DIAMOND BLACK MOTH (plutella xylostella) IN CABBAGE AND MUSTARD IN BOTSWANA

INTRODUCTION

The Diamond black moth (plutella xylostella) is a pest of brassicas and thrives mostly in hot and dry conditions and several generations may be completed on one crop in a field. The larvae cause the most damage as compared to the adult and so needs to be controlled inorder to get a better yield. Pheromone traps are used to catch adult the moth and the information obtained is used to assess the incidence of pests. Sampling for stages of the moth also provides information on pest incidence and effect of control measures. The monitoring of Diamond black moth and its natural enemies is a critical activity in its management especially when implementing integrated pest management and this can be done at different levels and this guides farmers to make timely pest control decisions. An effective monitoring programme for Diamond black moth at different developmental stages provides information about its status and so enables the farmer to define treatment thresholds and therefore effectively apply different levels of integrated pest management.

LITERATURE REVIEW

PEST STATUS

Diamond black moth (plutella xylostella) shortened as DBM attacks all species of brassicas including cabbage and mustard (Wikepedia 2009). Severe attacks occur especially in hot and dry weather. In deserts such as Namib, hot conditions enable the pest to develop rapidly almost throughout the year. Under these deserts conditions a generation can be completed in 2 weeks in summer and 3-4 weeks in winter. Several generations may be completed on one crop in the field and with pest population numbers increasing significantly (Keizer 1999).

DESCRIPTION
                                                                                                             DBM Larvae           

   





                  


                                                                                                                                                               

The adult moth is about 7-8mm long, grey in colour with pale triangular markings on the inner edge of each forewing forming a triple diamond pattern when the wings are folded over each other ( Wikepedia 2009). Eggs are white and cigar shaped and are laid singly or in small groups near the midrib or a strong vein of the leaf. Incubation period is 3-8 days depending on environment. Pupation takes place inside a gauze-like silken cocoon about 9mm long which is stuck to the underside of a leaf. The pupa is greenish at first and changes to brown as the moth develops and the pupal stage lasts from 5-10 days (Keizer 1999). The larvae are pale green with a black head capsule and sporadic black hairs. A single female moth may live for 114 days and lay more than 400 eggs. In the tropics, breeding is continuous with as many as 15 generations a year (Bellows 2002).

DAMAGE

Caterpillars of DBM feed on leaves usually on the undersides, eating into tissue, leaving only the veins and the upper epidermis giving the leaves a windowed appearance which grows in size and eventually forming holes as the leaves grow and the dead tissue tear. On seedlings the growing point is usually damaged often resulting in death of plants. In older plants, new shoots are produced and the attacked plants produce several heads of little value (Thacker 2002). The adult DBM feed on small amounts and damage is so light that it will go unnoticed. It is the larvae that causes significant economic damage and economic damage is most severe after heading begins where the larvae burrows into head destroying its marketability (Keizer 1999).

CONTROL

The recommended treatment threshold from seedling to cupping is when 20% of the plants are infested. After heading begins, the recommended threshold decreases to 5% infestation (Varela 1999).DBM can be controlled by natural enemies like diadgmasp, apanteles, cotesia, and tetrastichus species.Neem (azadirachta indica) based products give good control of DBM and are harmless to natural enemies and they kill the larvae and cause adults to be deformed and have reduced egg laying capacity. Keeping down weeds reduce egg laying sites and uprooting and burning cabbage and mustard remains avoids carry over of the pests to the next brassica crop (Wikepedia 2009).Use of pheromone catch traps which have pheromone substances on a sticky surface which attract insects like DBM can also reduce the DBM population but should be used with other control methods for it to be effective (Persley 1996).

