Abstract: Between 2009 and 2013 several field experiments were carried out in order to show efficiency and reliability of various application rates of the commercial product Dianem™ based on insect parasitic nematodes (Heterorhabditis bacteriophora). Artificial infestation of maize plants with 300 to 500 WCR eggs per plant resulted in about 40 beetles per plant emerging from soil (and was a basis for reliable results). Application of Dianem reduced populations of WCR up to 65% in some trials, however, it was less effective in root protection. In order to achieve high efficacy application rates of 2-3 x 109 nematodes/ha are necessary. Proper application of nematodes was crucial for good efficacy.