A survey on the response of the last instar larvae of acorn weevil, Curculio glandium (Col.: Curculionidae), to entomopathogenic nematodes Steinernema bicornutum and Heterorhabditis bacteriophora in the laboratory

The acorn weevil, Curculio glandium Marsham, is an important forest pest of oak trees in most of countries as well as Iran. The pest disturbs regeneration of host trees by feeding on their acorn. The necessitate to control the pest as well as limitations of chemical pesticide application in natural resources, different indigenous entomopathogenic nematodes (EPNs): Heterorhabditis bacteriophora and Steinernema bicornutum were tested under laboratory conditions upon the fifth instar larvae of acorn weevil collected from Arasbaran forest. In the first experiment, penetration assay was conducted using a suspension of 4000 IJs of the EPNs per 1 ml distilled water in multi-well plates. The plates were incubated for 40 h at 25ºC and dead larvae were dissected. Penetration percentage was %1.6 for H. bacteriophora and %0.55 for S. bicornutum. In the second experiment, H. bacteriophora and S. bicornutum were applied at different concentrations (0, 150, 250, 500, 1000 and 2000 IJs per 1ml of distilled water) in the 9 cm Petri plates lined with filter papers in order to test their capability of parasitization of the fifth instar larvae of C. glandium. The experiments were conducted at two temperature ranges (21-24ºC and 25-28ºC). Maximum mortality caused by H. bacteriophora and S. bicornutum were %58.3, %25 (at 21-24ºC) and %63.5, %30.5 (at 25-28ºC), respectively. Therefore, H. bacteriophora caused higher larval mortality comparing to S. bicornutum at both temperature ranges. Analysis of variance revealed significant nematode species and concentration as well as temperature effects on larval mortality. By increasing of concentration of nematode and temperature, larval mortality was raised. The highest penetration in larva and the highest mortality of fifth instar larvae of C. glandium was observed for H. bacteriophora under the both temperature ranges. Based on probit analysis, the LC50 of H. bacteriophora at two temperature ranges of 21-24ºC and 25-28ºC were determined 1331 and 1037 IJs/ml, respectively. Regression analysis showed significant relationship between concentrations and larval mortality in both nematodes and both temperature ranges. Therefore, H. bacteriophora comparing to S. bicornutum is more effective and can be suggested for complementary studies toward finding a suitable biocontrol agent of the pest.