3.3.4. Nematodes

Nematodes of the family *Mermithidae* parasitize arthropods, mainly insects with at least 15 different orders as hosts [123]. The use of *Mermithidae* has been discussed for biocontrol of mosquitoes [124], but they are occasionally also found in click beetles or wireworms [122,125]. Considering the low densities and propagation difficulties, they are not considered to be suitable candidates for wireworm control.

Entomopathogenic nematodes (EPN) from the genera *Steinernema* and *Heterorhabditis*(Nematoda: Steinernematidae, Heterorhabditidae) with their bacterial symbionts *Xenorhabdus* spp. and *Photorhabdus* spp. (Gram-negative Enterobacteriaceae) are bacterium–nematode pairs and pathogenic to a broad range of insects (Figure 4C). Species from both nematode genera have also been successfully implemented in biological control of insect pests throughout the world [126]. However, wireworms often show very low susceptibility [127] or are sometimes even considered to be resistant to EPN [128]. This may partly be due to unsuitable species combinations, as there are also several cases with successful infection by EPN and damage reduction in the field [129–131]. For example, larvae of *A. lineatus* were reduced by *Heterorhabditis bacteriophora* and *Steinernema carpocapsae,* but not by *Steinernema feltiae* [132,133]. Lehmhus [42] showed differences in mortality for the same three EPN when they attacked four common European wireworms *Agriotes lineatus, A. obscurus*, *A. sputator*, and *Selatosomus aeneus*. All EPN did cause mortality in the three *Agriotes* species, but *S. feltiae* failed to cause mortality in *S. aeneus*, which was also the least sensitive wireworm to the other EPN. According to Campos-Herrera and Gutiérrez [127], a Spanish isolate of *Steinernema feltiae* performed poorly against *Agriotes sordidus*. Ansari et al. [132] demonstrated that there were considerable differences in the mortality of a wireworm (*Agriotes lineatus*) caused by different EPN species and even by different strains of a single EPN species (0–67% mortality). Rahatkah et al. [134] showed that after injection of infectious juveniles, the immune reactions of the same wireworm species (*Agriotes lineatus*) to different nematode species (*Heterorhabditis bacteriophora* and *Steinernema carpocapsae*) differed with a higher encapsulation of infectious juveniles from the former, which may be one reason for nematode strains performing differently. Morton and Garcia del Pino [135]

found that the mortality of *Agriotes obscurus* in the lab was dependent both on nematode species and on infectious juvenile dose rates, while under field conditions, a dose of 100 IJs/cm<sup>2</sup> and the best performing strain *Steinernema carpocapsae* (Weiser) B14 still resulted in nearly 50% mortality. These results indicate that in entomopathogenic nematodes, the control achieved against wireworms is, besides the environmental factors discussed below, dependent on the concentration of infectious juveniles and on the combination of nematode strain and wireworm species.