USE OF PHEROMONE TRAPS

• Pest monitoring where information from trap catch is used for detection (early warning) of pest incidence and allows enough time for organisation of control measures and application before larvae have done too much damage.
• Used for surveying (define infested areas).
• Timing of treatments as a threshold catch should be established for any trapping system for before any method of control is justified.
• Timing of other sampling methods as trap catch information is often not sufficient by itself to take pest management decisions. A threshold catch in a pheromone trap triggers the taking of a sample for inspection for eggs or young larvae.
• Risk assessment where information from trap catches provides population trends and population density correlations (Howse et al 1998).
• Mating disruption
• Attract and kill
• Indicate effect of control measures where an increase in numbers shows that control measures are not being effective (Persley 1996).

OBJECTIVES

• To determine the seasonal abundance of diamond black moth.
• To assess the incidence of diamond black moth in cabbage and mustard.

METHODOLOGY

Sampling for developmental stages (adult, larvae, and pupae) of DBM incidence on cabbage was carried out at Mike’s farm and the Horticultural council’s farm on 13/02/09, 13/03/09 and 27/03/09 in the afternoons. Visual inspections for presence or absence of the moth were carried out along 2 diagonals of each farm at 5 different rates (1 plant, 2 plants, 3 plants, 4 plants and 5 plants per diagonal) and the numbers were recorded. The amount of time taken to complete the assessment was also recorded.

Sampling for DBM incidence on Mustard was also carried out on the Horticultural council farm but only on the 13^th of February since it was removed thereafter. The same method of sampling was used and the data was recorded.

Pheromone traps were set up at Mike’s farm (3a on cabbage and 3b on maize) and Horticultural council farm (5, 5a on cabbage and 5b on mustard). The number of adult DBM caught were counted and recorded on 13/02/09, 27/02/09, 13/03/09 and 27/03/09. The information obtained from the sampling and traps was then analysed and a report was prepared.

RESULTS

Number of adult DBM caught

Date	Mike ‘s farms			Horticultural council farm
				Cabbage			Mustard
	3b Maize	3a Cabbage		5		5a	5b
13/02/09	2		26	8	38		5
27/02/09	2		18	47	27		8
13/03/09	3		35	78	69		0
27/03/09	32		119	83	22		5
Total	39		198	216	156		18
Grand total	39		198	390

Average number of developmental stages of DBM from sampling

Date	Adult			Larvae			Pupae
	Mike farm	Council cabbage	Council mustard	Mike farm	Council cabbage	Council mustard	Mike farm	Council cabbage	Council mustard
13/02/09	0	40	0	0	8	12	0	112	0
13/03/09	0	34	-	2	0	-	0	0	-
27/03/09	10	-	-	0	-	-	0	-	-
Total	10	74	0	2	8	12	0	112	0
Grand total	10	74		2	20		0	112

Minor pests found in both Cabbage and Mustard were mostly aphids.

Average time taken to complete the assessment in both cabbage and mustard for

Adults                =  30 seconds per plant

Larvae                = 1 minute per plant

Pupae                 = 55 seconds per plant

Natural enemies = 1 minute per plant.

DISCUSSION

The number of DBM in Mike’s farm increased with time and there was a sharp increase in March especially in the cabbage field. This may be because the cabbages had now reached physiological maturity and so there was enough time for DBM to reproduce and increase in numbers. The other reason may be that the farmer had planted another plot of cabbage and so there was more food and breeding places for the pest. The number of DBM in the maize field was lower than those from the cabbage field but the number also increased sharply in March as in Mike’s farm. This lower numbers may be  because DBM is a pest of brassicas especially cabbages and so thrives more in a cabbage field as compared to a maize field. The increase in numbers in March was in conjunction with that of the cabbage field and this may be because the DBM from the cabbage field would flew to the maize field in search for  alternative breeding places since they are in close proximity.

The number of DBM in the Council farm was increasing with time with only 5a decreasing in March. This may be because the crop was removed in March and since plot 5a is in the middle, it had less DBM as compared to plot 5 which is at the field’s end and may be getting DBM which is breeding in weeds and other plants bordering the field. 5b however had the lowest number of DBM in the council field and this may be because mustard is less favoured by DBM as compared to cabbage. The other reason may be because the mustard was removed before the 13^th of February and so there was less food and breeding places. The number of DBM in 5b was increasing at the end of March and this may be because a new mustard crop was planted by then and so the DBM now had a supply of food to support their multiplication.

