One taxonomic group which is host to a fungal STD unfamiliar to most people is the beetle family Coccinellidae, the ladybirds, lady-beetles or ladybugs. In late September I was doing some conservation work, cutting willow on the wetland nature reserve of Winnall Moors on the edge of Winchester, southern England. During this work, several specimens of the Kidney-spot Ladybird Chilocorus renipustulatus were found, a scattered but sometimes locally abundant beetle associated with willow on wetlands. So, no great surprise there. I took a photo and when I got home, added it to my list of species records and also decided to add it to the iSpot site. At this point I noticed the yellowish tufts attached to the rear - I had seen two or three individuals with such tufts in the field but only now did they become clear.
|C. renipustulatus showing the tufts of what appears to be H. virescens on the rear of the elytra (wing cases).|
I had no idea what they were (they looked like lichen but that seemed unlikely!) and made a note to investigate, though I didn't have a specimen. However, a few days later, one of the recorders from the UK Ladybird Survey posted a response stating that the tufts looked like the sexually-transmitted fungal disease Hesperomyces virescens (in the order Laboulbeniales). This was interesting enough as I'd never heard of it, but he also mentioned that it might be the first record of H. virescens on this species of ladybird. A shame I didn't collect a specimen! So, what is this fungus and what does it do?
All the fungi in this order are obligate parasites of arthropods (i.e. they need live arthropods) and derive nutrition via a 'haustorium' which penetrates the cuticle (Evans 1988). So, although they are ectoparasites (living on the outside of their host), they do penetrate the surface and H. virescens is in fact one of the few fungi in this order to have haustoria forming a number of narrow branches radiating out into the body cavity (Batra 1979). In work on the first record of this fungus on the Harlequin Ladybird Harmonia axyridis, Garcés & Williams (2004) noted that H. virescens was more prevalent in autumn after ladybirds had been through a period of aggregation/clustering. With my observation having been made in late September in the UK, this tendency may explain that a few fungal tufts were noted even though they weren't being sought.
Other ladybird species such as Adalia bipunctata (the Two-spot Ladybird) and Olla v-nigrum (the Ashy-Grey Ladybird) have been recorded as hosts of this fungus, while research into a relative of the species considered here, C. bipustulatus (Heather Ladybird), has shown that the fungus can cause premature death of the beetle (Kamburov et al. 1967). More recent work in the US (Riddick & Cottrell 2010) has suggested a preference for H. axyridis as a host with few instances of the fungus infected its 'original' host Hippodamia convergens (the Convergent Ladybird), a common and widespread species in North America. However, this research notes that H. axyridis was also the most abundant ladybird, hence the high rate of fungal infection could be due to the higher frequency of sexual/social contact between individuals (this was also seen in the native O. v-nigrum) - something which is reduced in less abundant species. Both H. axyridis and O. v-nigrum overwinter is mixed-sex aggregations, again supporting the idea that host and fungal abundance are connected. This may seem obvious in hindsight, but is important as it also links to possible effects due to differing activity levels.
For instance, Riddick & Cottrell (2010) also showed that of the two main host species, H. axyridis males had a higher infection rate than females, something not seen on O. v-nigrum. This suggests a possible behavioural effect with H. axyridis males behaving in such a way (they mount males as well as females when seeking mates) as to experience greater levels of exposure to the parasite. When Welch et al. (2001) proposed their sexual transmission hypothesis to explain the distribution of H. virescens thalli on ladybirds (they looked at A. bipunctata), this was because they noted fungal structures on the upper surface of females and the corresponding underside of males, mirroring the mating position. As physical contact may initiate spore discharge, the position of the fungus on a beetle's body may reflect the type of contact that occurred
(Weir & Beakes 1996). Thus, fungal infection rate may be affected by the rate of sexual/social contact as well as simply the abundance of hosts.
So, we have an overview of some aspects of the relationship between the fungus and several ladybird hosts, but what about C. renipustulatus? Well, initial indications are that there are no published records of this ladybird as a host of H. virescens. This may turn out not to be the case even though I have searched quite thoroughly as has 'rimo' at iSpot, but for now this appears to be a potential new discovery worthy of a visit to Winnall to collect specimens!
*** UPDATE ***
After feedback from mycologists, this find does appear to be a world's first as a new host for the fungus and has now been published as a short paper in The Coleopterist:
D. Hubble (2011). Kidney-spot ladybird Chilocorus renipustulatus (Scriba) (Coccinellidae), a new host for the parasitic fungus Hesperomyces virescens Thaxter (Ascomycetes: Laboulbeniales). The Coleopterist, 20 (3), 135-136 Other: 0965-5794
Batra, L. (1979). Insect-fungus Symbiosis, Nutrition, Mutualism and Commensalism. Allenheld, Osmun & Co., New York.
Evans, H.C. (1988). Coevolution of Fungi with Plants and Animals. Academic Press, San Diego.
Garcés, S. & Williams, R. (2004). First record of Hesperomyces virescens Thaxter (Laboulbeniales: Ascomycetes) on Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae). Journal of the Kansas Entomological Society 77(2): 156-158.
Kamburov, S.S., Nadel, D.J. & Kenneth, R. (1967). Observations on Hesperomyces virescens Thaxter (Laboulbeniales), a fungus associated with premature mortality of Chilocorus bipustulatus L. in Israel. Israel Journal of Agricultural Research 17(2): 131-134.
Riddick, E.W. & Cottrell, T.E. (2010). Is the prevalence and intensity of the ectoparasitic fungus Hesperomyces virescens related to the abundance of entomophagous coccinellids? Bulletin of Insectology 63(1): 71-78.
Weir, A. & Beakes, G.W. (1996).Correlative light- and scanning electron microscope studies on the developmental morphology of Hesperomyces virescens. Mycologia 88: 677-693.
Welch, V. L., Sloggett, J. J., Webberley, K. M. & Hurst, G.D.D. (2001). Short-range clinal variation in the prevalence of a sexually transmitted fungus associated with urbanisation. Ecological Entomology 26: 547-550.