# The Current State and Future Potential of Microbial Control of Scarab Pests
## 9 Synergistic Interactions  
Given that most microbial controls are slower-acting than curative chemical options, many studies have explored the possibility of combining different microbes to improve infectivity, mortality, and/or mortality rate. Combining the use of nematodes, which are more mobile, with bacterial and fungal options can increase microbial contact with hosts and infectivity, while other combinations might simply weaken the host faster, leading to faster mortality rates. Few studies have looked at interactions between bacteria and fungi. However, in a lab experiment, [PERSON] [255] found that dual exposure of New Zealand grass grub larvae (_C. zealandica_) to _M. anisopliae_ and _S. entomoplila_ resulted in synergistic negative effects on larval mortality. Importantly, the dual treatment was not impacted by larval instar, while the single exposure treatments were. Given that larval instar strongly impacts the efficacy of most microbial pathogens, this result highlights how dual treatments may overcome some of the common obstacles found for microbial insecticides. Far more research has been performed on nematode-bacteria and nematode-fungi combinations and their impacts on scarab pests. [PERSON] et al. [256] found that dual infection with _B. popilliae_ bacteria and the nematode _H. bacterioplora_ increased the nematodes' ability to penetrate the midgut in of masked chafer larval hosts, greatly increasing the subsequent number infective juveniles emerging from _B. popilliae_-infected cadavers. However, in a subsequent pot trail, [PERSON] et al. [257] showed that _B. popilliae_-nematode combinations only resulted in increased grub mortality when _S. glaseri_ was used, not _H. bacterioplora_. This suggests that dual treatments of _B. popilliae_ and _S. glaseri_ may increase grub mortality in the field but also that pre-treatment with bacterial pathogens may improve the ability of other nematode species to become established by improving host utilization. Another pot study with _C. hirita_ and _C. pasadena_ showed that dual exposure to _B. thuringiensis japonicus_ (Buibui strain) and either _H. bacterioplora_ or _S. glaseri_ resulted in additive and synergistic effects on grub mortality [258]. No interactions were found for _S. kushidai_ and larvae had to be exposed to _B. thuringiensis japonicus_ for at least 7 days before nematode inoculation. Another study on the oriental beetle (_A. orientalis_) also showed synergistic effects of _B. thuringiensis japonicus-S. glaseri_ dual treatment [259]. While _H. bacterioplora_ and _S. glaseri_ are easy to find, unfortunately _B. thuringiensis japonicus_ is not currently commercially available in the U.S.  
[PERSON] et al. [260] tested interactions between the entomopathogenic fungi, _B. beauveria_ and _M. anisopliae_, with two nematode species, _H. megidis_ and _H. bacterioplora_, on the southern masked chafer (_C. lurida_) under lab and greenhouse conditions. In the lab, neither fungus had an impact on grub mortality alone, but when applied in combination with nematodes, they found additive or synergistic effects in all _M. anisopliae_-nematode combinations and mostly additive effects in all _B. beauveria_-nematode combinations. In the greenhouse, additive/synergistic effects were found for _M. anisopliae_-nematode combinations but only some _B. beauveria_-nematode formulations. In fact, the control seen in some combinations was comparable to that of imidaloprid application. Nematode infection rate and/or infective juvenile production, however, was not impacted, suggesting that, unlike bacteria, fungi do not facilitate nematode infection. _M. anisopliae_ was also found to increase monkey beetle (_H. philanthus_) grub mortality in lab and greenhouse experiments when combined with either _H. megidis_ or _S. glaseri_[261]. Effects were additive or synergistic, and longer incubation periods with _M. anisopliae_ enhanced effects.