IMPACTS OF THREE PESTICIDES ON ARTHROPOD BIODIVERSITY IN BRINJAL AGRO-ECOSYSTEM

A field study was conducted to find out the impact of three selected pesticides on arthropod biodiversity in brinjal agro-ecosystem during 2011-2012 at Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU). Twenty-six arthropod species were observed through visual searching method out of which 18 were insect pest species (from 14 families under 8 orders) and 8 were natural enemies (from 8 families under 6 families). Twenty one species of surface dwelling arthropods were caught in pitfall traps method, 12 of them were insect pest species (from 11 families under 8 orders) and 9 were natural enemies (from 9 families under 5 orders). Formicidae was the most abundant family among them. Bacillus thuringiensis (Bt) and Tracer 45SC (Spinosad) were less harmful to arthropod pest species as well as natural enemies in the brinjal agro-ecosystem. Proclaim 5SG reduced the overall abundance of arthropod species. Bacillus thuringiensis (Bt) and Tracer 45SC were relatively safe for natural enemies and therefore would be fit well into integrated pest management (IPM) against BSFB of brinjal crop.


Introduction
Brinjal, Solanum melongena L. is one of the most important and popular vegetable crops grown in Bangladesh as well as in South and South-East Asia.Bangladesh produced 0.238 million tons of brinjal from an area of 28631.509hectares with an average yield 8.00 t ha -1 , which is approximately 15 and 13% of the total vegetable area and production of our country, respectively (Anonymous, 2011).
One of the main constrains of brinjal production is its vulnerability to pest attack.In brinjal field in addition to Brinjal Shoot and Fruit Borer, Leucinodes orbonalis Guenee (Lepidoptera: Pyralidae) as the major pest, various arthropod species both pests and natural enemies prevail from seedling to harvesting stage.EL-Shafie (2001) observed 28 species of insect pests of 7 different orders from the brinjal ecosystem, while Nayar et al. (1995) reported 53 species of insect pests of brinjal.Many arthropod natural enemies of those obnoxious pests also exist in the same ecosystem (FAO, 2003).
In general, common insecticides are not specific and they frequently kill natural enemy populations and may causes upset and resurgence of other pest population (Pedigo, 2002).In addition, development of insect biotypes resistant to the commonly used insecticides is not uncommon (Pedigo, 2002).Many researchers reported that insecticides killed other non-target arthropods, caused serious ecological imbalance and affected arthropods biodiversity (Alam et al., 2006, Navntoft et al., 2006, Filho et al., 2004, Prijno et al., 2004, Rodriaguez et al., 2003).
The present studies were, therefore, conducted to know effect of different selected prospective insecticides on arthropod biodiversity including natural enemies for judging their suitability as eco-friendly component in the IPM package of BSFB.

Materials and Methods
The experiment was conducted during November 2011 to April 2012 in RCB design with four replications.Details of the insecticides used with their doses are given in Table 1.The whole field was divided into 4 blocks of equal size having space of 2 meters between blocks and 1 meter between plots.Each block was subdivided into 4 equal plots including one control plot.The unit plot size was 3m x 2m accommodating 10 pits per plot.The distance between rows was 1 m and that of plants was 60 cm.A total of 160 seedlings were planted in 16 plots @ 10 seedlings plot -1 .

Data collection
The number of species was the simplest measure of diversity.However, for limitation in species identification, concept was restricted to order and family level.The counting of individual was performed by using visual searching, sweeping net and pitfall trap method.The normal seasonal fluctuations and the population dynamics of arthropods were recorded from pesticides free plots of brinjal grown away from the pesticides treated plots, while for pesticide affected population the data on arthropods were collected from the pesticides treated plots.

Visual searching method
Arthropods species considering sessile, agile, adult, nymphs and larvae of flying insects on brinjal plants were counted from a random sample of 5 plants taken from each plot.Ten leaves were chosen randomly on each plant, four from the bottom (older leaves), two from the middle and four from the top (younger leaves).The lower surface of the leaf was thoroughly examined for the presence of insects.Counting was done before 08:30 hr (Bangladesh local time) to avoid the excessive mobility of the adult insects after this time, but the migration of the fast moving and mobile adults from one plot to the other could not be totally avoided.The data were pooled over the season's average was combined to provide an overall average density plot -1 .The population density of each insect is expressed as number of individuals per 10 leaves of the plant.Individuals were counted from five branches selected randomly from 5 plants of each plot at weekly interval.

