# The Current State and Future Potential of Microbial Control of Scarab Pests
## 7 Nematodes  
Entomopathogenic nematodes (EPN) are roundworms that live in the soil and are obligate parasites of insect hosts. Despite the existence of over 14,000 known species of nematodes, only two genera, _Steinernema_ and _Heterorhabditis_, comprising -120 species, are used for insect biocontrol. Many species of _Steinernema_ and _Heterorhabditis_ naturally occur globally. Both genera associate with bacterial symbionts, _Steinernema_ with _Xenorhabditis_ and _Heterorhabditis_ with _Photorhabtus_, that assist the nematodes in sequestering their host by producing toxins that kill the insect and inhibit other bacteria from utilizing the cadaver [205]. EPNs are an excellent option for insect biocontrol, as they are non-toxic to humans, and therefore not regulated by the U.S. Environmental Protection Agency (EPA). Most can be mass-produced and applied with common pesticide equipment. They can also be used therapeutically, as they can kill their hosts within days of infection [205; 206]. In natural systems, the most widespread EPN species infecting white grubs are _S. carpoaspase_, _S. Jeltiae_, _S. glaseri_, _H. bacteriorlora_, and _H. megidis_; however, studies reporting EPN distribution are limited and often not randomly sampled [207; 208]. _S. carpoaspase_, _S. Jeltiae_, and _H. bacteriorlora_ are generalist species but only _H. bacteriorophora_ has been documented to infect white grubs to any extent under natural conditions [207]. _S. glaseri_ and _H. megidis_ appear to have close natural associations with scarabs, and it is hypothesized that the less well-known _S. anomoli_ and _S. kushidiai_ species may also be scarab-specialists [207; 209]. Despite these natural associations, other EPNs have been tested on different white grub species under field and lab conditions [210; 211]. Table 3 shows summaries of these studies, indicating which EPN species have been tested on scarab species and whether the results showed high or low efficacy. Across field studies, _H. bacteriorlora_, _S. scarabaei_, and _S. carpoaspase_ show the broadest efficacy for scarab species. In the lab, results were more mixed for _H. bacteriorlora_, while _S. scarabaei_ maintained high efficacy (Table 3). Differences between field and lab results are likely due to many environmental factors but also to differences in methodology and the nematode concentrations tested. Across the range of existing EPN species, _H. bacteriophora, S. carpoaspase, S. feltiae_ are the most widely available commercial EPNs, followed by _S. kraussei_, and _S. glaseri_ (Table 1). Surprisingly, _S. scarabaaei_, which shows the most consistent infectivity on white grub species across studies, is only currently available in one product, Nemgard (Table 1). Although some white grub species have developed barriers to EPN infection [212], Japanese beetle, which is the most problematic species in the U.S., appears to be highly susceptible to EPNs [212; 213]. Overall, studies have shown that under the right conditions, EPN can control white grub populations as effectively as conventional insecticides [214; 215; 216]. However, results vary considerably across environments. Soil temperature, texture, and oxygen levels impact EPN activity and viability, while pH and salinity appear to be less important [217; 218; 219]. Selecting the appropriate EPN species for white grub control is also important, as different species employ foraging strategies that are optimized for pest occurring above-ground, at the surface, and below-ground. The timing of application is key, as grub instar also strongly impacts pathogenicity, with EPN effectiveness being better on early versus late instars [220; 221]. Despite being a fast-acting, biomedical option, EPNs continually constitute a small share of the biopesticide market [222]. Their short shelf life, cost, and requirement for technical knowledge in selecting and applying different strains are likely impediments to market expansion.