Photo: Feather-legged Fly, Judy Gallagher
Judy Gallagher is an ASNV board member and a regular surveyor of local wildlife who also captures photos of what she sees, in particular the less common species.
As I slap at mosquitoes on a warm summer night, it can seem as if there are limitless numbers of insects. In reality, insect numbers are limited by physical factors like temperature or pesticides, or biological factors like lack of food, competitors for resources, or natural enemies like predators, diseases and parasitoids.
Parasitoids are small insects that develop in or on another organism, called the host. The parasitoid eventually kills the host, but typically not until the parasitoid is fully grown when it is usually capable of living on its own, eating pollen, honeydew or nectar. Parasitoids are different from parasites like fleas and ticks, which feed on other organisms without killing them.
Parasitoids can develop inside the host, or outside the host sucking bodily fluids through holes in the victim's skin. They tend to specialize on one particular host species. Most parasitoids are wasps or flies, but there are also parasitoid beetles, moths and other insects.
Trissolcus species wasp on Stink Bug eggs, Judy Gallagher
Some parasitoids lay eggs directly on or in a host. Trissolcus species wasps, only about 1/16 inch long, are parasitoids of stink bugs and lay their eggs on stink bug eggs. The female wasp scent marks the eggs to indicate to other wasps that the eggs are already parasitized. She also guards the eggs until her larvae are well developed to prevent other wasps from getting in there. Male wasps sometimes guard stink bug eggs, but, in that case, they are waiting for a female parasitoid adult wasp to emerge, and will then mate with her.
Stink bugs infest many commercial crops including beans, squash and some fruits and nuts, and Trissolcus wasps are an important biological control of the stink bugs.
Modest Masked Bee with Blister Beetle larvae (the reddish stuff around the bee's thorax), Judy Gallagher
Blister beetles lay eggs on vegetation. Once the larvae hatch, they climb to flowers and wait for a solitary bee to come along. Some blister beetle larvae even emit fake pheromones to attract a male bee. They grab onto the bee, and when the bee finds a mate, they transfer to the female, riding her back to her nest where they eat pollen, nectar and bee larvae. Note that the blister beetle larva can survive on just pollen and nectar, so it does not always act as a parasitoid.
Catalpa Sphinx caterpillar with translucent wasp larva and white wasp cocoons, Judy Gallagher
As their name suggests, Catalpa Sphinx caterpillars eat the leaves of the Catalpa Tree. Fortunately, the tree can produce multiple sets of leaves in a year and is rarely seriously damaged by the caterpillars. The Catalpa Sphinx caterpillars only feed on Catalpa, so it's not in their best interest to have the tree die.
But if the tree senses that it is being harmed too much, it produces extra-floral nectaries on its leaves. This is just like having a flower with nectar, minus the flower. The nectar from the extra-floral nectaries attracts wasps and other insects. In this case, a parasitoid Braconid Wasp, attracted by the nectar, has laid eggs beneath the skin of the Catalpa Sphinx Moth caterpillar, and the developing wasp larvae feed inside the caterpillar. This will eventually kill the caterpillar, but not until the wasps are ready to pupate and emerge from inside the caterpillar. So, the tree attracts wasps, which parasitize some caterpillars, keeping the caterpillar population enough under control that the tree is not damaged too much.
Tachinid Fly egg on a Leaf-footed Bug (the egg is the white thing between the insect's eyes), Judy Gallagher
Tachinid or Bristle Flies are a large fly family. Their larvae are endoparasitoids, developing inside their hosts.
Feather-legged Fly , Judy Gallagher
The Feather-legged Fly, one of many Tachinid Fly species, selects a large nymph or an adult true bug from the Leaf-footed Bug or Stink Bug family to lay eggs on. It can find a host by following the pheromones secreted by its prey. The fly can lay many eggs on one host, but only one parasitoid larva will survive. The larva uses the host's respiratory system to breathe and eats the host's body tissues. The larva then emerges from the host and pupates in soil.
Perilampus hyalinus, a hyperparasitoid, Judy Gallagher
But one of the most bizarre examples of a parasitoid is a Chalcid Wasp in the family Perilampinae. The female wasp scatters her eggs on leaves, bark, buds, etc., anywhere that a host might be found. The hatched larvae wander around in search of any living thing. If one finds an appropriate host, it penetrates the host's cuticle. Then it looks for an organism that has already parasitized the host, and enters that larva, parasitizing it. It eventually pupates inside the host cocoon. This is a hyperparasitoid, or a parasitoid of another parasitoid. Interestingly, this particular hyperparasitoid can also complete its life cycle as a parasitoid of sawfly larva, so depending on conditions, it can be a parasitoid or a hyperparasitoid.
There are other local hyperparasitoids, but I personally have only seen a couple. Nature is fascinating and complex!
View more of Judy’s Observations from Meadowood articles here.