Two studies were conducted in farmer’s rice field at four upazila viz., Dumki, Bauphal, Mirjagong and Patuakhali sader of Patuakhali district and Patuakhali Science and Technology University (PSTU) farm during July 2010 to January 2011. Study 1: Four locations of Patuakhali district viz., Dumki, Bauphal, Mirjagong and Patuakhali sader were selected randomly to carry out the survey programme. Surveys were conducted in 10 randomly selected farmer’s rice fields of each location and Patuakhali Science and Technology University (PSTU) farm. Different rice varieties viz., lalmota, moulata, chinigura, HYV rice and Accession No. 20 were included in this study at PSTU farm. Data were collected from rice seedling to harvest. The total developmental period of rice plant was divided into four growth stages viz., Early tillering, mid tillering, maximum tillering and panicle initiation stage. The insect pests of rice and their natural enemies were observed and collected by a fine nylon cloth sweep net (30 cm diameter) and an aspirator. The aspirator was made up two pieces of rubber tubes, the length of the tubes were 37.5 cm and 9 cm respectively and their diameter were 0.8 cm and 0.7 cm respectively. Sweeping was done from the plant canopy level including the interspaces between plants as well as close to basal region of the plants as far as possible. In each field 10 complete sweeps were made to collect the insect pests and their natural enemies. Sampling was done at 7 days intervals at the above mentioned period during morning hours at all study fields on all sampling dates. Ten samples were taken in every growth phase of rice plant for recording insect pests and natural enemies. Immediately after collection, the samples were kept separately in labeled polyethylene bag. The open end of the polyethylene bag was closed with rubber bands. The collected samples of the insect pests and natural enemies of 10 complete sweeps from each field were preserved separately in labeled container/sample bottle. The samples were identified under magnifying glass. After that the collected samples were properly sorted and counted in the laboratory of the Department of Entomology, Patuakhali Science and Technology University, Dumki, Patuakhali. Study 2: The study was also conducted in farmer’s rice field at four upazila mentioned in study No. 1 including Patuakhali Science and Technology University (PSTU) farm. The local rice variety viz., Lalmota was grown in all the selected farmer’s fields by the farmers in aman season. All sorts of intercultural operations and crop management were done by the farmers. No pesticide was used in the fields during the period of study. To study the relative abundance and species diversity of pests and natural enemies, four habitats were considered viz., rice-banana, rice-homestead, rice vegetables and rice-rice. The insect pests of rice and their natural enemies were collected by the same way mentioned in study No. 1. The collected samples were properly identified, sorted and counted under magnifying glass in the laboratory of the Department of Entomology, Patuakhali Science and Technology University, Dumki, Patuakhali. Data were analyzed following MSTAT program and means were separated by DMRT.
Relative abundance of insect pests and natural enemies was calculated using the following formula:
Total no. of each species
Relative abundance (%) = _________________________ × 100
Total no. of all species
Calculation of diversity indices
The following indices were used to measure the diversity of insect pests and natural enemies from the original data recorded from different rice habitat.
Species diversity
Menzies et al. (1973) define diversity as a community ecological concept which refers to the heterogeneity in a community or assemblage of different organisms. Thus diversity is dependent upon the number of species present (Species richness, S) and the distribution of all individuals among the species (Equitability or evenness). To provide an overview of diversity, the Shannon-
Weaver Index of Diversity was calculated (Shannon & Weaver 1963). The index is expressed as-
ST
H= - ∑ Pi log Pi
i= 1 Where, Pi = the proportion of individuals in the ith species and
ST= the total species
ni
[ Pi =_______ ; ni = the number of individuals observed for each species and
N N = the total number of individuals in each study area].
This index is based on information theory, where diversity is equated to the amount of uncertainty which exists as to the identity of an individual collected at random from a community.
Species richness
To provide a cohensive overview of species richness, Margalef’s Index was also calculated along with S (actual number of species collected). Margalef's index (Margalef 1958) assumes a theoretical relationship between the number of individuals (N) and the number of species (S) in a sample and is expressed as follows:
S-1
M.I. =
logeN
The index logarithmically scales the value of S, and hence provides a means of comparison between stations with different ratios of S and N.
Equitability or Evenness
Equitability is considered a component of diversity, in that it provides an idea about the evenness of species distribution at a site. Usually a positive correlation exists between diversity and equitability (Delong 1975) i.e. a high equitability would indicate a high diversity and probably a 'healthy condition' of a fauna. Pielou's (1966) method of measuring equitability is most widely used.
The computational formula is:
H
J=
logeS
Where,
H=Shannon's index
S=Total species collected
The index value ranges from 0 to 1, with a value of 1 being the maximum possible evenness in the community.
Another diversity index was calculated using the Berger-Parker’s Dominance Index (d).
Nmax
d = _______
NT Where, Nmax = the total no. of individuals of the most abundant species
NT = Total no. of individuals of all species collected
Hence, the reciprocal form of the index is,
D= 1/d where, d= Berger-Parker’s Dominance index (Southwood 1978).