P.E. Ragama

In vitro antagonism of endophytic Fusarium oxysporum isolates against the burrowing nematode Radopholus similis

Radopholus similis is one of the key pests of banana worldwide. In this study, nine endophytic Fusarium oxysporum isolates were screened for the production of secondary metabolites antagonistic to R. similis in culture. Undiluted and diluted culture filtrates were tested against motile stages and eggs of R. similis. All isolates tested demonstrated in vitro antagonistic activity, causing paralysis of R. similis motile stages. The percentage of paralysed nematodes increased with increase in the length of exposure time to culture filtrates. After 24 h exposure in culture filtrates up to 100% of the treated nematodes were paralysed compared to 26.5% in the control treatments. Nematode mortality rates after 24 h exposure in culture filtrates ranged from 76.4% to 100%. Paralysis was reversible at lower filtrate concentrations. Radopholus similis males were more sensitive to culture filtrates than females. Culture filtrates of all isolates demonstrated inhibitory effects on hatching of R. similis eggs. The results demonstrate the potential for using endophytic F. oxysporum as biological control agents against R. similis and for toxic derivatives from their secondary metabolism to be used as potential nematicides.

The use of aggregation pheromone to enhance dissemination of Beauveria bassiana for the control of the banana weevil in Uganda

Abstract Candidate strains of Beauveria bassiana were identified for use in integrated pest management of the banana weevil Cosmopolites sordidus. Horizontal field transmission of B. bassiana between banana weevils using different delivery systems, including aggregation pheromones, was investigated. We observed that infected weevils could transmit the fungal pathogen to healthy individuals. Most dead weevils (52%) due to B. bassiana infection were found at the base of banana plants in the leaf sheath or in the soil near banana plants. Significantly more weevils died from the pathogen in plots where B. bassiana was applied in combination with the pheromone than where it was applied alone. Our data demonstrate that C. sordidus aggregation pheromone can be a valuable tool to enhance the dissemination of B. bassiana for the control of C. sordidus.

Host plant odours enhance the responses of adult banana weevil to the synthetic aggregation pheromone Cosmolure

Abstract Attraction of adult banana weevil, Cosmopolites sordidus to volatiles from banana pseudostem tissue and the synthetic pheromone Cosmolure+ presented singly or in combination, was studied in the laboratory and in the field. Olfactometric studies in the laboratory showed that 50 g of fermented banana pseudostem tissue was as attractive as pheromone but more attractive than 50 g of fresh pseudostem tissue. In the laboratory, volatiles from pseudostem tissue had an additive effect on attraction of weevils to the pheromone but the effect was not significant in the field. Field trials, however, showed that attractiveness to weevils was positively correlated with the amount of fermented tissue added to the pheromone. The results indicate that fresh or fermented pseudostem tissue may enhance pheromone trap catches but the effect was not large enough to warrant their deployment for the optimisation of the infochemical-based trapping system for the management of C. sordidus.

The influence of crop management on banana weevil, Cosmopolites sordidus (Coleoptera: Curculionidae) populations and yield of highland cooking banana (cv. Atwalira) in Uganda

A field study was undertaken in Uganda using highland cooking banana (cv. Atwalira) to test the hypothesis that bananas grown under stressed conditions are more susceptible to attack by Cosmopolites sordidus (Germar). Four banana treatments were employed to create different levels of host-plant vitality: (1) high stress: intercrop with finger millet; (2) moderate stress: monoculture without soil amendments; (3) low stress: monoculture with manure; (4) high vigour: monoculture with continuous mulch and manure. Adult C. sordidus were released at the base of banana mats 11 months after planting and populations were monitored for three years using mark and recapture methods. Cosmopolites sordidus density was greatest in the mulched plots which may have reflected increased longevity and/or longer tenure time in moist soils. Lowest C. sordidus numbers were found in intercropped banana. Damage, estimated as percentage corm tissue consumed by larvae, was similar among treatments. However, the total amount of tissue consumed was greater in mulched banana than in other systems. Plants supporting the heaviest levels of C. sordidus damage displayed bunch size reductions of 40-55%. Banana yield losses ranged from 14-20% per plot with similar levels in the intercropped and mulched systems. Yield reductions, reported as t ha-1, were twice as high in the mulched system as in the intercrop. The results from this study indicate that C. sordidus problems are not confined to stressed banana systems or those with low levels of management, but that the weevil can also attain pest status in well-managed and productive banana stands.

