ECOLOGY AND MANAGEMENT OF THE WHITE MANGO SCALE, Aulacaspis tubercularis Newstead (HOMOPTERA: DIASPIDIDAE) IN ETHIOPIA

Abstract

Mango is grown in more than 100 countries. However, many Arthropd insect pests attack mango, among which scale insects are the most devastating. Aulacuspis tubercularis Newstead (Homoptera: Diaspididae), commonly known as the white mango scale, is a serious insect pest of mango in many mango-growing countries, including Ethiopia. The ecology, host and cardinal direction preferences, presence of natural enemies (predators and parasitoids), and management aspects, including botanical extracts, are not well studied in Ethiopia. Therefore, the survey was conducted to determine the distribution, incidence, severity status, and knowledge of farmers’ occurrence and management practices of the pest in western, southwestern, northwestern, northeastern, central rift valleys, and eastern Ethiopia. The results of the field survey confirmed that the RAJ Agro Industry Loco mango commercial farm was the 1st locus of A. tubercularis accidental emergence and distributed to the neighboring administrative zones and regions of the country up to maximum air distances of 239, 277, 380, and 436 km in the western, southwestern, eastern and northern directions, respectively. Among the respondents, approximately 63% indicated that different cultural practices, such as tree smoking and pruning of heavily infested branches, are used for management practices. The incidence of A. tubercularis cluster (ATC) formation in the infested zones varied from 60 to 100% and was significantly (P<0.05) higher in the Assosa zone, followed by west Shewa. The lowest incidence was observed in the UAAI mango commercial farm in the East Showa zone, followed by the Bench-Sheko, Mizan Tepi & Kefa zones. The maximum average mean ATCs per leaf were recorded in the Assosa zone (39.24±7.56) and the minimum was recorded in the north Shewa zone (5.06 ± 0.66). The numbers of clusters and adult A. tubercularis females recorded per infested mango leaf showed large differences among the localities, with the maximum being 73 clusters and 821 adult females at the Algalea locality in the Homosha district of the Assosa zone, while the minimum was 5 clusters and 38 adult females at the UAAI locality. The results of the study on ecological aspects, cardinal direction preference and cluster population density confirmed that A. tubercularis were present in all sampled study areas throughout the study period with a fluctuating cluster population density. In all study areas, the highest population density of ATCs was significantly (0<0.05) concentrated on the upper side than on the underside of the infested mango leaves. The abundance and population density of ATCs were significantly higher at Bako than in the rest of the study areas (0<0.05). There was a marked increase in ATC population density from March to mid-June, with the general trend of slight to medium increased precipitation. Conversely, there was a swift decline in the ATC population from scarce to nonexistent from mid-June to September followed by prolonged heavy precipitation. The population fluctuation i.e., increase and decrease of the pest population may concides with the peak flowering and fruiting season so that the scale can exploit the peak time of photosynthesis, which needs future detailed study. The correlation coefficient between the ATC population and some basic weather factors, such as maximum and minimum temperature, rainfall, and relative humidity, showed weak to moderate positive correlations in the four study areas and a weak negative correlation with mean average temperature (TeM) in Nekemte city. The results of the study showed that ATC is distributed in all four cardinal directions with significant differences in population density (0<0.05), where the A. tubrcularis scale insect prefers the southern cardinal direction. Furthermore, the build-up and decline in ATCs were found to be affected by basic climatic factors, temperature and rainfall, and cardinal direction differences, whereas the other contributing factors need to be investigated further. The study on the presence of natural enemies was carried out in three study areas of the western and central rift valleys of Oromia and the Assosa zone of the Benishangul-Gumuze regional state. The specimens of A. tubercularis and associated natural enemies (predators and parasitoids) were collected and identified via DNA sequencing. The DNA sequences of the COI gene of all Ethiopian sampled scales were identical and confirmed as a single haplotype of A. tubercularis. This lack of genetic variation might be expected of an invasive species and suggests that a small number of insects instigated the invasion. Identifying the source xxiii of that invading population would be useful but is beyond