Experiências sobre o combate do Heliothis obsoleta Fabr., praga do milho (Lep. Noctuidae)

Resumo

This paper reports the results of some preliminary field experiments in the control of the corn earworm, Heliothis obsoletu Fabr. (Noctuidae, Lepidoptera) at the State Agricultural College of Minas Gerais, Viçosa, Brasil. 

A field of young corn in which 66% of the crowns were attacked by earworms was divided in four areas for treatment. Analyses of variance of field data showed no statistical difference between the number of plants attacked in the four plots, and no significant difference in the distribution of the worms. The area having the smallest number of worms per plant was taken as the control and leit untreated. Three statistically identical plots were treated as follows: Plot Nº. 2 received a small quantity of wet poisoned bait placed in the crown of each plant. This bait was made of wheat bran, sawdust, white arsenic (3% by weight), molasses, and water. Plot N°. 3 was dusted with a mixture of equal parts Paris green and wheat flour. The poisoned bran mash was spread broadcast in Plot Nº. 4, At two day intervals obser- vations and statistical counts were made of the number of plants attacked, number of worms per plot, and number of dead worms in the treated areas. The control showed a normal rise in number of ear- worms. There was no statistically significant reduction of the infestation in Plot Ne. 4 attributable to the broadcasting of poisoned bait. It is indicated that the corn earworm cannot be efficiently controlled by spreading poisoned bait, as is practiced against grasshoppers and army worms. Plot N.º 2 showed a larval mortality of 70% by the sixth day, and Plot N.º 3, 64% larval mortality in 6 days. There was no statistically significant difference between results from these two methods of treatment. It is suggested that in cases where the cost of treatment is a secondary consideration, either of these two methods may be employed to reduce earworm damage in young corn. It is emphasized that Uh ordinary field conditions, chemical control is rarely prac- tical. A table giving the arithmetical results of the experi- ment and three graphs showing normal infestation and sur- vival curves are included. All statistical analyses of variance are made in accord with the methods of Fisher and Snedecor. A short summary of the life history of Heliothis obsoleta is given, as well as a recapitulation of general control measures.