Cosmopolites sordidus (banana weevil)

The infestation by C. sordidus begins at the base of the dying outermost leaf-sheath and in injured tissues at the lower part of the pseudostem. Initially the young larvae make several longitudinal tunnels in the surface tissue until they are able to penetrate to adjacent inner leaf-sheaths; the larvae then bore into the pseudostem base and rhizome (in bananas, also into the base of suckers and into roots). Larval tunnels may run for the entire length of fallen pseudostems (Kalshoven, 1981).Larvae tunnelling in the corm and rootstock do the most harm. Rotting occurs through fungal decay in thoroughly riddled corms which are reduced to a blackened mass of tissue and the leaves die prematurely. The boring, if severe, so weakens the plant that it is easily knocked or blown over. Injury to the corm can interfere with root initiation and sap flow within the plant (Wright, 1976)Infested plants have dull yellow green and floppy foliage. Young infested suckers often wither and fail to develop. In a high wind more than average numbers of plants blow down, at times with severe losses.The young suckers attacked by the borers wither and die very quickly because of larval feeding and tunnelling between the lateral roots and the corm. An indication that a young plant is infested is the withering and drying of the curled roll of unopened leaves or growing part of the plant (Zimmerman, 1968a).The older plants infested by the weevil appear tall and spindly and no doubt succeeded in growing as much as they did due to the presence of numerous lateral roots surrounding the bulbs of the plants and because the attacks of the insects had been gradual (Zimmerman, 1968a).In northern Tanzania, the most obvious symptoms of damage observed were toppling, snapping, splitting, leaning, stunting and reduced fruit size, in that order (Reddy, 1988a). Observed weevil damage was snapping (in 27% of plants) and stunting (in 20%).In the Philippines, the presence of banana weevil infestation is said to be associated with bacterial rhizome head rot, caused by Erwinia carotovora (Juan, 1977), as well as in Honduras (Hord and Flippen, 1953). However, subsequent studies by Hord and Flippen (1956) showed this to be untrue.In Kerala, India in August 1984, larvae of C. sordidus are reported to have damaged the internal tissues of a banana peduncle (Charles et al., 1986), but these larvae could have been those of Odoiporus longicollis.

Cultural Control

Harvesting by cutting pseudostems at random out of the stools results in cut stumps and rhizomes which provide ample material for further infestation. Harvesting of all mature pseudostems at certain intervals, rather than continually, is suggested as a preventive measure of control. This discourages the continuous breeding of the weevil, as then there will be periods in which few young suckers are present. It is also necessary to destroy stumps of wind-damaged plants (Kalshoven, 1981).

The borers' affinity for freshly cut rhizome is used in surveys of adult weevil populations. At Ungoye, Kenya, Reddy (1988a) used split pseudostem traps and disc-on-stump traps, and the weevils were hand collected from them. The disc-on-stump traps were more attractive, with a maximum of 50 adult C. sordidus and 134 adult Temnoschoita nigroplagiata caught per week, as against 42 and 82, respectively, in split-pseudostem traps. Reddy (1989) stated that continuous trapping over one year reduced the population of weevils by 50%. However, trapping is not normally an effective control and has been replaced by insecticides applied to the base of the plant (See also section on Trapping, and Stover and Simmonds, 1987). In Honduras, over one million borers were collected and destroyed during a 2-year experiment but as many borers were collected during the last month as during the first. Thus trapping does not appear to be an effective control.

Planting infected rhizomes increases the damage. It is most important to use vigorous uninfested planting material under good planting conditions. Planting material should be obtained from plantations free of weevils and examined carefully by taking one or two slices from it. If larvae, pupae or tunnels are present, the material must be destroyed (Treverrow, 1983). Peeling the rhizomes free of lesions and immersing in hot water at 54°C for 10 minutes, as recommended for nematode control, is practical and effective (Stover and Simmonds, 1987). Then plant them as soon as possible in a new plantation. They should not be left overnight in heaps in the old plantation, where they could be infected with weevils' eggs.

