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Parasitoids

Promethea Pupae Parasitized

12-21-15 promethea cocoon 257Although a lack of snow makes tracks difficult to find, there are other, more permanent, animal signs such as bird nests and cocoons that are visible this time of year. Among the more obvious is the cocoon of the Promethea Moth – a giant silk moth. When the time for pupating arrives the Promethea caterpillar selects a leaf and strengthens its attachment to the tree by spinning silk around the petiole of the leaf as well as the branch it grows on (to assure that it doesn’t fall off the tree). With more silk it rolls the leaf up into a tube and then proceeds to spin its cocoon inside the rolled-up leaf, leaving a valve-like structure at the top of the cocoon through which the adult moth exits in the spring.

Unfortunately for silk moths, many are parasitized by flies and wasps (there are nearly 100 natural parasites that affect the 24 species of silk moths east of the Mississippi River). Frequently flies or wasps lay their eggs in silk moth caterpillars and then develop inside them. Eventually the fly or wasp larva secretes a substance that causes the caterpillar to pupate, at which time the fly or wasp also pupates and then exits the moth pupa and cocoon (see exit hole in smaller photo), causing the death of the moth pupa. Silk moth populations are decreasing, in part as a result of these parasitoids. Among others, a non-native parasitic tachinid fly, Compsilura concinnata, is wreaking havoc on silk moths.

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Pigeon Tremex Horntails Laying Eggs

horntails 239Horntails, also known as wood wasps, are non-stinging, wood-eating insects that lay their eggs deep within trees. Both male and female horntails have a pointed spine at the tip of their abdomen; females also have a long, slender ovipositor. (They get their name not from their spine or ovipositor, but from a knob (cornus) at the tip of their abdomen.)

Pigeon Tremex Horntails (Tremex columba) are active in late summer and early fall. A mated female inserts her ovipositor several inches into a dead or dying tree and lays an egg (where it is safe from most, but not all, predators). Along with the egg the adult horntail deposits some white rot fungus (Daedalea unicolor) which she stores in special abdominal glands. The fungus breaks down and softens the wood for the horntail larva to eat and is required for the successful development of the horntail. The larva typically begins consuming the soft, fungus-ridden wood around it, and then chews its way to the inner bark so as to provide a means of exiting the tree when it becomes an adult. The larva then returns to feed on inner wood. It completes its metamorphosis and emerges from the tree within a year as a winged adult horntail.

There is a parasitic wasp, the Giant Ichneumon Wasp (Megarhyssa macrurus), which possesses a long three-inch ovipositor capable of drilling into trees. There are several theories as to how this parasitic wasp detects the presence of horntail larvae deep within the tree. She may lay her antennae on the outside of a tree and pick up the vibrations of horntail larvae gnawing away in their wood chambers. Another theory proposes that the female wasp uses her antennae to smell the frass (droppings) of the horntail larva as well as the wood-softening fungus. Once she locates a horntail larva, the ichneumon wasp paralyzes it and then lays an egg on it. The ichneumon wasp larva feeds on the paralyzed horntail larva, consuming it completely within a couple of weeks. The ichneumon wasp then pupates and remains dormant under the bark until the following summer, when the adult emerges.

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Parasitoids & Parasites

9-10-15 pelecinid wasp with mites 071By definition, a parasitoid is an organism that lives on or in a host organism and ultimately kills the host. The pictured Pelecinid Wasp is a parasitoid. Its host is the grub, or larval stage, of June Bug beetles. The female Pelecinid Wasp uses its long abdomen to probe into the soil until it locates a June Bug grub and then it lays an egg on the grub. When the egg hatches, the wasp larva burrows into and feeds on the grub, eventually causing its death.

A parasite is much like a parasitoid, deriving nutrients from a host, but, unlike a parasitoid, a parasite does not usually kill its host. Often parasites are much smaller than their host, and frequently live in or on their host for an extended period of time. In this photograph, a parasitoid, the Pelecinid Wasp, is host to reddish parasitic mites (located on its thorax).

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