybugs and 73 with sticto­ coccids. The three most abundant species were from different families: a psyllid, a stictococcid and a mealybug. The most abundant species were not the most widely dispersed, as found by Bigger (1981b). For example, Phenacoccus hargreavesi (Laing) was less than one-twentieth as abundant as Planococcoides njalensis but was present at the same number of sites and on more trees. The most widespread species was Plano­ coccus citri (Risso), which was present at aH sites and was found on 89 trees. Ten species were found at more than half the sites.

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