Studies on Pest Management Practices for the sweet potato weevil (Cylas puncticollis) Boheman (Coleoptera: Appionidae) were conducted at the Institute of Agricultural Research (IAR) Farms at Njala and Ogoo Farm from September, 1994 to January, 1996. These experiments were undertaken to identify components for utilization in developing pest management strategies for sweet potato weevil Cylas puncticollis. Each experiment was reported over two seasons with planting at Njala (1994) and Ogoo Farm (1995) in Sierra Leone.
EVALUATION OF 15 SWEET POTATO CLONES FOR RESISTANCE/SUSCEPTIBILITY TO Cylas WEEVILS ATTACK INCLUDING AN ATTEMPT TO DERTERMINE THE BASIS OF RESISTANCE
Fifteen sweet potato clones were evaluated for resistance to attack by the weevil C. puncticollis. The clones differed significantly in numbers of damaged tubers, undamaged tubers and weight of damaged tubers. 82/123 (red), Madam and 87/131 had the highest number of damaged tubers with mean values of 15.8, 16.8 and 17.0 respectively. Clones with low numbers of damaged tubers included 82/144, 93/03 and 93/04 with mean values of 4.0, 3.8 and 3.0 respectively.
Tuber characteristics such as neck length, depth placement, tuber length, width, cortex thickness, numbers of tubers per plant and percent dry matter content were significantly different among the clones tested. 82/123 (red) and Njala White had long tuber necks with values of 7.6cm and 9.3cm respectively, while 87/06 and 87/37 had short necks with values of 3.2 cm, and 3.3cm respectively.
Njala white and Njala Wonder had deep tuber placement with values of 9.7 cm and 9.5 cm, respectively, while 87 / 06, 84/16 and 82 / 123 (white) had short tuber placement depth with values of 5.6 cm, 5.4 cm and 5.3 cm respectively. Clones with long tubers included 82/144 and 84/17 with values of 11.9 cm and 12.0 cm respectively, while 93/04 and Madam had short tuber lengths with values of 8.2 cm and 93.04 cm and Madam had short tuber lengths with values of 8.2 cm and 8.1 cm, respectively. Tuber widths were highest in clones 87/37 and 87/06 with values of 4.7 cm and 5.4 cm respectively, while Njala wonder and Njala White had smaller widths with values of 3.0 cm and 2.9 cm and respectively.
Tuber cortex was thickest in 87/131 and thinnest in 93/03 with values of 8.7 mm and 1.2 mm, respectively. Madam and 84/16 had the highest numbers of tubers per plant with mean values of 6.0 and 6.7, respectively while Ropot 11 and 93/03 had the least with values of 2.8 and 2.2, respectively. Percent dry matter content was higher in 93 / 04, TIS- 2532 and 93 / 03 with values of 36.9, 37.1 and 39.3 and lowest for Madam with a value of 29.7
The percent damaged tubers was negatively correlated with : (a) mean numbers of tuber per plant, (b) tuber neck length, and (c) tuber depth placement, while the percent damaged tuber was positively correlated with: (a) tuber cortex thickness, and (b) total number s of weevils. The correlation between percent damaged tubers and total number of weevils was highly significant (r= 0.759**). Thus clones having tubers with long necks, deep placement and high dry matter content were attacked less by the weevils.
Other tuber characteristics of the sweet potato plant which appeared to have influenced low weevil infestation and damage were: white fleshed tubers and skins, elongated tubers and dispersed tuber formation. Conversely, clones that were heavily damaged had tubers that were red fleshed and skinned, elliptical, short necked, shallow placement, clustered formation and low in dry matter content. Thus clones with tuber characters such as long necks, deep placement, high dry matter content, white colour and dispersed formation should be further evaluated for their bases of resistance with a view to using them either as indices for identifying resistant clones or in plant breeding programmes incorporating resistance in sweet potato to Cylas weevil attack.
