Biological control of Diatraea spp . ( Lepidoptera : Pyralidae ) in sugarcane crops in Central Venezuela

In 1990-1996, we evaluated the relative importance of the Amazonian fly (Metagonystilum minense) and the wasp (Cotesia flavipes), in sugarcane crops, in Central Venezuela. We observed a significant decrease in relative abundance of Diatraea spp. when the Amazon fly abundance increased but not with the wasp abundance. Diatraea spp. abundance in presence of both parasitoids was similar to its abundance throughout the last 45 years, when only inundations of M. minense occurred. However a net effect by using both parasitoids was not evident. M. minense is probably the stronger pest control, because its more efficient in its host searching than C. flavipes and is better as competitor than the wasp. Rev. Biol. Trop. 55 (2): 655-658. Epub 2007 June, 29.

Flooding agroecosystems with parasitoid insects is sometimes very effective in lowering the abundance of crop pest insects (Knipling and McGuire 1968, Knipling 1970, 1972, Knipling and Gilmore 1971).Thus, parasitoids are commonly reared in laboratories and periodically liberated in high-density populations as biological control agents of lepidopterous crop pests (Barclay et al. 1985).The effectiveness of this biological control technique may be improved by adjusting parasitic inundation rates, until the pest population reaches a very low level (Barclay et al. 1985), and by evaluating the stability of parasitic-host system after inundation stops.
The purpose of this study is to evaluate the role of two parasitic species, the Amazonian fly Metagonystilum minense Myers (Diptera: Tachinidae), and the wasp Cotesia flavipes Cameron (Hymenoptera: Braconidae), in regulating sugarcane borers Diatraea saccharalis Fabricius, and Diatraea rosa Heinr (Lepidoptera: Pyralidae) in sugarcane crops in Central Venezuela.

MATERIALS AND METHODS
To determine the relative abundance of Diatraea spp., M. minense and C. flavipes, we sampled sugarcane harvests yearly, from 1990 to 1996, in Carabobo and Aragua States, Central Venezuela.During each harvest, we counted the number of stem holes, larvae and pupae of Diatraea spp., and pupae of M. minense and C. flavipes on 100 randomly selected sugarcane stems from each of 100, one ha plots (total=10 000 sterns).
These methods employed to estimate population were similar enough to those used 1950-1989.Proportions of parasitized and nonparasitized Diatraea spp.were correlated with the abundance of M. minense and C. flavipes reared in the laboratory, and liberated on the farms.Laboratory work was done at the Entomology Laboratory of Santa Teresa Farm, El Consejo, Aragua State.
Stems were dissected in the laboratory to extract Diatraea larvae or pupae, and M. minense and C. flavipes pupae.Each Diatraea larva was placed in a vial and given a corn diet.Food was replaced every two days.Larvae were maintained in vials to molting pupae or to emergence of the parasitoid pupae.Diatraea spp., M. minense and C. flavipes pupae were placed in a Petri dish with absorbent paper.
We performed Kolmogorov-Smirnov and Schefee-Box tests to evaluate normality and homogeneity of variance (Sokal and Rohlf 1995), a parametric analysis of variance when was homogeneous (Sokal and Rohlf 1995), and Kruskal-Wallis test when variance was not homogeneous (Conover 1980).

RESULTS
The relative abundance of Diatraea spp. in sugarcane crops and proportions/ha, of laboratory reared M. minense and C. flavipes, are shown in Figure 1.The impact of mass liberations of these parasitoids on the sugarcane borer population has been important, particularly for M. minense.M. minense has controlled the pests for 50 years with a mean relative abundance range of 0.9 to 4.2 %.Changes in Diatraea spp.population levels showed a significant relationship with M. minense released between 1950 to 1989 (F=99.9, α=0.0001).
During the last six years of biological control (Fig. 2), changes in Diatraea spp.population density showed a significant relationship with M. minese in this period (F=6.17,α=0.05), but the relationship with C. flavipes was unsignificant (F=2.99,ns).However, the parasitization frequency of Diatraea hosts by C. flavipes was similar to that of M. minense, suggesting that C. flavipes is less efficient than M. minense as a parasitoid, because ten times more C. flavipes must be liberated to equal the effect produced by M. minense.

DISCUSSION
For six years, biological control by both M. minense and C. flavipes has used to control Diatraea spp.populations in sugarcane crops in Venezuela.A net effect by using both parasitoids is not evident.The recent trend of Diatraea abundance in presence of both parasitoids is similar to its abundance throughout the last 45 years, when only inundations of M. minense occurred (Fig. 1).However, M. minense probably has had a stronger effect as control agent because is more efficient as parasitoid (Data of Central El Palmar), and better competitor than C. flavipes (Weir and Sagarzazu 1998).
A reduction in Diatraea spp.abundance to lower levels by the parasitoids and partial or total parasitoid extinction may be a result of artificial inundation practices of both parasitoid species (Box 1956, this study).The final balance is an unstable interaction where parasitoid extinction occurs in many patches of sugarcane crops because of these low levels and to the intra and interspecific superparasitism caused by inundation practices (Ferrer et al. 1990, Weir 1991, Weir and Sagarzazu 1998).At present, recurrent inundation of parasitoids has avoided a population increase of Diatraea to epidemic levels in Venezuela (Ferrer et al. 1990, Micale 1995), Colombia (Gaviria 1990) and Brazil (Botelho et al. 1980).Regulation of artificial production of parasitoids, as well as parasitoid release in sugarcane crops, might avoid the intraspecific and interspecific superparasitism and stabilize parasitoid-host interactions.This action would allow a reduction of parasitoids released in sugarcane crops.

ACKNOWLEDGMENTS
This research was supported by the entomology Laboratory of Santa Teresa Farm.We thank Roque Morejon for his assistance in this study and Ivan Penzo and Clark Casler for reviewing and improving the manuscript.

Fig. 1 .
Fig. 1.Changes in host and parasitoid abundance in sugarcane field.A. Relative abundance of Diatraea sp.(estimated by the percent of stems bored).B y C. Shows the abundance of M. minense y C. flavipes respectively, liberated in sugarcane fields.