Effects of crop sanitation on banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae), populations and crop damage in Uganda M. Masanza1, C.S. Gold1*, A. van Huis2 & P.E. Ragama1 1International Institute of Tropical Agriculture, P.O. Box 7878, Kampala, Uganda 2Laboratory of Entomology, Wageningen University, P.O. Box 8031, Wageningen, 6700 EH, the Netherlands Crop sanitation, i.e. destruction of crop residues, has been hypothesized to lower banana weevil damage by removing adult refuges and breeding sites. Although it has been widely recommended to farmers, limited data are available to demonstrate the efficacy of this method. The effects of crop sanitation on banana weevil populations and damage were studied in an on-station trial in Uganda. Treatments included low, moderate and high levels of sanitation. Banana weevil populations, estimated by trapping and mark and recapture methods, were lowest in the high sanitation treatment. However, banana weevil damage was either not significantly different among treatments or lower in low sanitation treatments. Similarly, increases in crop sanitation level were not reflected in higher yields. The data from this trial suggest that crop sanitation is not an effective means of managing banana weevil and contrasts with results from an on-farm study in which sanitation reduced both weevil numbers and damage. Possible factors explaining the different outcomes of the two studies are discussed. Key words: banana weevil, Cosmopolites sordidus, crop residues, crop sanitation, highland banana. INTRODUCTION Highland cooking banana is an important food and impedes water and nutrient uptake. Damage and cash crop in Uganda. Most highland banana is results in plant loss through snapping and top- grown in small plots by resource-poor farmers pling and premature mortality before bunches are with limited inputs. As a perennial, highland developed, reduced bunch weights, mat disap- banana is important in reducing soil erosion on pearance and shortened plantation life (Gold et al. slopes. Well-maintained stands can last 50 or more 2001). Yield losses of up to 100 % have been reported years. In recent years, serious yield declines in (Sengooba 1986). The insect is disseminated traditional banana-growing areas in central through infested planting material. Population Uganda have led to the rapid disappearance of build-up is slow and banana weevil damage and banana in this region (Gold et al. 1993, 1999a). Early associated yield losses are most pronounced in stages of yield decline have also been reported ratoon crops (Rukazambuga et al. 1998). in commercial banana production zones in the The adult weevil lives up to four years. It remains country’s southwest (Gold et al. 1993, 1999b). The reproductive throughout its lifetime, although it decline in yield was attributed, in part, to the produces only a few eggs per week (Abera et al. banana weevil Cosmopolites sordidus (Germar) 1999). The adult is free-living and most often (Coleoptera: Curculionidae) (Gold et al. 1999a). In found in or near banana plants and crop residues central Uganda, farmers attributed high levels of (Gold et al. 2004). Treverrow & Bedding (1993) re- banana weevil to abandonment of crop sanitation ported crop residues to host 60 % of banana weevil practices, i.e. removal or destruction of crop adults in Australia. In Uganda, 25–32 % of adults residues, as these were seen by many as a breeding were found in cut residues, while 10–12 % were site for this insect. associated with standing stumps (Gold et al. 2004). Banana weevils oviposit in the base of the plant. Abera et al. (1999) found that 33 % of banana wee- The larvae tunnel mostly in the corm but will also vil oviposition on highland cooking banana was in attack the true stem and crop residues. Banana the stumps, while Vilardebo (1960) reported that weevil damage weakens the stability of the mat infestation of the dessert variety, Gros Michel (AAA), in Ecuador occurred mostly in the residues. *To whom correspondence should be addressed. E-mail: [email protected] Gold & Bagabe (1997) observed limited infesta- African Entomology 14(2): 267–275 (2006) 268 African Entomology Vol. 14, No. 2, 2006 tion on recently harvested plants of the resistant The soil is an isohyperthermic Kandiudalfic Eutro- cultivar, Kisubi (Ney poovan subgroup, AB), but dox (USDA taxonomy) with high water holding that infestation increased over time. Recapture of capacity, medium acidity (pH 5.9–6.3), low organic marked weevils showed considerable movement matter content and nitrogen and phosphorus defi- between adjacent stands of Kisubi and the suscep- ciency (McIntyre et al. 2001; O. Semalulu, pers. tible highland banana cultivar, Kibuzi. Thus, pop- comm.). ulation build-up in a ‘resistant’ Kisubi plantation Four strips of land, one hectare each, were used may have contributed to banana weevil damage in in the study. Two strips had been in fallow for two the highland banana stand. Similarly, no larvae years, following earlier plantings of maize and were found in recently harvested Pisang