 There were more adults in cabbage at Council farm as compared to Mike’s farm. The reason may be because Mike was controlling the pest better than at Council’s farm. There were also more larvae and pupae at Council farm as compared to Mike’s farm. The difference may be a result of poor management in Council farm since decision making involves a lot of people and so it takes time to pass a decision and so DBM will have reproduced and increased in numbers by the time a decision to effect a control is passed. Aphids were the predominant minor pests at both farms and although they were not the main pest being monitored, the farmer should control them since they can also result in yield losses if not controlled.

The average time taken to assess the plant for presence of adult DBM in both cabbages and mustard was less or almost half that taken to assess the presence for larvae, pupae and natural enemies. This maybe because it is faster to see if the adults are present since u just have to disturb the plant and they will fly around. It however takes more time to search for larvae, pupae and natural enemies since a person have to look under every leaf and correctly identify them. The amount of time taken to assess the plant for presence or absence of DBM is generally low.

CONCLUSION

Pheromone traps are very useful in detection of pest incidence and so trap catch information can be used to time treatments and other sampling methods. It can also be used to indicate if control measures are being effective and trapping the DBM is a control measure in itself as it attracts and kill and so disrupts mating. The information provided by the pheromone traps and sampling indicates that there is poor DBM control at Council farm as compared Mike’s farm farms. Both methods however showed the same trends and so shows that they are both effective in determining DBM incidence in Brassicas. Since the time taken for assessing the plants for absence or presence of DBM is very low it will therefore not cost the farmers much of their  time to monitor  crops and use the information to decide when to effect a control method. DBM can therefore be controlled by integrated pest management programmes.

REFERENCES

Bellows, T.S.2002.Entomology notes. Department of Entomology.Pennysylvannia State University.

Howse, P.E, Stevens, I.D.R and Jones, O.T.1998.Insect pheromones and their use in pest management. Chapman and Hill.

Keizer, M.J.1999.Manual of integrated pest management course for extension staff.Mashire Agricultural Development Institute, Rundu.Namibia.

Persely, G.J.1996.Biotechnology and IPM.CAB International.

Thacker, J.R.M.2002.An introduction to arthropod pest control. Cambridge University Press.

Varela, A.M and Seif, A.A. 1999. Crop protection manual for brassicas, ICIPE, Nairobi.Kenya.

Wikepedia, The free encyclopaedia. Whiteflies. www.wikepedia.com.Adapted on 7 March 2009.

GROUNDNUT PRODUCTION IN BOTSWANA

Local names - Manoko, Matonkomane ( Tsuaneng and Maphanyane 2000).

OVERALL OBJECTIVE

Assessing the production levels of oil crops in Botswana

SPECIFIC OBJECTIVES

• To assess the production level of groundnuts in farming regions of Botswana.
• To compare the production levels of traditional and commercial farming systems of Botswana.

INTRODUCTION

The groundnut  (Arachis hypogeae L.) originated in South Bolivia and North Western Argentina is grown as an annual crop in warmer regions of the world including Botswana, principally for its edible oil and protein rich seeds (42-51 % oil and 40-50% protein) (Protabase 2009). In most tropical and subtropical areas where groundnut is grown, the climate is seasonally arid and the staple crop is a cereal usually maize. To ensure survival farmers plant the cereal as soon as they can when the rains start and only plant groundnuts when cereals have established (Summerfields and Roberts 1985). Groundnuts are usually grown by poor smallholder farmers and are considered a women’s crop thus it is given lesser priority for when it comes to inputs such as certified seed fertilisers and irrigation (Okumu 1999).

