SUSVEG-Asia Country bean IPM

Flowers and fruits of country bean, Bangladesh

**Country bean (Lablab purpureus)
Family : Leguminosae**

Legumes are second only to cereals as a source of food for man and his animals. The family Leguminosae contains 600 genera with 18,000 species in three subfamilies, Papilionoideae, Caesalpinoideae and Mimosoideae. In south Asia legumes are more usually referred to as pulses, derived from the Latin word puls, meaning pottage. While many parts of the plants are edible most crops in South Asia are grown for their seeds, often referred to as grain-legumes. Importantly the roots of many legume species contain nodules which provide a habitat for bacteria that are capable of fixing atmospheric nitrogen and so crops can be grown in soils with poor fertility and indeed are used to increase fertility for the next crop. For this reason some pulses are grown as short-duration crops, notably the indigenous blackgram (Vigna mungo), which is grown in rotation with rice, and the related greengram (Vigna radiata) grown as a green manure, cover or forage crop. In recent years hyacinth bean or lablab (Lablab purpureus) has become very popular in Bangladesh and grown in a significant acreage alongside brinjal and tomato. L. purpureus was included in the project because the crop was subject to considerable insect damage and farmers who applied pesticide to L. putpurteus were more likely to spray their tomato or brinjal as well. By endeavouring to develop non-pesticide strategies for controlling all the crops it was surmised that the overall consumption of pesticides could be reduced, but more importantly farmers would be more able to break the habit of using them.

At the commencement of the project L. purpureus was thought to have one major insect pest species, the legume pod borer, Maruca vitrata, a pan-tropical pest of legume crops. The project envisaged developing pheromone baits to control M. vitrata by mass trapping, building on the surveillance trap system developed by NRI working with IITA in West Africa. However, although project surveys confirmed that M. vitrata was the major pest of L. purpureus there were two other species of pod and flower feeding pests, H. armigera and a previously unknown plume moth, Sphenarches anisodactylus (Walker). Infestation status of the different borer species was found to be location specific and changed over the three study years. During 2002-03 and 2004-05 three species of borer were observed at Joydebpur but during 2003-04 one species, H. armigera was completely absent, although in Jessore H. armigera represented between 18 and 28% of the borer population in flowers and comparable levels in pods. The infestation of S. anisodactylus was high in flowers during 2002-03 at Joydebpur but decreased in the two subsequent years and was not present at all in Jessore, with similar results observed in pods. The predominant species at both locations studied over all seasons was M. vitrata with infestations ranging between 71 and 99% in pods. Infestations were found to be lowest in cool months, January, and highest in September and October when temperatures are high. Flowers and buds were always present and damage levels of 52 and 84% were found in 2005 season despite high use of insecticides.

According to the Bangladesh Pesticide Association more than 70% of the total insecticides used in vegetables were applied only against four pests during the 2001-2002 cropping season, brinjal shoot and fruit borer, L. orbonalis, white fly, B. tabaci, fruit borer, H. armigera and pod borer, M. vitrata, suggesting that farmers also perceived these species as the major pests. Project surveys in Bogra, Norsingdi, Jessore and Comilla showed that farmers typically used between 80 and 100 applications of insecticide per year, frequently employing cocktails of two or three toxic insecticides. However, even after such a high frequency of pesticide use the farmers cannot control those pests economically.

The situation is compounded by the fact that farmers have been encouraged to grow the crop during the humid summer season on a large scale because of high demand and price. As a result of the availability of the crop, infestation by a range of pest species is increasing. Infestations were always found to be higher in hot and humid months of September and October and pod infestations were always higher than flowers.

Year round large-scale production of country bean, Jessore District

Given the technical problems experienced by NRI researchers endeavouring to identify an attractive bait for M. vitrata in India, the cost and instability of the major sex pheromone component, (E,E)-10,12-hexadecadienal, and the economic importance of other lepidopterous pests, alternative approaches to solving the pest problems of L. purpureus had to be developed by the project.

