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.