My plan for 1982 for the DWR Project as a whole was that we would do best to concentrate efforts on a few representative areas and laying down trials of agronomic practices in farmers' fields. These trials would be overlaid with trials of crop protection methods, including insecticide use. In suitable areas, a practice that might prove ideal from the point of view of insecticide use was transplanting the deepwater rice. The protection of the young plants in a discrete area, such as a seedbed, would have obvious advantages.

General observations of stem-borer incidence

Field trips to several areas of Bangladesh were made before flooding and between flowering and harvest. The pre-flood stem-borer activity was very low, with around 3% of stems infested. Although S. incertulas was predominant, Chilo polychrysus (Meyrick) and Sesamia inferens (Walker) also were found. After flowering, infestation had risen to 31%, but S. incertulas was found almost exclusively. At harvest, an arbitrary threshold of 40% stems infested was exceeded in 27% of the fields sampled.

Evaluation of the use of insecticides

Pre-flood insecticide application

Plans were made to follow the 1981 work by incorporating a superimposed insecticide treatment in agronomic factorial trials at five geographically separated sites (Daudkandi, Comilla District; Dubail, Tangail District; Rajbari, Joydebpur, Dhaka District; Manikganj (south), Dhaka District; and Narshingdi, Dhaka District) representative of the major river floodplains. The timing and frequency of spraying would have been determined from a long-term stem-borer population monitoring study initiated at Dubail.

The early season, however, was notable for an almost total absence of stem-borers. Stem samples taken from all 64 plots at each of the factorial trials in the period 21 June to 10 July, coinciding with the first build-up of floodwaters, revealed mean infestation rates of no more than 2-5% at four of the sites. At the fifth site, infestation was higher, 7-3%, but this was due primarily to C. polychrysus. In 1981, the comparable levels had shown a peak of infestation in May of 19%, with an associated deadheart level of 18%. Thus, there was no justification for any pre-flood applications of insecticide in 1982.

Insecticide application during flooded conditions

When both the monitoring study and field surveys showed considerable increases in adult S. incertulas numbers in mid-August, it was decided that application of insecticide, as a means of increasing the mortality of egg masses and first-instar larvae (before they penetrated the rice stems), should be evaluated. Part of each of five farmers' fields at Dubail and three fields in the uncontrolled flooding area of the Dhaka-Narayanganj-Demra (DND) Project were sprayed during September and October. The spraying was done from a boat punted through the field. In most instances, a motorized mist-blower was used, but on one occasion a spray boom with spinning disc applicators (Herbi) (Catling et al., 1980) was used.

Sprayboom with Herbi applicators

Motorized mist-blower in use

Both methods gave a spray swathe some 12 m wide. Monocrotophos (Azodrin 40 WSC) was applied at a dose rate of 250ga.i./ha. About one week before harvest was expected, panicle and whitehead densities were estimated in the sprayed and unsprayed areas of each field by making 20 counts of 1m² quadrats in each area. Between 100 and 200 stems were sampled from each area and dissected. The effect of insecticide application on borer infestation and on whitehead numbers is shown in Table III.

Because there were two sites and four rice varieties, the data were not grouped for analysis, but the whitehead and infestation levels in the sprayed and unsprayed areas of each field were compared by ttest. In six of the eight fields and for three of the four varieties, whitehead numbers were significantly higher in the unsprayed areas. There were significantly higher levels of infestation in the unsprayed areas of six of the eight fields. Only four of the fields had significant reductions in both infestation and whiteheads.

Yield estimation, by harvesting two 16-m² portions, was possible in only two of the fields. Both of these were at Dubail and both were of the variety Boron Bawalia. In Field 3, the results were: yield from unsprayed area 1.756 t/ha, yield from sprayed area 1.894 t/ha, yield saved 0.138 t/ha or 7.3%. In Field 5, the results were: yield from unsprayed area 2.081 t/ha, yield from sprayed area 2.311 t/ha, yield saved 0.230 t/ha or 10.0%. These yield savings were of the same order as the reductions in whitehead numbers of 8.3% and 7.4%, respectively.

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