Development of methods for the establishment of economic threshold level for corn borer Ostrinia furnacalis (Guenee)

Infestation in the field mostly starts during the early whorl stage of corn. Peak densities of egg masses were found to fall within 40-45 DAP (days after planting) and one week thereafter is the peak of larval populations. Egg mass production of the corn borer in different corn composites and hybrids differed significantly at high borer popular densities but not during relatively low infestation. Corn composites and hybrids, however, did not significantly differ in the distribution pattern of egg masses, i.e. random. The egg masses were randomly distributed with most plants having one egg mass/plant. The percentage of plants laid had direct relationship with the total number of egg masses indicating that the frequency distribution index (FDI, percent plants having one or more egg masses) is a good method of ETL (economic threshold level). Nevertheless, a specific ETL can not be recommended at this point and time because the egg mass density and FDI at which control should be given are not yet ascertained in the absence of yield loss versus egg mass density. With actual count of corn borer larvae, the ETL of 1.37 = 1.00 larvae per plant has been established although the method in itself may not be too practical in the farmers field. The larval ETL will be very useful in timing the release of larval parasites and predators. Four estimated ETLs from the present studies were evaluated: a. 75% having slight leaf injury (leaves with pin-size holes, leaf injury rating (LIR) of 3); b. 25% plants having moderate leaf injury (leaves with match-size and larger holes, LIR of 5-7); c. 50% damaged plants regardless of injury; d. more than 100 larvae per 100 plants. The use of ETL reduced insecticidal applications to a maximum of two as compared to four times by calendar spraying from whorl to silking stage. Although insecticidal applications based on ETL did not significantly reduce the number of borers as well as borer holes per plant yet there was a significant difference in yield between calendar spraying and ETLs