Natural parasitism of the European corn borer (ECB) eggs by Trichogramma (Hym., Trichogrammatidae) was assessed in 1993 to 1996 in south-western Slovakia. No parasitized eggs were found in 1993. In 1994, parasitized ECB eggs were observed on July 7th, July 1 lth and July 13th. The average percentage of egg parasitism was 3.86 and 1.54 at two locations. Only one parasitized egg cluster was observed at the beginning of the ECB egg laying in 1995. At the end of the egg laying period (loth, 13th, 17th and 24th July) parasitism varied between 1.29 and 100% and averaged 4.15% at the location Nitra-Malanta. Parasitism was high in 1996 at the location Nitra-Malanta and reaching an average of 15.21 YO. Parasitized eggs were detected throughout the ECB egg laying period. At Nitra-Janikovce in the same year, average parasitism reached 2.46%. The authors assume that extremely dry weather probably reduced the egg parasitoid populations in 1993-1995. The egg parasitoid species was identified as Trichogramma evanescens. The study showed that the egg parasitoide appears sporadically in corn fields, was often absent but sometimes occurred in low numbers in spring early summer and increased towards the end of the season. Therefore, the release of mass reared Trichogramma can be recommended to insure predictable biological control of the pest.
The intensity of symptoms on infested leaves was also different depending on treatments and years. The average biological activity was 82.4% in plots receiving releases of T. pyri and 58.8% in plots treated with Polysulphide-Ca.
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