Effects of two pheromone trap densities against banana weevil, Cosmopolites sordidus, populations and their impact on plant damage in Uganda

Abstract:  An on‐farm study to evaluate the effect of pheromone trap density on the population of the banana weevil, Cosmopolites sordidus (Germar) (Col., Curculionidae) was conducted in Masaka district, Uganda. The pheromone used was Cosmolure+, a commercially available weevil aggregation pheromone. Forty‐two farms were assigned to one of three treatments: 0, 4 and 8 pheromone traps/ha. Pheromone lures were changed monthly at which time the traps were moved to a different location within the stand. Adult weevil population densities were estimated by using mark and recapture methodology at 0, 6, 12, 18 and 21 months, while damage to the banana corm was assessed at 0, 3, 6, 12, 18 and 21 months since the start of the experiment. Pheromone trap captures were generally low: about 10 weevils per trap per month. There were no significant differences in mean catches of C. sordidus per trap per month except for February 2002 when doubling the pheromone trap density decreased weevil catches. Although not significant, decreased efficiency was also the trend in higher trap densities over all the data sets. Doubling the number of traps increased the number of weevils caught per hectare per month from 0.4 to 0.6%. There was no significant difference in plant damage between the pheromone treatments in low‐ compared with high‐trap densities. There were generally no significant differences in weevil populations and plant damage between pheromone‐treated and control farms. Possible reasons for the low‐trap efficacy in this study are discussed.

Factors influencing pheromone trap effectiveness in attracting the banana weevil, Cosmopolites sordidus

Abstract Studies were conducted in Uganda to evaluate the influence of distance, environmental factors, trap location and trap type on catches of Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae) in pheromone-baited traps. Marked weevils were released at recorded locations within plots. Trap efficiency was low and decreased with distance from the pheromone trap. When weevils were released in close vicinity to the trap, significantly more weevils were recaptured from 0.5 m compared to those recaptured from 4 m within 30 days of trap placement. Relative humidity showed a significant positive relationship to C. sordidus catches, while wind speed, temperature and rainfall had no effect. Covering pheromone-baited traps with banana leaves increased capture rates. In one experiment, more weevils were captured in pheromone-baited traps placed in alleys than at the base of mats, while no differences were found in the second. This study provides information on how various factors influence the effectiveness of pheromone-baited traps that will assist in designing a pheromone-based mass trapping strategy for the control of C. sordidus.

Selection of assessment methods for evaluating banana weevil Cosmopolites sordidus (Coleoptera: Curculionidae) damage on highland cooking banana (Musa spp., genome group AAA-EA).

Cosmopolites sordidus (Germar) is an important pest on bananas and plantains. Population build-up is slow and damage becomes increasingly important in successive crop cycles (ratoons). Yield loss results from plant loss, mat disappearance and reduced bunch size. Damage assessment requires destructive sampling and is most often done on corms of recently harvested plants. A wide range of damage assessment methods exist and there are no agreed protocols. It is critical to know what types of damage best reflect C. sordidus pest status through their relationships with yield loss. Multiple damage assessment parameters (i.e. for the corm periphery, cortex and central cylinder) were employed in two yield loss trials and a cultivar-screening trial in Uganda. Damage to the central cylinder had a greater effect on plant size and yield loss than damage to the cortex or corm periphery. In some cases, a combined assessment of damage to the central cylinder and cortex showed a better relationship with yield loss than an assessment of the central cylinder alone. Correlation, logistic and linear regression analyses showed weak to modest correlations between damage to the corm periphery and damage to the central cylinder. Thus, damage to the corm periphery is not a strong predictor of the more important damage to the central cylinder. Therefore, C. sordidus damage assessment should target the central cylinder and cortex.

Field distribution of banana weevil Cosmopolites sordidus (Germar) adults in cooking banana stands in Uganda

The banana weevil, Cosmopolites sordidus (Germar), is a nocturnal insect that is not readily observed in banana fields. Knowledge on its distribution is essential for the design of management strategies that target the adult stage. Marked weevils were released into seven-year-old banana plots. Ten days later the plots were systematically sampled for adult weevils by first removing the banana leaf mulch and residues, searching subsections of the soil to depths of 3 and 25 cm and, finally, removal and dissection of the banana mats. Of the marked weevils, 62, 56 and 82% respectively, were recovered in three trials. Most weevils were found in close association with either the banana mat (46–66%) or with cut residues lying on the ground between the mats (24–31%). Lesser numbers were found in the leaf mulch and in the top 3 cm of soil away from the mat. The distribution of weevils is likely to be influenced by crop management practices and the availability of residues and by the presence of mulch. Implications for banana weevil IPM are discussed.RésuméLe charançon du bananier, Cosmopolites sordidus (Germar), est un insecte nocturne qui n’est pas facilement observé dans les plantations de bananiers. La connaissance de sa distribution est essentielle pour élaborer des stratégies de lutte contre le stade adulte. Des adultes marqués ont été lâchés dans trois plantations de bananiers âgées de 7 ans. Un échantillonnage systématique des adultes a été réalisé 10 jours plus tard dans les résidus de feuilles au pied des troncs, dans le sol jusqu’à une profondeur de 25 cm ainsi que dans les gaines foliaires; 62,56 et 82% des adultes marqués ont été retrouvés dans les trois essais. La plupart des charançons ont été retrouvés dans la gaine foliaire (46–66%) ou dans les débris de gaines foliaires reposant sur le sol (24–31%). Peu d’adultes ont été retrouvés dans les débris de feuilles ou dans les 3 premiers centimètres de sol. Il apparaît que la distribution des charançons est influencée par les pratiques culturales et la disponibilité en résidus ainsi que par le paillage. Les conséquences de ces observations sur la lutte intégrée contre le charançon du bananier sont discutées.