DETERMINATION OF AN OPTIMUM TIME FOR PLANTING SWEET POTATO WITH REGARD TO CYLAS INFESTATION AND DAMAGE DURING THE SECOND CROPPING SEASON
An experiment to determine the optimum time for planting second season sweet potato with regard to weevil infestation was undertaken using three dates of planting and two clones. The number of larvae, adults and total weevils and percent damaged tubers increased as planting was delayed after 1st September. The mean values for numbers of larvae and numbers of larvae and adults for tubers of Njala White planted on 1st and 30th September was 8.7 and 128.7 and 9.0 and 152.7, respectively. Comparable values for 82 / 123 (red) were 257.3 and 647.3 and 316.0 and 730.7, respectively. The percent adult weevils and tuber length were not affected by delayed planting.
Plant characters such as tuber width, number of tubers per plant, total weight of tubers and dry matter content of tubers reduced as planting was delayed. This would indicate that delayed planting exposed the crop to higher weevil infestation and also reduced tuber yields.
For each of the planting dates, 82 / 123 (red) had higher mean values than Njala White for most of the parameters measured i.e. numbers of larvae, numbers of adults, total numbers of weevils (larvae and adults), number of damaged tubers, percent damaged tubers, tuber width, tuber length and total weight of tubers, thus indicating that Njala White was less susceptible to weevil attack than 82 / 123 (red).
Significant positive correlations were obtained between planting dated and weevil incidence in both clones (except number of adults for 82 / 123 (red) in the Njala experiment) while significant negative correlations were obtained for both clones between planting date and total weight of tubers. The correlation for Ogoo Farm were similar to those obtained at Njala except that percent dry matter content of the tubers was positively correlated with planting time for both clones. These further confirm that delay in planting the sweet potato crop exposed it to higher weevil infestation and reduced tuber yields. Thus it is suggested that for the second cropping of sweet potato in Sierra Leone, planting should be restricted to within the first half of September.
DETERMINATION OF AN OPTIMUM TIME FOR THE PROXIMATE HARVEST OF SWEET POTATO WITH REGARD TO CYLAS INFESTATION AND DAMAGE DURING THE SECOND CROPPING SEASON
A study to determine the proximate time for harvesting second season sweet potato crop with regard to weevil infestation was undertaken using three dates of harvesting and two clones. For each of the two clones, the mean numbers of larvae, adult weevils, total weevils (larvae and adults), the weight and percent of damaged tubers, the total weight of tubers, and percent dry matter content of tubers increased as harvest dates were delayed. For example, the numbers of larvae and adults for tubers of Njala White harvested at 13 and 17 weeks after planting were 7 and 142 and 0 and 23, respectively. Comparable values for 82/123 (red) were 175 and 701 and 15 and 114, respectively. Delaying harvest time had no effect on tuber width, tuber length and numbers of tubers per plant. Thus early harvesting had the twin advantages of reduced weevil infestation on the crop and at the same time tuber characteristics were not significantly reduced compared with late harvested crops. For each harvest date 82 / 123 (red) had higher levels of weevil infestation and damage than Njala White. For example, at 17 weeks after planting, the mean numbers of larvae and total weevils counted from harvested tubers of 82 / 123 (red) were 701.3 and 815.7, respectively. Comparable infestation levels for Njala White were 142.0 for larvae and 165.0 for total weevils. Again confirming that Njala White was less susceptible to weevil attack than 82 / 123 (red). Regression analyses indicated significant positive correlations between harvest dates and (a) (weevil incidence, and (b) damage levels for both clones. These taken together would suggest emphasis on early harvesting of sweet potato tubers as a way of minimizing weevil infestation. In this study 13-15 weeks after planting would appear to be a suitable time for proximate harvesting of tubers.
IDENTIFICATION OF THE DIFFERENT SPECIES OF CYLAS AFFECTING SWEET POTATO IN SIERRA LEONE
Adult weevils were collected at Njala and Ogoo farm for determination of species composition. All the weevils collected were identified as C. puncticollis. Distinguishing features were: a distinctly arched protonium in the lateral view, a very evident posterior constriction, narrowly separated eyes in the dorsal view, a uniformly black thorax and the possession of four pairs of sclerites on the internal sac of the aedeagus. These features were common to all the adult Cylas weevils collected from the two experimental sites.