awak beans. The other two strips, 200 m distant, had (AB) plants in Sumatra, Indonesia, but high infes- supported a banana germplasm collection for tations were observed on cut residues left pros- 10 years (1989–1998). The plants were uprooted trate in the field (C.S. Gold & A. Hasyim, pers. several months prior to our field trial and, based obs.). Antibiosis has been reported as the major on observations of fresh damage in crop residues, means of resistance to banana weevil (Kiggundu appeared to have a small residual banana weevil 2000; Gold et al. 2001). These data suggest that population at the time of ploughing. antibiotic mechanisms may break down after harvest and that cut residues may suffer higher Experimental design attack because of increased exposure of the corm The experiment had two sets of variables: sanita- and true stem. tion level and plant stress. The original design had Crop sanitation has been widely recommended six treatments: (1) low sanitation, low plant stress; as a measure for managing banana weevil in (2) moderate sanitation, low plant stress; (3) high Uganda. Indeed, in a survey of banana farms sanitation, low plant stress; (4) low sanitation; in Ntungamo district, Uganda, crop sanitation high plant stress; (5); moderate sanitation, high appeared to be the agronomic factor most strongly plant stress; (6) high sanitation, high plant stress. related to reduced banana weevil damage (Gold These were laid in a randomized block design with et al. 1999b). However, survey data for banana- four replications. based cropping systems are rarely conclusive be- Low levels of sanitation involved leaving both cause of the complexity of banana stands includ- the residual pseudostems and corms in the field; ing differences in topography, soils, intercrops, pseudostems of harvested plants were cut 1 m banana cultivars and agronomic practices. There- above the collar, the cut portion placed prostrate fore further studies are needed to determine the on the ground. Moderate sanitation involved role of crop sanitation on banana weevil popula- cutting the pseudostem 1 m above the collar and tions and damage. Crop residues could reduce shredding the cut portion so that it would dry out banana weevil damage by acting as a ‘trap crop’ rapidly. High sanitation involved digging stumps for ovipositing females and drawing them away from the soil and chopping both residual pseudo- from standing plants; and by providing a habitat stems and corms, denying ovipositing banana for natural enemies, especially arthropod preda- weevils access; moderate sanitation prevented tors that thrive in banana debris. access only to residual pseudostems; while low The primary objective of this research was to sanitation did not prevent access to pseudostem study the effects of crop sanitation on banana and corm residues. In all treatments, the corms weevil population levels and damage to growing were left in situ, but under high sanitation, the plants through a controlled on-station study. corms were covered with 3–5 cm of soil. Sanitation treatments were implemented with harvest of first MATERIAL AND METHODS bunches in the first crop cycle, April 2000, and continued until the end of the trial. A field trial was conducted at the Kawanda High plant stress was created by infesting the Agricultural Research Institute (KARI) (0o25’N plots with the banana nematodes, Heliocotylenchus 32°32’E, 1190 m a.s.l.), 13 km north of Kampala, multicinctus (Cobb) Golden and Radopholus similis Uganda, with 12-hour day length throughout the (Cobb) Thorne. Subsequent monitoring of root year, a mean annual precipitation of 1190 mm, and necrosis levels on flowered plants showed that an average daily temperature range of 16 to 29 °C. nematode infestations were low and not signifi- Masanza et al.: Effects of crop sanitation on banana weevil populations in Uganda 269 cantly different from those in non-innoculated of Kampala) were used as a source of innoculum. plots. Therefore, we reduced the treatments to Nematode-infested roots with necrotic discolor- three sanitation levels, high, moderate and low ation were chopped into 0.5 cm-lengths, mixed sanitation, with eight replicates. thoroughly to obtain a uniform concentration of The four strips were ploughed in February 1999. nematodes, and placed in depressions dug at the Plots (787.5–900 m2) contained 105–140 plants base of banana mats near actively growing roots. spaced at 3 × 2.5 m. Blocks and plots within The nematode populations did not establish well blocks were separated by grass alleys of 11 to 15 m. and root necrosis levels (<2 %) determined at These alleys minimized banana weevil movement flowering were similar among all treatments. between plots (Gold et al. 1998). The trial was planted in March 1999 at the onset Monitoring banana weevil populations of the rainy season. Two highland cooking banana Banana weevil adult populations were monitored cultivars, Atwalira and Namwezi, susceptible to by pseudostem trapping and the mark and recap- banana weevil (Kiggundu 2000) were selected ture method.