Response of cassava to nematicide application and plant-parasitic nematode infection in East Africa, with emphasis on root knot nematodes

Abstract Cassava (Manihot esculenta Crantz( is attacked by numerous species of plant-parasitic nematodes, among which, Meloidogyne spp. are the most damaging. However, data on the level of damage occurring under field and farm conditions are largely lacking. To examine the impact, upon cassava production, of plant-parasitic nematodes, field experiments were conducted at four sites in East Africa in Uganda (Serere and Sendusu( and Kenya (Mtwapa and Msabaha( during 2002 and 2003. We focused on Meloidogyne spp. (mainly M. incognita and M. javanica(, attacking two popular cultivars in each country. Nematicide (Nemacur® [fenamiphos]( was used against nematodes in control plots. Meloidogyne spp. were the most common nematodes encountered and caused galling damage at harvest at all sites, with Scutellonema spp. and Pratylenchus spp. being the next most commonly observed. The highest root densities of Meloidogyne spp. were recorded at Serere, while Mtwapa had the highest galling damage and intensity. At harvest, nematicide-treated plots had significantly lower densities of the most commonly occurring nematodes and fewer galls on feeder roots (galling intensity(. However, the root galling index score (damage( was not significantly lower compared with untreated plots. The majority of crop growth and yield variables measured at harvest across experiments were unaffected by nematicide application, but fewer storage roots suffered rot. The overall number of variables was minimized using Principal Components Analysis, which identified three major groups. A bivariate plot of the two larger groups across experiments showed that galling intensity, Pratylenchus spp. and Meloidogyne spp. density were negatively correlated with cassava yield (tonnes/ha( and marketable storage root number. Linear regression analysis showed that Meloidogyne spp. density at harvest was negatively associated with yield (tonnes/ha( for cv. Migyera and cv. SS4 in Uganda, but not for cv. Mtwapa1 or cv. Kibendameno (which was positively correlated( at Kenya sites. A strong negative correlation between marketable tuber number and Meloidogyne spp. for cv. Migyera was observed in Uganda, but not for cv. SS4. Meloidogyne spp. density and total nematode species density were each negatively correlated with non-marketable storage root number on both cultivars in Uganda, but not for either cultivar in Kenya.

Density Effects of the Banana Weevil, Cosmopolites sordidus (Germar), on Its Oviposition Performance and Egg and Larval Survivorship

The influence of banana weevil, Cosmopolites sordidus (Germar)(Coleoptera: Curculionidae), population density on oviposition rates and larval survivorship was studied in the laboratory at the Kawanda Agricultural Research Institute in Uganda. This study was undertaken to elucidate the relationship between density-dependent factors and rates of banana weevil increase in the field. Oviposition substrates (corm pieces) were offered to a range of densities of weevils maintained in buckets. Mean oviposition rates per female at densities of 10, 20 and 40 females per bucket were 29, 37 and 53%, respectively, lower than that at a density of 5 females. Nevertheless, total oviposition for the same groups was 1.4, 2.5 and 3.7 times higher than that of the 5-female group. Providing fresh corms daily instead of every five days did not increase oviposition. Larval survivorship was slightly higher at lower densities of immatures following insertion of different densities of eggs or first instars into banana corms. The results suggest that density-dependent factors can influence oviposition rates of individual weevils and survivorship of immatures, but appear to exert only modest influence in reducing banana weevil population growth. More likely, high mortality of weevil immatures under field conditions and /or higher rates of adult mortality and emigration than previously postulated contribute to the slow population build-up of this pest.RésuméL’influence de la densité de population du charançon du bananier Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae) sur le taux d’oviposition et la survie des larves a été étudiée au laboratoire à l’Institut de Recherche Agricole de Kawanda en Ouganda. Cette étude a été menée afin de comprendre la relation entre les facteurs de densité-dépendance et le taux d’accroissement du charançon du bananier au champ. Les substrats d’oviposition (des morceaux de rhizomes) ont été présentés à différentes densités de charançons maintenus dans des seaux. Les taux moyens d’oviposition par femelle aux densités de 10, 20 et 40 femelles par seau sont respectivement de 29, 37 et 53% inférieurs à celui observé pour une densité de 5 femelles. Cependant, la fécondité totale pour ces mêmes densités est 1,4, 2,5 et 3,7 fois plus élevée qu’à la densité de 5 femelles. La fécondité n’augmente pas si l’on fournit des rhizomes frais chaque jour plutôt que tous les 5 jours. La survie larvaire est sensiblement plus élevée aux faibles densités de jeunes larves juste après l’infestation des rhizomes. Ces résultats suggèrent que les facteurs de densité-dépendance influencent le taux d’oviposition des charançons et la survie des larves, mai ne réduit pas significativement le taux d’accroissement des populations du charançon.Il est vraisemblable que la forte mortalité larvaire en conditions naturelles et/ou le taux de mortalité élevé des adultes et l’émigration que nous avions évoqués contribuent au ralentissement de croissance des populations de ce ravageur.