Legume pod borer, Maruca vitrata

The female moth lays eggs individually on the shoots of young brinjal plants. The small larvae that emerge soon eat their way into the tender growing shoots where they are protected from larval parasitoids and natural enemies such as ants and beetles. The feeding activity of the larvae causes the young shoots to droop in a characteristic manner


Maruca vitrata - larva	Maruca vitrata - adult

Analysis of M. vitrata pheromone using insects from India and Bangladesh

The results of a linked gas chromatography - electroantennography (GC-EAG) analysis of male Maruca vitrata EAG responses to a blend of synthetic compounds separated on a polar Wax GC column is shown in Figure 1. The synthetic blend contained 10ng each of dodecyl, tetradecyl, hexadecyl and octadecyl acetate standards and (E)-10-hexadecenal (E10-16:Ald) and EE10,12-16:Ald. The GC outlet was split 50 : 50 between the EAG and GC detectors, thus the male EAG preparation was exposed to 5 ng of each compound. The EAG preparation was exposed to material eluting from the GC at 5 sec intervals for 3 sec resulting in a series of EAG responses (negative peaks). The only EAG responses larger than the background were elicited by E10-16:Ald and EE10,12-16:Ald at 14.2 and 17.5 min.

EAG trace

GC trace

Figure 1. Linked GC-EAG chromatograms of male Maruca vitrata EAG responses to a blend of synthetic compounds separated on a polar Wax GC column.

Related experiments were conducted with a series of ovipositor extracts prepared from field collected insects provided by BARI. Significant EAG responses were observed at the GC retention times of EE10,12-16:Ald (Figures 2a and b) and E10-16:Ald (not shown) and these were confirmed by analysing synthetic compounds against the same male EAG preparation (Figure 2c).

GC-MS analyses of the natural samples were unable to confirm the presence of any pheromone compounds suggesting the amounts present were less than 100 pg per female equivalent. The reasons for the low pheromone titer are uncertain. No additional EAG responses were identified although there was a suggestion of a response in the hexadecen-1-ol position.

GC trace
EAG trace
Figure 2.a

GC trace
EAG trace
Figure 2b

Figure 2c. Three GC-EAG linked analyses showing EAG responses to both natural pheromone extracts (a and b) and synthetic (c) EE10, 12-16:Ald.

Preliminary field trials with putative sex pheromone components were conducted in India by Downham et al. (2003) and although only a few insects were caught they suggested that blends of E10-16:Ald, EE1012-16:Ald and EE10,12-16:OH were attractive. Field trials were conducted in farmers’ country bean crops near BARI RARS, Jessore and Joydebpur during 2003 and 2004, to test the putative pheromone components at 100 µg dose with or without aluminium foil around the lures to minimise the effects of sunlight. Significant but low catches were obtained in traps baited with a number of the blends tested, and catches were higher in traps baited with protected lures. However, there were no clear trends between results from each of the trials or the composition of blends tested although a binary blend of 3 : 100 E10-16:Ald and EE10,12-16:Ald caught consistently in all trials.

Development of floral attractant for M. vitrata

Previous research conducted at NRI identified attractant compounds for H. armigera derived from African marigold, T. erecta and sweet pea, Lathyrus odoratus (Bruce and Cork 2001; Bruce et al., 200*). The T. erecta blend contained benzaldehyde, (±)-linalool, phenylacetaldehyde and (S)-(-)-limonene, and the L. odoratus blend contained (-)-linalool, phenylacetaldehyde, benzyl alcohol and diacetone (4-hydroxy-4-methyl-2-pentanone) in their natural ratios. Given that mature crops of L. purpureus is perpetually in flower in order to attract either H. armigera or M. vitrata to an odour-baited attractant a blend or blends of compounds had to be identified that was sufficiently distinct from the background odour to remain attractive. Inevitably such a blend would not necessarily be effective in other crops, thus trials were conducted in L. purpureus, chickpea, Cicer arietinium and tomato,Lycopersicon esculentum in order to assess the prospects for developing a blend or blends of compounds that would be effective in target crops. Table 1 shows typical data with blend 1 attracting almost as many H. armigera as the male pheromone and M. vitrata being attracted equally to all blends tested. Interestingly, three M. vitrata were caught by the H. armigera pheromone trap.