Froggatt (1928) and Sein (1934) were the first to recommend paring and heat sterilization. Paring allows for superficial inspection of the rhizome surface and rejection of suckers containing weevil damage. However, significant internal damage may be present on suckers in which the rhizome displays little evidence of attack (Ogenga-Latigo and Bakyalire, 1993). Paring and removal of outer leaf sheaths also removes most weevil eggs and nematodes (for example, Radopholus similis and Pratylenchus spp. (Gold et al., 1998 a,b)). Hot water treatment kills remaining eggs and a high percentage of larvae. For example, Gettman et al. (1992) reported greater than 99% mortality of weevil eggs and larvae when suckers of dessert bananas were placed in a water bath of 43°C for 3 h. Immersing suckers in hot water baths of 54°C for 20 min has been demonstrated to be very effective at eliminating nematode species and has been recommended for banana weevil control. Seshu Reddy et al. (1998) have developed a simple method for farmers to control temperature by tying wood to wax with a melting point of 55°C. Banana weevils are attracted to cut rhizomes, so clean planting propagules should be planted before they are reinfested. Clean planting material will be most effective in the planting of new stands. Otherwise, weevils from nearby mats will quickly attack cleaned material.

It is advisable not to replant previously infested areas while old corms remain in the ground. Various methods are used for destruction of the corms and it could take up to 2 years to remove all of them. Maintain a cover crop on the land during this period, to minimize soil erosion. After removing all corms allow 3 months for the infestation to die out, before replanting (Treverrow, 1983).

Weevil numbers may be kept low by digging out and removing old corms, trash and other materials in which weevils may breed. Desucker and remove water suckers regularly and keep the plantation free of weeds at all times. As soon as the bunch is harvested, cut down the spent pseudostem close to the ground. Cut it into lengths of up to 60 cm and split each along its length. These cut lengths act as egg-laying traps to which weevils are attracted for shelter and feed, and to lay eggs. When the eggs hatch, the life cycle cannot continue as the cut pieces dry out and the larvae die from desiccation (Treverrow, 1983). This method of control, called 'count, cut and dry' was also advocated by Peasley and Treverrow (1984).

The impact of predators on larval populations of C. sordidus could be increased by applying deep cuts to the residual stumps directly after the harvest of the bunch, thus enabling the predators to penetrate the stumps earlier and giving them additional breeding sites (Koppenhöfer and Schmutterer, 1993).

Good husbandry practices, such as clean weeding, manuring and mulching produce vigorous banana plants which have improved weevil tolerance (PANS, 1977). The use of clean planting stock, rigid sanitation, strict quarantine measures and meticulous cultural methods are universally advocated to reduce the pest status of C. sordidus. Villagers adopting the traditional method of shifting cultivation generally suffer little loss from this pest.

Biological Control

Possibilities and considerations for classical biological control of banana weevil have been reviewed by Greathead (1986), Greathead et al. (1986), Waterhouse and Norris (1987), Neuenschwander (1988), Kermarrec et al. (1993) and Koppenhofer (1993b,c); Schmitt (1993) provides a partial list of arthropod natural enemies. Approaches include the use of endemics, exotics (classical biological control), secondary host association and microbial control. Based on the weevil's biology, Greathead et al. (1986) gave a ca 30% chance for complete success in biological control programmes. Koppenhöfer (1993a,b) studied endemic, opportunistic predators (histerids, hydrophilids, staphyinids and Dermaptera) of banana weevils in Kenya. Several species lowered weevil numbers in drum experiments.

In Fiji, C. sordidus became a very destructive pest of bananas following its introduction in about 1901 and this led to the first attempt at its biological control. The introduction of Plaesius javanus in 1918 led to its establishment (Veitch, 1926; Bennett et al., 1976). Simmonds (1935) reported a marked reduction in weevil damage, but Pemberton (1954) considered that only partial control had been achieved. It is now known that, in addition to P. javanus, the similar P. laevigatus was also introduced (Walker and Deitz, 1979).

P. javanus was released in Queensland (Australia) from 1921 to 1928 but established only briefly and establishment in New South Wales, Australia during later years failed. However, Wilson (1960) reported that Dactylosternum hydrophiloides, liberated in 1939 became established, but had only a minor controlling influence. P. javanus and P. laevigatus established in the Cook Islands after release from 1937 to 1940 but the banana weevil borer is still an important problem there (Walker and Dietz, 1979). P. javanus established in Chiapas, Mexico and was confirmed in 1973, 1984 and 1992, some 40 years after its release (Barrera and Jimenez-Jimenez, 1994).

