Rickettsiella spp. are tiny obligate intracellular bacteria frequently pathogenic to a range of arthropods. As a consequence of being difficult to diagnose, little is known about their biology and ecology, and the importance of Rickettsiella diseases in insect population regulation has been under estimated. Land use change to increase agricultural productivity has produced unintended consequences by generating wide scale pest outbreaks that threaten economic viability of the development initiatives. On the West Coast of the South Island in New Zealand, land improvement through ‘flipping’ and ‘hump & hollow’ earth movement has created productive pasture land, but produced a widespread outbreak of manuka beetles (Pyronota spp.) reaching unprecedented densities and causing severe damage to pasture. Over time a reduction of manuka beetle densities back to ‘normal’ levels was observed, and it was determined to be the result of an epizootic of bacterial disease caused by the Rickettsiella popilliae pathotype ‘Rickettsiella pyronotae’, which spread through the outbreak pest group.

Marshall SDG, Townsend RJ, Kleespies RG, van Koten C, Jackson TA (2017) An Epizootic of Rickettsiella Infection Emerges from an Invasive Scarab Pest Outbreak Following Land Use Change in New Zealand. Ann Clin Cytol Pathol 3(3): 1058.

•    Rickettsiella
•    Pyronota spp.
•    Manuka beetle
•    Biocontrol

Bacteria of the genus Ricketsiella have been long known as insect pathogens infecting a wide range of insect hosts [1]. The genus Rickettsiella has been assigned to the taxonomic family Coxiellaceae in the order Legionellales of the Gammaproteobacteria [2]. The Rickettsiella popilliae type species consists of a number of synonymized pathotypes that have been associated with specific insect diseases, and seem particularly related to soil dwelling species, but surprisingly from different insect orders (Coleoptera, Lepidoptera, Diptera). Rickettsiella spp. are also associated with crickets and ticks; R. gryllii causes behavioural fever in crickets and Rickettsiella spp. form mutual associations with ixodid ticks and can be transmitted to mammals [3,4]. It is also notable that strains of Rickettsiella are also being identified as maternally-inherited endosymbionts [5]. Rickettsiella popilliae is known as a pathogen of soil dwelling Scarabaeidae in New Zealand [6]. It is a fastidious intracellular pathogen that typically target the fat body and hemolymph cells of the host [1]. The infective cells are small, dense rods (approximately 0.2 by 0.6 µm) that are ingested during feeding and subsequently pass through the midgut epithelium to the hemocoel where they enter host cells (e.g. fat bodies) through endocytosis. Once within the cell, pleiomorphic forms develop within the cytoplasmic vacuoles, varying from bacteria-like secondary cells to large, round stroma. As the disease develops, characteristic protein crystals form and the bacterial cells revert to small rods that fill the host cells, which appear as rapid Brownian motion under light microscopy observation. Eventually, the infected insect cells lyse, releasing the multitude of Rickettsiella cells and crystals into the hemolymph, producing the white to blue coloration of infected larvae. Diseases of insects caused by R. popilliae are frequently chronic. The infected host loses vigour over time, becomes whitened, and eventually dies.

In this case report we document an epizootic of R. popilliae disease that occurred during an outbreak of manuka beetles (Pyronota setosa and P. festiva) causing beetle numbers to decline and the pasture to recover from damage.

The outbreak of manuka beetles occurred in an area of large-scale land development from swamp to pasture on the West Coast of the New Zealand South Island [8]. Pastures were monitored to assess pest numbers and R. popilliae (pathotype ‘R. pyronotae’) was isolated from manuka beetle larvae [7,8]. Five pasture paddocks were systematically sampled from 2009- 2012 to monitor insect numbers and possible infections with approximately 12,500 manuka beetle larvae collected over this period. Pest enumeration assessments were made by taking 20 soil spade samples (15 x 15 x 20 cm) per paddock along transects, and insect larvae extracted and counted to provide an estimate of manuka beetle density. Larvae were identified to species level in the laboratory, while estimates of disease rates were determined from sub-samples of collected insects by a combination of observation for characteristic symptoms of infection and microscopic examination. Confirmation of the ‘R. pyronotae’ identification was carried out via PCR amplification and DNA sequence analysis of partial 16S rRNA and ftsY gene sequences as described in [8].

