DENSITY DEPENDENT PARASITISM AND HOST-KILLING OF LIRIOMYZA TRIFOLII (DIPTERA: AGROMYZIDAE) BY DIGLYPHUS INTERMEDIUS (HYMENOPTERA: EULOPHIDAE)

Abstract

Liriomyza trifolii (Burgess) is an important leafmining pest of numerous ornamental and vegetable crops. The pest is attacked by many species of parasitoids which inflict heavy mortality in the absence of insecticides. The functional response, as well as parasitoid-induced mortality, of one major parasitoid, Diglyphus intermedius (Girault), was estimated over a range of densities of third instar L. trifolii at 25-27°C in the laboratory. The functional response of D. intermedius was given by the equation Y = Kp·n2·(1-exp(-n1/Kh)) where Y, the rate of parasitism, is the number of hosts parasitized per day; Kp is a constant of 7.3908 hosts parasitized per parasitoid-day; n1 and n2 are the densities of leafminer larvae and parasitoid adults (numbers/cm2 leaf area), respectively, and Kh is a constant of 0.0144 leafminer larvae/cm2 leaf area. The relationship between host and parasitoid density and parasitoid-induced host mortality was given by Z = Cp·n2·(1-exp(-n1/Ch)) where Z, the rate of parasitoid-induced mortality, is the number of leafminer larvae killed per day; Cp is a constant of 9.2064 hosts killed per parasitoid-day and Ch is a constant of 0.0165 leafminer larvae/cm2 leaf area. The observed rate of parasitism at a particular parasitoid density was always lower than the observed rate of parasitoid-induced host mortality at that density. Lower leafminer larval densities resulted in increased multiple oviposition by D. intermedius. When eggs of the parasitoid were placed at increasing densities on leafminer larvae in artificial mines, the number of parasitoid eggs surviving to adulthood decreased while the number of individuals surviving per host tended to remain at about one.

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