According to Waterhouse and Norris (1987), though weevils as a group seem to be poor candidates for biological control, the establishment of P. javanus and P. laevigatus in Fiji appears to have reduced the pest status of C. sordidus there. Introductions of P. javanus have resulted in successful establishment in French Polynesia, Jamaica, Marianas, New Caledonia, Western Samoa and Trinidad, but no information is available on the effects it has produced. Two other predators have been established, one in Australia and two in Jamaica (Dactylosternum hydrophiloides and D. abdominale) (Edwards, 1942) and one (Hololepta quadridentata) in St Vincent (Bennett et al., 1976) but apparently without much effect. Results were uncertain even when predators became established; they are not specific predators on the banana weevil and as weevil tunnels do not extend to the outside of the plant, the efficiency of predators on root borer larvae and pupae is much reduced (Cuillé, 1950).

Although several beetle predators have established successfully in a number of instances, no significant reductions of the pest have been achieved (Koppenhöfer and Schmutterer, 1993). A major reason for some of these failures might have been that the pest's biology was not taken into consideration sufficiently, and neither the biology of the predators, nor their quantitative impact had been studied.

In greenhouse trials, the nematodes Steinernema carpocapsae, S. glaseri and S. bibionis reduced the number of tunnels made by larvae in plantain corms at rates of 400, 4000 and 40,000 nematodes per 4-month-old plant. At the higher two rates, the nematodes caused 100% larval mortality (Figueroa, 1990). Two large-scale field trials in New South Wales, Australia using the nematodes S. carpocapsae All and S. carpocapsae NC513 gave acceptable levels of larval C. sordidus control (Treverrow et al., 1991).

In Cuba, the fungus Beauveria bassiana is believed by the Cubans to be effective against C. sordidus (D. Greathead, Centre for Population Biology, Ascot, UK, personal communication, 1996). According to Koppenhöfer and Schmutterer (1993), fungi have shown useful efficacy as control agents of this pest and might become substitutes for insecticides in the future (Delattre and Jean-Bart, 1978; Castineiras et al., 1991). However, in many tropical countries where bananas are an important staple food, distribution and application of biocontrol agents are still restricted by lack of facilities. They are also too expensive for the resource-limited subsistence farmers who represent the majority of the banana producers. Researchers in Cuba have also reported successful control of banana weevil using the ants Tetramorium guineense and Pheidole megacephala in combination with B. bassiana (Roche, 1975; Roche and Abreu, 1983; Castineiras et al., 1991). Although the ants are generalists, high populations in banana stands make them formidable predators. The ants will enter crop residues and living plants in search of weevil immatures.

The introduction of antagonists in classical biological control and augmentation is more feasible than periodic releases as a sustainable solution to crop pest problems in developing countries (Yaninek and Herren, 1989), although they may not be as effective as pesticides or formulations of pathogens. Therefore in spite of previous failures, further searches for effective natural enemies should be conducted, especially in the Indo-Malaysian region. At the same time, indigenous natural enemies should be studied in areas more recently infested with C. sordidus, since it is possible that they may have escaped attention by occurring only locally, their potential impact may have been hampered by suppression from insecticides or interference with cultural practices, and some might still be adapting to their new host (Neuenschwander, 1988).

Host-Plant Resistance

Fogain and Price (1994) field tested a total of 52 varieties of Musa for C. sordidus damage by assessing the corm for galleries. Of the varieties tested, AAB plantains as a group showed the highest susceptibility, while AAA bananas generally escaped attack. In South Nyanza, Kenya, Speijer et al. (1993) showed that damage caused by C. sordidus was higher on a plantain used for roasting (cv. Gonja) and on an East African Highland cooking banana (cv. Lusumba) than on sweet and multi-purpose cultivars. These two studies highlight the possibility that resistant varieties could be developed. (See also Seshu Reddy and Lubega, 1993; Speijer et al., 1993; Fogain and Price, 1994; Gold et al., 1994; Musabymana, 1995; Ortiz et al., 1995; Pavis and Lemaire, 1996; Abera et al., 1997).

The most important resistance mechanism appears to be antibiosis. In a study comparing six clones (Pisang awak, three highland cooking and two highland brewing banana clones) Abera et al. (1997) found little difference in host-plant attraction (based on trap catches at the base of mats) or acceptance (number of eggs per plant), but found large differences in larval survivorship indices.

