Infection-like markings in S. pyri larvae

[saye]

I've been keeping (it's the season after all) a few caterpillars inside, of which in some, and of a particular species, I've noticed something odd.

The photographs below, of flimsy quality and subpar cropping (sorry, smartphones), show two third instar Saturnia pyri larvae, which are immobile as they are set to moult, that very quickly developed a series of tubercular marks which, to me, appeared like necrosis and signalled one of three things: either fungal or viral infection, or, of course, endoparasitism of the likes of wasps or flies.

Additionally, one other caterpillar, in apolysis/ecdysis as well, seemed to apparently leak heavily, from between the last two pairs of prolegs, as it hung head down with less and less grip, seemingly nearing its death - here even more did I expect a parasitic larva to appear, or something of that sort - though this larva itself didn't seem to have the same development of markings at all (sorry for the out of focus exuvia by the way, couldn't quite catch it).

To my surprise, both the sick-looking larvae and the bleeding one moulted without any markings or apparent sickness.

I've since wondered, for the first two, if it was any sort of melanizing process, as I've noticed other caterpillars with the same colouration, but only on one to three tubercles, as can be seen in the fifth and sixth photos.

Anyone that has reared this species, or any saturnids, experienced this?

Attachment DeletedAttachment DeletedAttachment Deleted

[irisscientist]

Looks like a viral infection to me. I would quarantine these larvae immediately and clean/sterilise ALL of your equipment. If you have any unaffected larvae also keep these away from any other stocks that you have. This kind of infection can wipe out entire collections.

[saye]

I quickly feared the same, but since signs of pathology have apparently decreased instead of increasing, I have kept the same and usual spectating scope. I haven't taken any quarantine measures because, as could become unfortunate for the sixteen larvae, there is at least one infected individual per box and in that sense I consider myself late for any proper action. Thank you for the advice - will update on what follows.

[Adam Cotton]

I have had similar problems when I was rearing a friend's Saturniidae in boxes. I think it is probably bacterial. When larvae are kept in unnatural conditions, crowded and no airflow, the naturally occurring bacteria on the larval skin surface take over (due to lowered natural resistance) and kill the larvae.

Always rear these singly from hatching in as large a box as possible (sleeved on growing foodplant is better). Try spraying the larvae with a 1/2 strength Milton solution. It won't kill them, but it will reduce the number of bacteria on the skin of the larvae which may help.

I do agree with Mark though, sterilise all equipment and also WASH your hands every time you handle each box. Oh, and always remove frass as soon as possible. You should also put kitchen tissue in the bottom of the box to absorb moisture, it will help a little.

Adam.

[saye]

That does indeed make sense. I have kept and observed that four individuals per container (13.5 x 6.5 x 6cm) is somewhat admitted until third instar, but with fourth it cannot be helped that they are simply too big. I also deduce, from the information you provide on naturally occurring causes of pathogenic distress to be part of the very larvae, that there is correlation with the obvious stasis they adopt in order to moult; they are inactive, and under unnatural conditions, so the bacteria spread faster.

I am currently keeping watch of a static fourth instar who's markings are increasing by the hour as it shares the box with two other large fourth instars, while an older one that I separated on a side, clean, frassless, dry box is developing the markings much more slowly and much less extensively - though this does not take into account other factors, such as individual fitness.

However so, I do regularly wash my hands and keep the boxes padded with kitchen paper, which effectively gets wet and thus does help a little, though I am sloppy with the frass removal sometimes.

Thank you for the advice.

[Adam Cotton]

It is really about allowing the larva's natural immune system from stopping the bacteria from taking a detrimental hold. I strongly recommend separating each larva as soon as they reach 2nd instar, or even better from hatching, and make sure there is the largest possible airflow (I assume the boxes have a net in the lid rather than being completely closed). Of course this only works for solitary species, naturally gregarious larvae need to be in groups, so it is important to know the normal habits of the species you are rearing. However, you should keep the gregarious larvae in small groups in a box, rather than the very large assemblages in nature.

Adam.

[Adam Cotton]

Oh, I forgot to mention that Papilio machaon is extremely vulnerable to this problem, even when reared outside on plants, particularly in hot climates. It is important to raise these with one larva per plant, so they never interfere with each other. This interference causes the stress that allows the bacteria to take over. Usually the bacterial infection increases as the rearing progresses, and in P. machaon I expect that the latter half of each generation will show increasing symptoms.

I recently had some Italian visitors who asked me how I can stop disease in P. machaon. They said that whenever they rear it the larvae all die, so this problem obviously also occurs in Europe with this species.

Adam.

[saye]

I had read a few times that S. pyri was a challenge to rear, and one of the more care-needing saturnids, at least in comparison, supposedly made by those who rear saturnids, to other species of general interest, like S. pavonia or A. luna. I've already begun separation of all later larvae, and the idea is to have each one alone before fifth instar (they are all fourths). I appreciate the recommendations, which are pertinent since I've almost never reared more than one to three or four larvae of any species.

But I did not know that at all about P. machaon.
I've taken P. machaon on several occasions, but only single fifth instars after observing their growth in situ, and at times already in the wandering stage. There is much opportunity to collect eggs and first instars, but I've always deemed it quite troublesome to cut or pot either of the two local hostplants; fennel and rue. And as known, numbers can suffer great blows from predation very quickly, eggs included. I have, however, noticed odd instances of parasitism and even the oddest occurrence with the species, a self-canabalizing larva. Behaviour I, unfortunately, did not record. But nothing of what I am seeing in these S. pyri larvae at the moment.

In which conditions were the Italians rearing their larvae though?

[Adam Cotton]

May 9, 2015 14:35:00 GMT -8 saye said:

In which conditions were the Italians rearing their larvae though?

I don't know, they didn't say.

[saye]

@ Adam
I see, thank you.

@ radusho
Unless you're excluding the option by default, I would have to disagree and say that the best way to rear the species is on a sleeved plant, as with many if not all saturniids, or any large species who's larvae become large. More so because S. pyri is indeed a "tree" species, and cutting branches for the bigger larvae would be a big beat for any (fruit) tree. In any case, my locality supports the existence of the species in the wild, as I am in (temperate) Europe (Iberia). On the other hand, yes, plastic boxes are the worst places to rear them as far as the referenced conditions go; and as said, the species is repeatedly mentioned as one of the most sensitive.

I got a hint of the origin of the pathology, which was the larger point of the thread. So thank you guys.
Will post if anything gets worse or turns unexpected.

[saye]

The context in which I set for it being "the best" referred to the conditions in which the larvae grow, those being their naturally growing hostplant in favourable climate, except with a protection that extends both ways (for the rearer and for the larvae), that's all. I have never used sleeves at all, but from the simple mechanism I easily imagine it capable of being tied around a single large branch, instead of a very young tree. At the very least the hostplant is intact as far as cutting anything goes.

I'm assuredly not arguing for what I haven't experimented, nor with your experience, and I appreciate your recommendation.