The pastures sampled had been established on renovated land in 2002 with the manuka beetles reaching damaging levels by 2005/2006 [8], and after several seasons of very high densities of beetles had passed that the first manuka beetle larvae infected with Rickettsiella (Figure 1) were recognised in 2009. R. popilliae pathotype ‘R. pyronotae’ infection was first observed in larvae from a small patch of pasture with a high density of manuka beetles and developed from an initial average infection rate of 0.5% in 2009 to 28.6% of larvae in 2012 showing advanced stages of Rickettsiella infection (Table 1).Consequently the numbers of manuka beetles dropped significantly (p< 0.001) from an average of 465 larvae/m2 in 2009 to 223 larvae/m2 by 2011 (Table 1). The density of manuka beetles remained low in subsequent years with widespread ‘R. pyronotae’ infection and the pasture had recovered from insect damage.

The spectacular outbreak of Rickettsiella and ensuing local population collapse may be related to specific site conditions [9]. The pest outbreak occurred where a new soil profile was created with an abundance of freshly established, fertilised, grass roots for the insect larvae. Peak densities of manuka beetle larvae (P. setosa and P. festiva) were reached in 2009, but no significant diseases were recorded in the pest group for the first five years following sowing. Consistent rainfall provides ideal conditions for pasture growth and insect population development. In these conditions, a small inoculum of ‘R. pyronotae’could initiate infections, lead to insect death, and thereby build up propagules in the soil. The subsequent generations of manuka beetle larvae were exposed to this inoculum and became infected by horizontal transmission from the soil. Inoculum would be spread by the surviving beetles as they emerged and flew for mate searching to adjacent local populations.

The combination of a spectacular pest outbreak, the ability of the project team to recognise Rickettsiella infections, and the concern of the farmers to support a monitoring project, has meant that this episode demonstrating the actions of Rickettsiella disease in stabilising a pest population could be documented. A similar impact of Rickettsiella disease was noted by Niklas on May beetles (Melolontha spp.) in Germany in the 1960’s [10], but the difficulty of diagnosis and the association with soil insects means that the importance of Rickettsiella diseases in insect population regulation is vastly underestimated.

Table 1: Annual mean larval density and estimated level of ‘R. pyronotae’ infection.

We thank Jessica Shaw (Landcorp Farming Limited, New Zealand) for assistance with field collection of manuka beetle. Funding for this work was provided by the Sustainable Farming Fund (contract SFF 90-080), New Zealand.

1. Jurat-Fuentes JL, Jackson TA. Bacterial entomopathogens. Insect Pathology, 2012; 265-349.

2. Garrity G, Brenner DJ, Krieg NR, Staley JR. Bergey’s Manual® of Systematic Bacteriology Volume 2: The Proteobacteria, Part B: The Gammaproteobacteria: Springer US, 2005.

3. Louis C, Jourdan M, Cabanac M. Behavioral fever and therapy in a rickettsia-infected Orthoptera. Am J Physiol. 1986; 250: 991-995.

4. Anstead CA, Chilton NB. Discovery of Novel Rickettsiella spp. in Ixodid Ticks from Western Canada. Applied and Environmental Microbiology. 2014; 80: 1403-1410.

5. Tsuchida T, Koga R, Horikawa M, Tsunoda T, Maoka T, Matsumoto S, et al. Symbiotic Bacterium Modifies Aphid Body Color. Science. 2010; 330: 1102-1104.

6. Glare TR, O’Callaghan M, Wigley PJ. Checklist of naturally-occurring entomopathogenic microbes and nematodes in New Zealand. New Zealand Journal of Zoology. 1993; 20: 95-120.

7. Leclerque A, Kleespies RG, Schuster C, Richards NK, Marshall SD, Jackson TA. Multilocus sequence analysis (MLSA) of ‘Rickettsiella costelytrae’ and ‘Rickettsiella pyronotae’, intracellular bacterial entomopathogens from New Zealand. J Appl Microbiol. 2012; 113: 1228-1237.

8. Regina G Kleespies, Sean D G Marshall, Christina Schuster, Richard J Townsend, Trevor A Jackson, Andreas Leclerque. Genetic and electron-microscopic characterization of Rickettsiella bacteria from the manuka beetle, Pyronota setosa (Coleoptera: Scarabaeidae). Journal of Invertebrate Pathology. 2011; 107: 206-211.

9. Jackson TA, Townsend RJ, Dunbar JE, Ferguson CM, Marshall SDG, Zydenbos SM. Anticipating the unexpected – managing pasture pest outbreaks after large-scale land conversion. Proceedings of the New Zealand Grassland Association. 2012; 73: 119-124.

10. Jackson TA, Glare TG. Rickettsial diseases of scarabs. In T. R. Glare and T. A. Jackson (eds), Use of Pathogens in Scarab Pest Management. Andover, Hampshire: Intercept. 1992; 33-42.