According to Waterhouse and Norris (1987), there is some debate about the pest status of the banana weevil. Opinions range from "the most serious pest of the banana" (Purseglove, 1972) and "one of the more important pests of the banana growing regions of Brazil from the economic point of view" (Suplicy Filho and Sampaio, 1982), to "a troublesome pest of neglected plantations" (in Fiji) (Swaine, 1971). In Western Samoa, C. sordidus is a serious localized pest of bananas, but generally remains at a low-to-moderate infestation level (Waterhouse and Norris, 1987). In New South Wales, Australia, the major banana-producing state of Australia, Wallace (1937) concluded that the borer was not a serious pest of normal established plantations, and that its economic importance was often exaggerated; also Braithwaite (1963) concluded that the importance of infestation is aggravated by poor culture. However, it is always likely to be important in areas that experience strong winds. PANS (1977) included C. sordidus amongst the major pests of bananas.According to Stover and Simmonds (1987), except for damage to new plantings, 10 years of observations and experiments in Central America showed that the borer is not as important as assumed in permanent plantations. It appears to prefer rhizomes of harvested plants to healthy rhizomes and this is supported by some reports from Australia (Smith, 1939; Wallace, 1937). The economic threshold of borer populations remains to be determined. In almost all the plantations in Central America, insecticides for borer control have not been applied for many years. In heavily infested plantations, smaller than average bunches of undersize fruit may lower production (PANS, 1977).Larvae tunnelling in the corm and rootstock do the most harm. Weevil damage, expressed as percentage coefficient of infestation, was studied by Reddy (1988a, 1989) in 22 districts in Kenya. Severe damage of >80% was recorded in ten districts. In Tanzania, corm damage ranged from 52-95% among different cultivars. In general, cooking types were found to be more damaged than sweet bananas by the weevils. Apparently, however, percentage coefficient of infestation is also directly related to altitude and plant age.A survey was conducted in the major banana-growing areas of Kagera, Arusha and Kilimanjaro Districts in northern Tanzania to evaluate the interrelationships between the occurrence of insect damage, plant parasitic nematodes and agronomic factors as they relate to the current decline in banana production (Reddy, 1988a). The results showed that this decline in banana production is caused in order of importance by the banana weevil, C. sordidus, the nematodes Pratylenchus goodeyi, Helicotylenchus multicinctus and Radopholus similis, and poor agronomic practices. The high prevalence of toppling, snapping and root necrosis due to weevil and nematode damage was found to be due ultimately to poor agronomic practices, such as inadequate sanitation of planting material, lack of mulching, insufficient propping, severe wind exposure, improper destruction of old pseudostems and low fertilizer inputs.Economic importance depends, in part, upon genome group and management. In Uganda, plantains and highland banana are more susceptible cultivars than Kayinja and Kisubi to attack, while Gros Michel suffered peripheral attack without much penetration in the central cylinder. Gold et al. (1997) found 100-fold differences among weevil densities among farms in a single watershed, suggesting that management may play an important role in determining weevil levels. Analysis suggested that crop sanitation might be the most important factor in determining weevil levels.Although plantain is widely considered susceptible to banana weevil attack, damage levels tend to be low in Ghana. This may reflect the short duration of the plantain stands and the population dynamics of the weevil. In Ghana, farmers normally abandon plantain stands after two or rarely three crop cycles. This suggests that the weevil has a limited time to build up its population, wheras planting material used in new stands has very low levels of weevil eggs and larvae.Sponagel et al. (1995) found that plantain yields in Honduras did not increase (compared to controls) following chemical control of banana weevils. From this, they concluded that banana weevil was not a pest in plantain in Honduras. However, this study followed plantain yields for a single cycle, while the effects of weevil damage may be more severe in subsequent ratoon cycles.In Uganda, yield loss in highland cooking banana to banana weevil increased from 5% in the plant crop to 44% in the third ratoon. This reflected high levels of plant loss and substantial reductions in bunch size. Weevil damage also affected the vigour of followers: mats which sustained heavy weevil attack in preceding cycles had lower yields than mats which had not. Effects of weevil damage on developmental rate and plant size were limited, perhaps reflecting weevil preferences for flowering (and recently flowered) plants (Abera et al., 1999).Between 1970 and 1990, Uganda witnessed the decline and disappearance of highland cooking banana from traditional growing areas in the central region. Farmers cited increases in banana weevil pressure (attributed to reduced management) as a leading cause of this decline. Remaining farms in these sites had weevil levels more than 3 times the national average and at levels for which estimated yield losses might be 20-60%.