WHY GROW GROUNDNUTS

• Chief source of edible oils and plant protein in human diet.
• Low input-high value crop.
• Cost of production drawn from purchasing seed but farmers can preserve seed from previous crop unlike most crops.
• Most costs in groundnut production are labour and can be supplied by family in scarcity of cash (Finn and Kenny 2004).
• Groundnut is an income substitute or supplement for maize.
• Unlike vegetables and fruits which are highly perishable items, groundnuts can be stored for a long period of time and sold when prices are high.
• Farmers can also double its price by roasting it hence earning even higher income.
• Since groundnut takes a shorter time to mature, the land can be used for another possible cropping thus utilising the second rains. This increases gross margins per unit area of land per year.
• Groundnuts are nitrogen fixers and so improve soil fertility ( Okumu, 1999).

CLIMATIC AND SOIL REQUIREMENTS

Groundnut requires warm tropical and subtropical environments and is well adapted to sandy and sandy loam soils. The optimum mean daily  temperature for Botswana is 27-30˚c, optimum soil temperature for seed germination is 25-30˚c and temperatures lower than 20˚c slows development and those higher than 35˚c adversely affect development. A rainfall range of 500- 1000mm is ideal for groundnut production but a crop can produce on as little as 300-400mm thus it has some degree of drought tolerance. A dry period is required for ripening and harvesting and a soil pH of 5,8-6,2 is ideal for production (Tsuaneng and Maphanyane 2000).

LAND PREPARATION

Groundnuts should be sown in a seedbed with a fine tilth since large clods may hamper emergence. Due to its pod forming habit, deep ploughing should be avoided since it encourages development of pods in deeper layers of the soil making harvesting difficult (Bass, 2000).

SELECTION AND TREATMENT OF SEED

Quality seeds are of utmost importance for establishing the optimum plant stand. Pods for seed purposes should be stored unshelled in a cool dry and ventilated place and pods should be shelled by hand one week before sowing as hand shelling ensures little  damage to the seeds. Seeds shelled before sowing times are liable to suffer from loss of viability and storage damages. Shrivelled and diseased seeds should be discarded and only bold large seeds should be used for sowing (Bass 2000). Seeds should be treated with thiram or captan before sowing to check various seed and soil borne diseases. Seeds should also be inoculated with a proper strain of rhizobium culture particularly in those places where groundnut is grown for the first time ( Tsuaneng and Maphanyane 2000).

PLANTING

In Botswana groundnuts  should be sown from late November to mid December and soil moisture reserves are often adequate at this time to sustain good vegetative development and produce strong plants. Planting later than the end of December often exposes the crop to good moisture reserves but mid season drought usually experienced in January can affect pod formation and filling. The season also becomes too short to allow enough time for maturity. Planting in November exposes the crop to risks of early season drought. Low temperatures at this time also adversely affect germination resulting in poor or delayed emergence. Groundnuts in Botswana are planted 4-6cm deep at a rate of 30kg/ha for row planting (Tsuaneng and Maphanyane 2000) but in other places it is planted 4-7 cm deep in moist soil at a rate of 60-80kg/ha (Protabase 2009).

PLANT POPULATION

The recommended plant population in Botswana is 50-100 000 plants/ha depending on moisture availability and 133 000 plants/ha under irrigation. An inter row spacing of 0.75m is recommended with variation of 0.1-0.2m within row depending on plant population required (Tsuaneng and Maphanyane 2000). In other places optimum plant populations are 133 000 to 167 000 and Spanish types are spaced at 60-75cm between rows and 10cm within rows while large seeded Virginia types are spaced at 75cm between  rows and 15cm within rows giving a plant population of 89 000 plants/ha (Summerfield and Roberts 1985).

CULTIVARS

The recommended groundnut varieties in Botswana are given in the table below

Cultivar	Growth habit	Maturiy (Days)	Yield (t/ha)	Seed		Pod
				colour	size	Size (mm)	Beak	Constriction	Reticulation
Sellie	Upright	120-130	1-2	Tan	Medium	26	Slight	Moderate	Prominent
GC8-13	Upright	115	1-2	Tan	Small	25	Moderate	Slight	Prominent
S46	Prostate	120-130	1-2.5	Tan	Small	30	Slight	Deep	Slight
ICGS-31	upright	120-130	1-2.5	Tan	Small	25	Absent	Slight	Moderate

  (Tsuaneng and Maphanyane 2000).

