When conducting a routine screening of the air potato leaf beetle for disease, MWLR technicians discovered gregarines in the intestines of the beetles. This spurred a thorough investigation of where these gregarines came from as well as a lengthy process to get rid of the gregarines in the leaf beetles.
Image: A gregarine through a microscope.
But first, what are gregarines? Just as humans have single-celled organisms living inside them, so too do insects. Gregarines are single-celled eukaryotes that live in the gut of invertebrates. They can be symbiotic, commensal or parasitic. They are host-specific and can be transmitted to the offspring of individuals via faeces. Gregarines have a very diverse morphology, taxonomy, and genetics. There’s limited research on them because they are difficult to study; they cannot live outside of their host, they have a high diversity, and standard genetic markers cannot distinguish them from other eukaryotes and their insect hosts.
Because we did not know much about the gregarines we found in the gut of the leaf beetles we had to treat them as if they were parasitic. We therefore increased our hygiene measures in our insect containment facility by sterilising the eggs of the beetles and keeping them separate from the infected adults. We also trained our staff in sample preparation and microscopy to identify the presence of gregarines.
We continued to assess the colony health of the beetles and tested to see if the infection had transferred to other insects in the containment facility. Thankfully we did not find gregarines in the other insects, indicating that they had not transmitted to other insect species in the facility. It’s therefore likely that the gregarines we found in the air potato leaf beetles are host specific.
Image: air potato leaf beetle larvae.
We also investigated where the gregarines came from. We contacted our collaborators in Florida from where we had imported our colony of air potato leaf beetles, and they also found gregarines in their lab colony, as well as in wild populations of the air potato leaf beetle. It is thus highly likely that the gregarines in our colony of air potato leaf beetles came from the hosts in Florida. Interestingly, both our New Zealand and Florida colonies of the air potato leaf beetles were healthy and performing well. They were free of disease symptoms such as high mortality, low fecundity, and deformities, so we have concluded that these gregarines are not having a negative effect on their host beetles.
Current gregarine levels in the air potato leaf beetles are very low, but we have not been able to prove their absence from the colony yet because the infection load of gregarines may be too low to isolate enough gregarines from the host gut. From here our approach is to continue to control the gregarine infection to avoid overburdening the insects. We have also adjusted our release strategy for this biocontrol agent. In the previous newsletter issue we discussed the release of the air potato leaf beetle in Niue. To conduct this release safely we exported surface-sterilised eggs to Niue and then released the freshly hatched larvae.
Throughout this process we gathered information from our collaborators and the literature to improve our methods for clean rearing, microscopy, and molecular analysis. Because not a lot is known about gregarines, in the future we would like to study their transmission mechanisms and host range, as well as the effects of different concentrations or loads of gregarines on the host health. It would also be useful to develop specific molecular methods to identify the species of gregarines detected.
While some mysteries relating to gregarines remain unsolved, this experience has improved our safety measures for biocontrol and our understanding of the complexity of insects and their gut. The discovery of gregarines in the air potato leaf beetle taught us that routine disease screening is essential. We also learned the importance of routinely recording colony health and performance so that we can effectively monitor future populations.
This investigation of gregarines is part of MWLR’s Beating Weeds Programme, funded by the Ministry of Business, Innovation and Employment's Strategic Science Investment Fund.