the scope of the current study, that would needed a significant collection trip or some other way of acquiring many more samples of A. tubercularis, which in turn, this requires a significant source of funding. There are currently only three COI sequences available for A. tubercularis in public databases: one from the Philippines and two from Egypt. Three Coccinellidae (Coleoptera) predators of A. tubercularis were recovered from the specimens. These included two Coccinellidae, Rhizobius lopanthe (Blaisdell), Platynaspis species, and a third unidentified beetle species from the family Nitidulidae. The DNA sequences of the parasitoid specimens identified two species of Encarsia, E. lounsburyi Berlese & Paoli and E. citrina Craw (Hymenoptera: Aphelinidae). These natural enemies were identified for the first time as resident natural enemies of A. tubercularis in Ethiopia. During specimen collection, the newly associated adults and larvae of the predators were found to be voraciously feeding on live A. tubercularis. The parasitoids were found parasitizing all stages of sessile A. tubercularis. In addition, one newly associated Coccinellidae predator Twice-stabbed lady beetle, Chilocorus stigma, was identified morphologically. The beetle was observed preying voraciously on sessile A. tubecularis. The novel association of resident predators and parasitoid wasps with A. tubercularis has likely developed recently in Ethiopia. A host preference study on A. tubercularis was carried out on nine mango cultivars through 12 successive months at RAILMCF and MARC. The results of the study revealed that the pick maximum mean ATCs were recorded on local cultivars during the month of June at RAILMCF & MARC, with values of 50.97±4.62 and 49.22±5.13, respectively. The minimum mean average ATC aggregation was recorded on the Dodo cultivar (0.47±0.56) at RAILMCF and on the Apple cultivar (0.33±0.48) at MARC during the month of November. At both study sites, the mean annual minimum ATC formation was recorded on the Sabre (2.14±0.41) and Vandyke (2.29±0.33) cultivars, followed by the Dodo (4.26±0.63) and Apple (5.20±1.02) cultivars, which were more resistant to A. tubercularis. Overall, ATC populations on the tested mango cultivars showed statistically significant positive correlations (0<0.01) with Tmin, while all mango cultivars showed moderately positive correlations (p<0.05) with Tmax. RH had slight significant positive correlations, and Rf showed moderately significant positive correlations with ATC populations on all mango cultivars except Van Dyke, which had slight negative correlations. The experiment was carried out to test the efficacy of Azadirachta indica L. (A. Juss) seed powder water extract (SPWE) & leaf powder water extract (LPWE) against A. tubercularis under field conditions at two experimental sites in the Arjo Gudetu and Uke experimental sites of the East Wollega zone. The experiment was laid out in a randomized complete block design in four replications. The amount of neem seed & leaf powder required per liter of water was 5, 10 and 15g, which was for each parts of neem powder. The treatments were applied 3 times at 10-day intervals after complete infestation was observed. The mortality count was recorded 10 days after each of the 1st, 2nd, and 3rd-round treatment applications with different spray concentrations of 0.05, 0.1, and 0.15 mg/ml water. For field application 12 l of each concentrate was applied on one mango tree considered as a plot. The results of the study revealed that aqueous A. indica SPWE at a 0.15 concentration significantly (p<0.05) reduced the population of A. tubercularis at both experimental sites. Male adults and crawlers were more affected than females. A related study was conducted at the same study sites to test the potency of A. indica leaf powder water extract (LPWE) against A. tubercularis under field conditions. The results obtained revealed that water extracts of A. indica LPWE at 0.15 concentration significantly (p<0.05) reduced the population of A. tubercularis at both experimental sites. Crawlers and males were more affected than females. Hence, based on the findings, A. indica aqueous LPWE & SPWE could be useful as botanical insecticides in the management of A. tubercularis on mangoes. As a summary & conclusion, the current study concluded reviled that that A. tubercularis was spreading across all directions of the country from its first locus, present in all surveyed areas abundantly with higher infestation and damage level while temperature, relative humidity and rain fall affected and determine its population dynamics. The identified natural enemies (predators & parasitoids) and resistant cultivars as well as the results of neem botanical extracts are promising findings for planning future management practices by incorporating with IPM activities