DETERMINATION OF THE NUMBER OF INSTARS FOR CYLAS SPP. FOUND IN SIERRA LEONE
Cylas weevil larvae were collected for larvae instars determination from both Njala and Ogoo farm using head capsule width measurements. Five non-overlapping larval instars were obtained for larvae collected from (a) Njala and (b) Ogoo farm. The mean head capsule widths were 0.26 mm, 0.46 mm, 0.90 mm and 1.17 mm for the 1st ,2nd 3rd ,4th and 5th larvae instars, respectively for those collected from Njala. Head capsule width increments from one instar to the next ranged from 0.19-0.27 mm with a mean of 0.23 mm. These increments were not significantly different (t=23.56; P<0.05) and they conformed to Dyar’s rule (t=28.09; P<0.05). Similarly, for the Ogoo Farm collection, mean head capsule widths were 0.26 mm, 0.46 mm, 0.65 mm0.92 mm and 1.14 mm for the 1st, 2nd, 3rd, 4th and 5th larval instars, respectively. Head capsule width increments from one instar to the next ranged from 0.19-0.27 mm with an average of 0.22 mm. these increments were not significantly different (t=24.73; p<0.05) and also conformed to Dyar’s rule (t=26.24; P<0.05).
THE EFFECT OF MANAGEMENT SYSTEMS (TETHERED AND FREE- RANGE) ON THE REPRODUCTIVE PERFORMANCE AND DISEASE INCIDENCE IN THE WEST AFRICAN DWARF SHEEP IN SIERRA LEONE
The effects of grazing management on reproductive efficiency
and disease incidence were investigated. The two grazing management systems
were tethering and free-range. The following results were obtained:
Under the free-range system, the number of single births per year, mature
ewes without birth, percentage lambing, death of lambs before weaning,
number of weaned lambs per year per three ewes, percentage twining and
disease frequency were 2.66, 1.18, 0.56, 146.35, 0.64, 4.38, 30.67 and
0.62 respectively.
Under the tethered system, the corresponding figures were respectively
2.54, 0.78, 0.74, 106.66, 0.9, 3.2, 26.16 and 0.76.
Reproductive efficiency was apparently higher under the free-range system
but disease incidence was lower. This was both for lambs and ewes.
ENVIRONMENTAL AND HUMAN RISK OF PESTICIDE USE IN BANANA PLANTATIONS IN CAMEROON
Computer models were used to evaluate the effects of protective
equipments, number of days per year and events per day on the chronic
concentration exposures and risks on humans of some pesticides used in
banana plantations in Cameroon.
Chronic exposure concentrations and risks increased with the number of
days per year and events per day for all pesticides used in the various
simulations. The best protection from chronic exposures was provided when
all 3 protective equipments (coverall, gloves and mask) were used at the
same time.
Mixing, loading and application of Carbofuran for 8 hours work per day
was possible for a maximum of 3 days in a year by one operator with chronic
exposure concentration of 0.03mg/kg bw/day compared to the Accepted Operated
Exposure Level (AOEL) value of 0.05. The risk level was 0.623.
Simulations results show that Imazalil is not a toxic fungicide. With
all protective equipments used, mixing and loading lasting 10 minutes
could be performed for up to 250 days/year with chronic exposure concentration
of 0.0356mg/kg bw/day compared to the AOEL value of 0.05. The risk was
0.7113. However, dipping in Imazalil for up to 50 days/year for 8 hours/day
was very dangerous and toxic for a single operator. Chronic exposure concentration
at this stage was 5.162mg/kg bw/day compared to the AOEL value of 0.05
with a risk of 103.237.
Using all protective equipments, results evaluate Paraquat as a very toxic
herbicide. A single day’s application for 8 hours shows very high
chronic exposure concentration of 0.0341mg/kg bw/day over exceeding the
AOEL value of 0.0004 with a risk of 85.1589.
Mixing and loading Propiconazole lasting 10 minutes could go on for up
to 120 days/year and 10 events in a day. Chronic concentration exposure
at this level is 0.07mg/kg bw/day compared with to the AOEL value of 0.1.
Better protection is secured at lower number of days per year with increased
events per day than otherwise.