FERTILISATION

Groundnuts meet major part of its Nitrogen requirement through nitrogen fixation. However an application of 20-40kg/ha as a starter dose is given to meet the nitrogen requirement of the crop in the initial stage in poor fertility soils. About 50-60kg P2O5 and about 30-40kg K2O/ha should also be added (Wyne and Rowe 1998). In Botswana a basal application of N, P and K at 23; 10.5; and 12.5 kg/ha respectively, benefits the crop by improving plant establishment and pod production. Calcium is an important element for pod development  and filling and should be applied as gypsum at 500kg/ha at flowering. Farmyard manure/compost can also be added at 10-15t/ha before sowing (Tsuaneng and Maphanyane 2000).

MANAGEMENT PRACTICES

Weeding should be done thoroughly at the early vegetative stage and 1 or 2 hoeings should be done depending on soil type and extend of weed infestation. Earthing up (hilling) should also be done to promote easy penetration of pegs in soil and also provide more area to spread. To control diseases, plant debris should be buried or burnt and infected plants should be rouged as soon as they appear in the field. Tolerant cultivars should be grown and insecticides should be used to control high incidence of pests. Groundnut can also be intercropped with maize, millets, sorghum, and sunflower (Finn and Kenny 2004).

PESTS AND DISEASEAS

Groundnut is attacked by various pests and diseases and some of the pests  include thrips, aphids, groundnut leafminer, termites, and maize rootworm. Diseases include early leaf spot, rust while viruses include groundnut rosette virus which is transmitted by aphids. Aspergillus fungi also attack pods and seeds producing alfatoxin (Summerfields and Roberts 1985).

HARVESTING AND STORAGE

Groundnuts should be harvested when mature because premature harvesting lowers yield and kernel quality. The plants should not be allowed to dry before harvesting as this tends leads to pod losses at harvesting. To determine maturity, scratching the groundnut pod from outside will show a dark brown colour for a mature pod while a cream white colour is indicative of immaturity. Mature pods also show brown to dark marks on the inside while immature pods are white and pulpy or fibrous (Tsuaneng and Maphanyane 2000). Groundnuts should be harvested in dry weather to avoid rotting and development of mould due to wet weather. Harvesting is done by hand or by machines using a digger shaker inverter. The plants should then be stoked to dry before threshing. Threshing is done by hand or machine when plants are dry and pods should be stored in stacked bags until required (Bass, 2000).

USES

• The seed is eaten raw, boiled or roasted and is made into peanut butter and oil is also extracted from the seed.
• After oil extraction the press cake which is high in protein is used as animal feed, makes flour, glue, starches and wool.
• Shells are used to make building blocks and are a source of reducing sugars, organic chemicals, combustable gas, activated carbon and are also used for livestock feed.
• Plants may be used to feed animals as fodder or silage.
• Young pods and leaves are consumed as vegetables in some parts of West Africa and pod extracts are used for medicinal purposes like as eyedrops to treat conjuctivis and also have aphrodisiac properties. Macerations of seed coats treat syphyllis and ophthalmia and leaf infusions treat female infertility and eye injuries and cataracts.
• The groundnut plant fixes nitrogen and so improves soil fertility and the haulms are applied as green manure and shells are used as fertiliser or mulch (Protabase 2009)

Groundnut yield in kg/ha per region

Region	Year
	1989	1990	1993	1995	1996	1997	1998	1999	2001	2002
Southern	57	200	196	-	342	127	56	18	-	157
Gaborone	250	250	-	2033	191	90	54	-	22	16
Central	250	100	10	1352	115	224	24	4	73	36
Francistown	-	-	90	382	-	173	8	191	114	46
Maun	-	-	1364	-	-	-	-	-	35	195
Western	-	-	-	-	-	-	-	-	17667	-
Traditional	108	100	64	856	240	193	10	73	100	64
Commercial	500	500	193	200	-	143	16	-	-	630
Total	232	233	79	853	240	193	11	72	100	72