Sweeping net method
This method was used for counting insects on brinjal plants to know the abundance pattern of insects in the present study.Five times return sweeping was done in each plot to make a composite sample by a sweeping net at 15 days intervals.Each sample was examined separately without killing the insects and released them in the same plot immediately after counting.The individuals of each sample were counted by family.

Pitfall trap method
The pitfall trap method was used for the species that roam in the soil surface.Small plastic pots having 10 cm diameter and 18 cm deep were used as pitfall traps.Three traps were placed in soil in each of the plots and the mouth of the pot was kept at ground level, so as not to obstruct insect movement.Each pot was then half filled with water and detergent as trapping fluid.After 24 hours of setting traps, the trapped arthropods were emptied with a sieve and funnel into small plastic bottles filled to the half with 70% alcohol.The samples were labeled and stored until sorting, counting and finally were identified.Traps were set at 15 days intervals throughout the cropping season and insects were collected and counted separately from each plot.

Measurement of diversity index
To assess the abundance pattern and the species richness in different insecticides treated plots, Simpson's diversity index was used (Simpson, 1949).8 (1): 55-61, June, 2018 Where, 'Pi' is the proportion of individual for the 'i'th insect family and 'S' is the total number of insect family in the community (i.e. the richness).The value of index depends on both the richness and the evenness (equitability) with which individuals distributed among the families.Equitability was quantified by expressing Simpson's index, 'D' as a proportion of the maximum possible value of D.

Statistical analysis
Data were analyzed by MSTAT-C software for proper interpretation.The data recorded on different parameters were subjected to analysis of variance (ANOVA) and the means were separated by Duncan's Multiple Range Test (DMRT) at 1% level of significance (Gomez and Gomez, 1984).

Plant dwelling insect pests' species
Total of 18 insect pests' species from 14 families (under 8 orders) were recorded through visual searching method from insecticides free plots (Table 2).Most of the insect pests' species belong to the order Homoptera, Hemiptera,Coleoptera and Lepidoptera.Among insect pests species the highest abundance (185.67) was recorded in the species jassid followed by mealy bug (99.69), white fly (67.00), epilachna beetle (12.00), green leaf hopper (7.00) and less abundance species were leaf beetle, aphid, BSFB, flea beetle, thrips, hooded hopper, leaf roller etc.The frequency of BSFB adult was very low in the field though it was major pest of brinjal in terms of damaged shoots and fruits.The number of BSFB was very few because its adult is nocturnal in habit and no trap was used to collect the adult during night.

Plant dwelling beneficial arthropod species
Total of 8 beneficial arthropod species from 8 families (under order 6) were recorded through visual searching method from insecticides free plots (Table 3).Most of beneficial arthropods belong to the order Coleoptera, Hymenopera, Diptera and Araneae.Among natural enemies the highest abundance (126.67) was recorded in the species black ant followed by spider (18.33), ladybird beetle (11.33) and other available natural enemies were carabid beetle, mirid bug, staphylinid beetle and syrphid fly etc.

Surface dwelling insect pests' species
Total of 12 insect pests' species from 11 families (under 8 order) were recorded as surface dwelling insect pests through pitfall trap method from untreated control plots.Most of the insect pests' species belong to the orders Homoptera, Coleoptera, Hemiptera, Orthoptera and Isoptera.Available pests' species were field cricket, termite, red ant, short horned grasshopper and leaf beetle etc. (Table 4).

