Larval disease in butterfly house

[mothmaniac at nospam.com]

John, I have had much experience with pathogen problems and possible
treatments, in rearing Saturniidae; the pathogens which attack
butterflies are generally the same, so the information here will apply
in your case. There are three major types of disease-causing organisms
which attack larvae;  bacteria, fungi and viruses. The first two are
treatable when they occur, however, viruses are best "treated" by
prevention programs, rather than intervention, as there are no
commonly available drugs that are antiviral in nature. Symptoms of
viral infection are obvious and well-known; and occur late in the
progress of the infection, when treatment would be ineffective - were
any treatment available.  The most common viral affliction is called
NPV (Nuclear Polyhedrosis Virus) and it affects many, many species of
Lepidoptera. Symptoms are: Loss of larval appetite, followed by a
"bleeding" or oozing of hemolymph (larval "blood") through the skin,
and a concomitant loss of turgor (fluid "plumpness" or "firmness") of
the larva. The larva becomes very sluggish, and its color may become
dull. The loss of bodily fluids continues until what is left is
essentially a soft, mushy "bag" of skin, usually with the larva
assuming a characteristic "hanging by the claspers" (or sometimes a
proleg) posture. At this point death is imminent. The "baggy" hanging
larva will usually become a dull brownish color, although some larvae
do retain their normal colors, only dulled. The rate of progress from
initial symptoms until death is extremely rapid, usually within 24 to
48 hours, so nothing can be done even if treatment were available,
because of the massive internal destruction caused by the virus
rupturing cell membranes as it reproduces (this causes the
aforementioned loss of fluids, by permeation through the skin, and by
the spilling out of the cytoplasm of the cells themselves). Prevention
is really the only way to control this disease - sterilization of all
surfaces in the greenhouse with a chlorine-bleach/water solution, as
well as all sleeves and tools, plus washing leaves of foodplant in
bleach/water as well (larvae do not seem to mind this, at least in
Saturniidae). I also recommend irradiation, for 4 hours daily, with
ultraviolet radiation in the mediumwave spectrum (so-called UV-B) from
natural sunlight (NOT in a greenhouse, as typical glass and plastic
glazings absorb most energy at these wavelengths). This is one problem
of greenhouse rearing versus outdoor rearing; larvae are much more
prone to disease from lack of sufficient UV radiation inside a
hothouse. If exposure to natural UV cannot be arranged, use an
ultraviolet "sunlamp" (mercury vapor; NOT the ordinary "streetlight"
type), emitting in the 2570 Angstrom wavelength band, a 500 w unit
held 1 meter away from the larvae for 15 minutes per day would be a
typical application. A lower wattage lamp or a lamp at greater
distance from the larvae will need to be left on longer. Use of
germicidal shortwave UV lamps to sterilize the upper air in the
greenhouse (as is done in hospitals) is also recommended, however, do
not expose the larvae (or yourself) directly to these (they should be
shielded from below)- the radiation from these lamps, in the 2200-1800
Angstrom band is dangerous to eyes and skin. Good ventilation is
essential with these lamps; they may produce appreciable ozone. All of
these measures are also very effective in preventing bacterial and
fungal infections as well. Good air circulation, prevention of
excessive heat (which stresses larval immune systems) and low humidity
(as low as the larvae able to tolerate) are also good measures for
disease control. Any larvae which have been infected with NPV should
be immediately destroyed- they cannot be saved. Do not simply throw
them on the ground, where the virus could spread into the environment
and harm native Lepidoptera; soak them in pure chlorox bleach until
they dissolve. Any confinement sleeves in which the infected larvae
were held must also be sterilized; soak them in a strong bleach/water
solution for several hours. Wash the leaves of the plants on which
these larvae were feeding with this also; better yet, if possible,
destroy the plants. Only in very rare circumstances have larvae
recovered of their own accord, or subsequent to irradiation
treatments, from NPV, in my experience. And these larvae often die in
the pupal stage, anyway, so trying to "cure" them of NPV is almost
always an exercise in futility. (Although I am very interested in the
mention by someone on this NG of an antiviral agent derived from
Polyporus (Piptoporus betulinus) fungus- I have not heard of this
before, and wonder if it might be effective against NPV). Bacterial
infections of larvae, unless advanced, are treatable, as are fungal
infections. Fungi usually cause dark spots or blotches to form on the
skin; bacterial afflictions are usually characterized by a loss of
appetite followed by a very rapid blackening and deliquescence of the
larva, but without the typical hanging, "baggy" posture of virally
infected caterpillars. The most common bacterium responsible in many
areas of the US is Bacillus thuringiensis (BT) which is sprayed by
townships and state agencies in many areas to control Gypsy Moth
populations. Unfortunately, although dubbed as "safe, killing only
pest species", it is a very lethal pathogen to hundreds of Lepidoptera
species, good and bad, and is a typical marketing conspiracy which
places profit over the environment and has had many state and local
government agencies as well as uninformed citizens fooled for many
years, to the detriment of nonpest and even endangered species. The
best defenses against this pathogen (and other bacteria as well) are
(aside from an informed citizenry, or avoidance of exposure) the
various antibiotics such as  Kanamycin sulfate and Terramycin, etc.
These can usually be purchased for aquarium use in any good pet shop,
and are sold in capsule form. A capsule, emptied into a small (clean)
flit gun filled with water, and sprayed on the leaves of the foodplant
daily works well, somewhat more effectively as preventative than cure.
However, it does seem to be actually toxic to certain Saturniid
larvae, in my experience, but if your larvae are dying and you have
nothing to lose, then try it.  (I don't know if it is harmful to
whatever butterflies you are trying to rear;  if experiments show it
not to be, then make it part of your daily care regimen, to help
prevent anything from even starting). As far as fungal infections are
concerned, many are preventable by offering the larvae good
(non-stressful) conditions (keeping heat and humidity down and free
airflow in the greenhouse) and proper foodplant choices. And practice
the sterilization measures I have mentioned above. As far as
treatments go, I have not had occasion to use antifungal agents in the
treatment of larvae (no problems with fungus) but I believe that UV
irradiation would be effective (and even prevent it to begin with).
Hopefully, this brief overview of pathogen control will have been of
some use to you. Happy Rearing!