Table 1. Mean number of insects attracted and caught with floral attractants in country bean fields, Jessore, 2004

Treatments	Number of days	Total number of H. armigera (total of 3 reps.)	Total number of M. vitrata (total of 3 reps.)
Floral blend 1	32	12	15
Floral blend 2	32	5	16
Floral blend 3	32	3	16
Floral blend 4	32	9	18
Floral blend 5	32	6	17
HA Pheromone	32	19	3

Significant progress has been made since the early trials but a fully optimised blend has not yet been developed.


H. armigera larva on bean	H. armigera larva on bud

H. armigera moths in pheromone trap	Floral bait in water trap

Development of a bio-rational IPM package against pod borer complex

Farmers spray different types of insecticides at very high frequency and dose on country bean to control the pod borer pest complex, typically every alternative day. Nevertheless, it was observed that at the same time as applying insecticide some farmers hand-picked infested flowers and pods. Earlier research had suggested that the application of insecticide did not significantly reduce infestation but in order to develop a management strategy that would be acceptable to farmers a combination of insecticides, hand picking and a biopesticides, neem kernel seed extract (NSKE) and H. armigera nucleopolyhedrovirus (HaNPV), were tested. These trials were conducted during the summers of 2004 and 2005 at Joydebpur and at Jessore. The experiments were laid out in randomised complete block design with three replications. There were six treatments during 2004 and seven treatments during 2005 which were as follows:

T1 = Insecticide spray (Cypermethrin) @ 1.0 ml/L of water at 15 day intervals
T2 = Insecticide spray (as T1) + hand picking (twice a week)
T3 = HaNPV spray @ 0.4 ml/l of water at 10 day intervals
T4 = HaNPV spray + hand picking (twice a week)
T5 = Hand picking (twice a week)
T6 = Untreated control
T7 = Neem seed kernel extract was applied ( 500 g/10 l) at 10 day intervals + hand picking every alternate day (2005 only).

During each harvest the number and weight of healthy and infested fruits were counted and recorded. The percent fruit infestation by number and weight was calculated from recorded data.

Percentage infestation in the untreated check was found to be significantly higher than that recorded in treatment plots apart from the HaNPV application alone (T3) at Joydebpur which probably reflected the fact that HaNPV had no effect on M. vitrata. Significantly lower infestation levels were observed in the insecticide treated plots (T1) and plots where hand picking of infested flowers and pods was practiced as the sole means of control. Combining hand picking with either insecticide or HaNPV appeared to result in significantly lower damage levels compared to insecticide or HaNPV alone. NSKE was effective when used in combination with hand picking although there was no significant difference between hand picking alone and in combination with application of NSKE.

Hand picking of infested pods and flowers resulted in yields double the untreated check and they were further increased when combined with insecticide or NSKE. HaNPV in the absence of hand picking was the least effective means of control of the pod borer complex, as might be expected bewcuase M. vitrata was the predominant pod borer pest in all plots.

Although labour intensive, hand picking requires no input costs, apart from labour, and given that NSKE is inexpensive impacts on all species of the borer complex the combination provides the basis for a low-cost, sustainable intervention that would be acceptable to farmers. Research is continuing to develop flroal attractants to control the adult stages of the pests however the research has already identified the basis of a technology package that will provide farmers with a viable alternative to the use of toxic pesticides.

References

Bruce, T.J. & Cork, A. (2001) Identification of attractants from flowers of African marigold, Tagetes erecta, for female bollworm, Helicoverpa armigera (Lepidoptera, Noctuidae). Journal of Chemical Ecology, 27, 1119-1131.

Burguiere, L., Marion-Poll, F & Cork, A.. (2001) Electrophysiological responses of female Helicoverpa armigera (Hubner) (Lepidoptera, Noctuidae) to synthetic odours. Journal of Insect Physiology, 47, 509-514.

Downham, M. C. A., Hall, D. R., Chamberlain, D. J., Cork, A., Farman, D. I., tamò, M., Dahounto, D., Datinon, B. & Adetonah, S. (2003) Minor components in the sex pheromone of legume podborer, Maruca vitrata (F.) (Lepidoptera: Pyralidae): Development of an attractive blend. Journal of Chemical Ecology, 29, 989-1011.