Surface dwelling beneficial arthropod species
Total of 9 beneficial arthropod species from 9 families (under 5 order) were recorded through pitfall trap method from insecticides free plots.Most of the beneficial arthropods belong to the order Coleoptera, Hymenoptera and Aranae (Table 5).In summer (Table 7) the highest number of natural enemy families ( 9) was recorded in Tracer 45SC treated plots followed by untreated control plots ( 8) and the lowest number of families ( 6) was recorded in Proclaim 5SG treated plots.The highest number of natural enemy abundance (163.33) was recorded in untreated control plots followed by Tracer 45SC (118.00) and Bt (104).
The lowest number of natural enemy abundance 63.00 was recorded in the plots treated with Proclaim 5SG.The diversity index was the highest (2.36) in the plots treated with Proclaim 5SG and the lowest diversity index (1.58) was obtained from the plot treated Tracer 45SC followed by untreated control (1.62) and Bt (1.72) and their diversity index increased with the increase of total abundance except Bt and Tracer 45SC.
The highest equitability 0.39 was recorded from the plots treated with Proclaim 5SG (0.39) which indicates less richness of natural enemy species of this treatment.On the other hand the lowest equitability was obtained from Tracer 45SC (0.18) followed by Bt (0.25), which indicates species richness of natural enemy of these treatments.
Table 7. Diversity of natural enemies in three selected prospective pesticides treated plots and a control plot of brinjal agro-ecosystem through visual searching method.From sweeping net method (Table 8), the highest number of arthropods families (24) was recorded in untreated control plots followed by Bt (23.00) and Tracer 45SC (22.00).The lowest number of families (12.00) was recorded in Proclaim 5SG.The highest number of arthropods (250.67) was recorded in the untreated control plots followed by Bt (193.67) and Tracer 45SC (177.00).The lowest number of arthropods 142.00 was recorded in the plots treated with Proclaim 5SG.The diversity index was the highest (6.32) in the plots treated with Bt followed by untreated control plot (5.86) though the total abundance of Bt was less than the total abundance of untreated control plot but it showed higher diversity index than the untreated control plots, which indicates more abundance of some families than the other treatments.The lowest diversity index was obtained from in the plots treated with Tracer 45SC (2.17) followed by Proclaim 5SG (3.06).
The highest equitability 0.27 was recorded in the plots treated with Bt followed by Proclaim 5SG (0.26) and untreated control (0.24), on the other hand the lowest equitability was obtained in Tracer 45SC (0.10), lower equitability indicates higher species richness of the treatment.

Conclusion
From the study, it may be concluded that collected arthropods through visual searching, sweeping net and pitfall trap methods revealed that Bacillus thuringiensis (Bt) and Tracer 45SC (Spinosad) were less harmful to arthropod species and natural enemies in the brinjal agroecosystem and thus would be suitable for IPM program for successful management of BSFB.On the other hand, Proclaim 5SG (Emamectin benzoate) was more harmful to arthropod biodiversity.

Table 1 .
List of pesticides and their groups, trade names, chemical names and concentrations used in various treatments.

Table 2 .
Abundance of insect pest species in insecticides free plots through visual searching method.

Table 3 .
Abundance of beneficial arthropod species in insecticides free plot through pitfall trap method during summer.

Table 4 .
Abundance of insect pest species in insecticides free plot through pitfall trap method.

Table 5 .
Abundance of beneficial arthropod species in insecticides free plot through pitfall trap method.

Table 6 .
Diversity of arthropods species in three selected prospective insecticides treated plots and a control plot of brinjal agro-ecosystem through visual searching method.

Table 8 .
Diversity of arthropod species in three selected prospective pesticides treated plots and a control plot of brinjal agro-ecosystem through sweep net method.

Table 9 .
Diversity of natural enemies in three selected prospective pesticides treated plots and a control plot of brinjal agro-ecosystem through sweep net method.

Table 10 .
Diversity of arthropod species in three selected prospective pesticides treated plots and a control plot of brinjal agro-ecosystem through Pitfall trap method.During summer in pitfall trap method (Table11) the highest number of natural enemy families (12) were recorded in Tracer 45SC & untreated control plots followed by 9.00 in the plots treated with Bt, the lowest number of families (6) were recorded in the plots treated with Proclaim 5SG.The highest number of natural enemy abundance (95.67) was recorded in the plots treated with Proclaim 5SG followed by Bt (83.00).The lowest number of natural enemy abundance 81.00 was recorded in the untreated control plots.The diversity index was the highest (3.39) in the plots treated with Proclaim 5SG, on the other hand the lowest diversity index (2.56) was Int.J. Agril.Res.Innov.& Tech.8 (1): 55-61, June, 2018 obtained in the plots treated with Tracer 45SC followed by Bt (2.66).The highest equitability 0.57 was recorded in the plots treated with Proclaim 5SG, on the other hand the lowest equitability 0.21 was obtained in the plots the treated with Tracer 45SC followed by untreated control plots (0.22).Lower equitability indicates higher species richness of natural enemies of these treatments.

Table 11 .
Diversity of natural enemies in three selected prospective pesticides treated plots and a control plot of brinjal agro-ecosystem through Pitfall trap method.Data are the average of 3 replications from 12 observations.