Sphaerularid Nematodes are highly virulent parasites of Insect hosts. While most parasites seek to keep their host alive for as long
as possible, enabling them to live and produce offspring for as long as
possible, Sphaerularids begging to reproduce at a high rate shortly after
entering a new host, producing a large
number of young which quickly overcome the host. While some of these young may
exit the host while it still lives and start seeking out new hosts, the
majority will exit the host after its death.
In a paper published in the journal Systematic Parasitology on 1 November
2014, George Poinar of the Department of Zoology at Oregon State University and
Donald Thomas of the United States Department of Agriculture-Agricultural Research Service Cattle Fever
Tick Research Laboratory, describe a new species of Sphaerularid Nematode
infecting the Arundo Gall Midge, Lasioptera donacis.
Gall Midges lay their eggs within the tissues of plants, where they
cause the plant to form a gall, in which the larval Midge lives, surviving on
the plant’s natural fluids and protected from predators by the gall. The larval
stage of the Insect lives its entire life within the gall, emerging from the
egg, and growing till it is ready to pupate, which it does within the gall.
Only the adult stage emerges to mate and lay eggs on new hosts. High levels of
Gall Midge infections are harmful to plants and may even kill them, and species
of Gall Midge are often capable of infecting only a single species of plant,
making them of great interest to biologists looking for control species for
invasive plants (plants which have been introduced to new environments by human
activities, and lacking natural predators in these environments, become to
dominate them and exclude native flora and fauna).
The Arundo Gall Midge, Lasioptera donacis,
is a natural parasite of the Old World Giant Reed, Arundo donax, a species native to waterways of the Mediterranean
Basin, but introduced to North America by European settlers in the sixteenth
century, where it has become a serious invasive pest, as it lacks natural
predators or parasites in the New World, and has an ability to modify and
dominate aquatic ecosystems. As such the Arundo Gall Midge is considered to be
a potential biological control for the Giant Reed, and is being studied at a
quarantined laboratory run by the United States Department of Agriculture at
Moore Air Base at Edinburg,
Texas, with a view to releasing
it into American ecosystems.
Midges were imported to Moore Air Base inside the stems of infected
Giant Reeds harvested in France,
Italy and Greece. Many of
these Midges were found to be infected with a virulent Sphaerularid Nematode,
which suppressed the reproductive ability of, and eventually killed its hosts.
Such parasites have important implications when trying to establish control
species for invasive pests. On the plus side, if the biological agent proves
capable of switching to unintended hosts and itself becomes an invasive
species, the parasite can be used to control it, but on the minus side, if the
parasite is accidentally releases with the control species, it can prevent it
from becoming established, thereby undermining the control effort, or worse
still, the parasite may itself prove capable of switching to a new host (or
hosts) in its new home, further damaging an already vulnerable ecosystem (and
eradicating a new parasitic Nematode, once released, is almost impossible).
The new Nematode is placed in the genus Tripius, and given the specific name gyraloura, meaning ‘round-tail’. The species has a free-living
sexual stage, which seeks out new hosts to infect, but does not feed
independently or infect a secondary host (many parasites have more than one
host in their life-cycle, and thus have primary and secondary, and sometimes
even tertiary hosts). The free-living females reach 297–360μm in length and
have a well-developed stylet (a hollow mouthpart similar to a hyperdermic needle), while the males reach 415–428μm and
have an underdeveloped stylet. These mate in the environment, whereupon the
males die and the females seek out a new host, using secretions to penetrate
the body wall of juvenile Midges shortly before pupation. The infective female
everts her uterine cells into the host’s haemocoel, giving birth to secondary,
parasitic females, which can reach 0.6-2.0 mm in length, and which produce
asexually, giving birth to larvae 290-430 μm in length. Sexual reproduction in
infected Midges emerging form their pupas appeared to be supressed, and the
Midges soon after, revealing body cavities packed with Nematodes. Some of these
may have escaped through the ovipositors of female hosts, but most are thought
to have escaped after the host’s death.
Tripius gyraloura. (1) Cluster of parasitic juveniles (arrow) in a dissected female Lasioptera donacis (arrowhead shows anterior
end of a mature parasitic female); (2) free-living infective stage female.
Scale-bars: (1) 144 μm; (2) 39 μm. Poinar & Thomas (2014).
As well as being of interest due to its potential impact on the
biological control program for the Old World Giant Reed, the discovery of Tripius gyraloura is interesting because
it isonly the second such infection in a living Gall Midge discovered, one
other member of the same genus Tripius gibbosus infects
the Gall Midge Cecidomyia pini, while
a third member of the genus infects Sciarid Flies. A Gall Midde infected with
parasitic Nematodes is also known from Eocene Baltic Amber.
See also…
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