DISCUSSION

The average yield (kg/ha) of groundnuts was very low from 1989 to 1993 and this may have been because of poor rains which are usually received in Botswana. The yield then increased from 1993 to 95 especially in Maun, Central and Gaborone regions with Maun reaching its peak in 1993 and Central and Gaborone regions reaching theirs in 1995. This maybe because there was a lot of rain in the 1995 season. There was however a deterioration in yield after reaching the peak up to 1999 but it started increasing again in 2001 up to 2002 though at a slower rate. This may be because the conditions of poor rains had started prevailing again.

The average total yield of groundnut was constant in 1989 and 90 and increased to reach a peak of just above 800kg/ha in 1995 coinciding with the year in which there was an increase in yield in Gaborone and Central regions. The total yield started declining in the subsequent years to levels below 300kg/ha.

Commercial yield showed constant yield in the first 2 years and declined to 200kg/ha and below only to increase sharply and reaching a maximum of above 600kg/ha in 2002. Traditional yield was more or less constant averaging around 100kg/ha from 1989 to 93 and sharply increased to a maximum of above 800kg/ha in 1995 before declining in the subsequent years to levels below 200kg/ha. The decrease may have been due to drought conditions prevailing again since there may have been more rain in 1995 which had caused a sharp increase. However the commercial farming system generally has higher yields than traditional and this maybe mainly because of use of improved hybrids, fertiliser, more weed and pest control and maybe supplementary irrigation.

CONCLUSION

Yields of oil crops are consistently low in all farming regions as they are below the expected Botswana yields and this is probably due to poor rains and substandard management. The subsistence farmers are  getting lesser yields as compared to the commercial farmers and most of these small scale farmers who are growing oil seed crops have limited access to resources and markets. Most of them cannot at present afford expensive capital developments or purchased inputs. The physical, financial, technical, commercial and administrative support which farmers in the industrialised countries are getting is not mostly available to them. However significant improvements are already technically possible as new cultivars with durable and stable resistances, tolerances and adaptations to biological and environmental constraints have been and are being developed. Establishment of local industries that use oil crops as raw materials can also help to provide market for the oil crops produce and thus gives farmers an incentive for spending more on inputs and thus produce more.

RECOMMENDATIONS

The market demand should be sustained so that an efficient and reliable seed industry can deliver improved seeds to  farmers. Farmers should  also purchase seed from reputable seed companies or dealers with a good technical service record if production problems do occur and assistance or consultation is required. The cultivars which the seed companies must offer should provide a measure of insurance against pests, diseases and drought and this would encourage both small scale farmers and large scale farmers who are entering agriculture to use more productive methods including timely sowing and control of weeds to ensure higher productivity. The farmers should always seek for assistance from the nearest offices of the Department of Agriculture.

REFERENCES

 Bass M.H. 2000. Groundnut: Production guideline: www.ficciagroindia.com. Agribusiness information centre.

 Finn K and Kenny A. 2004. Groundnuts, women and development.

Okumu M.O. 1999. Economic evaluation of groundnut for smallholder farmers. Ministry of Agriculture and Rural development, Kenya.

Summerfield R.J and Roberts E.H. 1985. Grain legume crops. Cpllins Professional and Technical Press.

Tsuaneng M.M and Maphanyane G.S. 2000. Field crops reference Handbook in Botswana Department of Agriculture Research.

Van de Vossen H.A.M and Fagbayide J.A. 2007. Helianthussannus L. Plant Resources of Tropical Africa. http://database.protabase.org. Accessed 17^th April 2009.

10. Wynne J.C and Rowe R.C. 1998. Advances in Agronomy.