The first phase - 1978-80

Much of the first-phase work, led by H.D. (David) Catling, on stem borers, principally the yellow stem borer (or yellow rice borer), Scirpophaga incertulas (Walk.), was described by Catling & Islam (1979) and subsequently by Catling (1980, 1981).

Quite rightly, the concentration was on the population biology, the life cycle and natural enemies. Much information was acquired, however, about the incidence of yellow rice borer, its importance as the major pest of deepwater rice in Bangladesh, and its population dynamics. Catling postulated that there are six broods, or generations, of yellow rice borer each year. The first infestation of deepwater rice is by brood 2 in the preflood period (May to mid-June). Brood 3 tends to coincide with the onset of flooding (mid-June to the end of July). Brood 4 is at the height of flooding (August to late September), and brood 5 infests the rice at the heading stage, as the floods subside (late September to the end of October). The seasonal patterns, however, were variable in terms of density of infested stems and in numbers of larvae and pupae. In general, from Catling's conclusions, a typical year has a fairly high infestation in brood 3, a decline in the numbers in brood 4, and a peak in brood 5. The incidence of natural enemies of yellow rice borer, both predators and parasites, is highest in brood 5, relatively low in brood 3, and depressed probably by the onset of flooding in brood 4 (Catling 1981).

Relatively little work was done to investigate possible control of S. incertulas. Basal applications of carbofuran at sowing time were used in 1979 but these gave little or no protection into the flooding period and there were no differences in yield between treated and untreated plots. In 1978, an attempt was made to compare the effect of intensive insecticide use with a no-treatment regime. Although some 22 applications of insecticide (carbofuran granules twice before flooding and then 20 sprays ofdiazinon at intervals of 6-7 days) did not give total protection from yellow rice borer attack, stem infestation at harvest was only 9.0% in the treated area compared with 33.2% in the nonsprayed area. The yield from the treated area was 26.7% higher than that from the control area. In 1979, a similar experiment (with only 20 sprays of diazinon and no carbofuran) resulted in 33.2% infestation in the nonsprayed control area and 13.5% in the treated area. The yield improvement, however, was only 15.4% and it was concluded that the trials showed the futility of trying to control yellow rice borer with insecticides alone (Catling et al 1980, Catling 1981).

Writing in 1979, however, Catling postulated that feeding by S. incertulas larvae within elongated stems must cause yield loss (Catling, 1979). The assumption underlying his decision was that larval feeding causes 'damage'.

At this point in time, we come the Mid-Term Review Mission of H.F. van Emden, S.M.H. Zaman, D. Hocking and A.S. Davis who, in a divergence of opinion, did not accept the assumption and commented on the failure to demonstrate 'what would appear to be self-evident - that panicle weights of affected stem should be lower than those of unaffected stems' and emphasised the need to establish 'the true host-plant relationship leading to yield loss' (van Emden et al., 1979).

Resolution of this issue has added importance because of the modern practice of producing simple guidelines to enable pest and disease symptoms to be identified, incidence to be assessed, and, that incidence to be used to predict potential yield loss as a basis and justification for implementing control procedures.

David Catling decided to move on the lead an IRRI team in Thailand and I was recruited as Entomologist, with responsibilities as Team Leader. I was urged to start a research programme specifically targeting the 'damage' issue.

So, in early 1981, after reviewing the first phase findings, I decided that there could be scope for depressing brood 2 and, maybe, brood 3, and thus lower the late-season population potential. The use of insecticides in the early season would avoid the possible major problems of logistics and of contamination in spraying deeply flooded rice. Such a timing was also expected to have a minimal effect on the natural enemies of yellow rice borer. A literature search showed that there had been little evaluation of insecticides for yellow borer control and almost none in deepwater rice areas. As a first step, with my colleagues, Badiul Alam and, later in the season, Quazi M.A. Razzaque, we tested a range of insecticides to see which would control yellow rice borer. Particular emphasis was on trying to establish the effects on the egg and first-instar stages.

Methods

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