Insect Parasitism [2] in This Unit, We'll Differentiate Between the Three

Unit 11 in Entomology

[2] In this unit, we’ll differentiate between the three types of symbiosis, describe the different ways to classify parasites, discuss the costs and benefits of endo versus ecto parasitism, differentiate between parasite and parasitoid, define hyper, multiple and gregarious parasitoids, and explain in insect terms why being small is actually good. We’ll discuss more about parasites, but for the time being, take a look at the photographs below. Both of these are lice that can be found on chickens. One is found on the body and one is found on the feathers. Take a look at the morphological differences. Why do you think they’re shaped so differently? These are some things to think about as we discuss insect parasitism.

[3] In this unit, we’ll discuss insects that parasitize humans, mammals, and other arthropods. You will first learn the difference in three symbiotic relationships, and between parasite/parasitoid, parasitoid/predator, and about various endo and ecto parasites. Prepare yourself for some repugnant pictures that illustrate some damage that insect parasites can do to other animals.

[4] Symbiotic relationships refer to interactions between two different species. In fact, when broken into its Greek roots, the word symbiotic means “life together”. Sym means together and bio means life. In this unit you’ll first learn about three different types of symbiotic relationships: mutualism, commensalism, and parasitism. The balance of the unit will then focus on various aspects of parasitism. These terms are generally used to describe the relationship between species, but can also be used for interactions within a species, particularly humans. Mutualistic relationships are those in which both species involved benefit. For example, most friendships are a mutualistic relationship. As you can see in this picture, ants and aphids have a special mutualistic relationship. The aphids feed the ants by secreting a sugary solution called honeydew through cornicles on the tip of their abdomen. These ants, in return, protect the aphid by attacking predators such as ladybugs.

[5] The second type of relationship is commensalism. In commensal relationships, one species benefits but the other neither profits nor is harmed. In this picture, orchids are growing on the side of a tree. The orchid needs the tree for support and protection, but the tree doesn’t benefit, not is it harmed, by the orchid’s presence. Relationships in which one species benefits at the expense of the other is a parasitic relationship. A good example of this is a mosquito and a human relationship. As pictured here, this mosquito has inserted her mouth part into a human to draw out a blood meal. The human will have a nice itchy lump where the mosquito has injected its poison, but the mosquito will use the protein it got from the blood to make her eggs.

[6] Insects, more than any other arthropod class, practice parasitism. We humans are mostly aware of the insect parasites that affect us. Mosquitoes seem to be everywhere we go. Your household pets are commonly affected by fleas and schools often have to check for outbreaks of lice among the students. Not only do insects harm us, but they also harm other mammals, vertebrates, invertebrates, and even members within their own class, as depicted in the table below. Some orders are 100% parasitic. Some of these include Mallophaga and Anoplura and Siphonaptera. Hymenoptera are the next highest, with 50% of its members being parasitic. You’ll learn about a few wasps that use other insects as their host later in the unit.

[7] There are two types of parasites we will be discussing in this unit: endoparasites and ectoparasites. Endoparasites are parasites that live within the body of its host, and an ectoparasite lives outside the body of its host. What type of parasite would a mosquito be, endo or ecto? It would be an ectoparasite, because it lives off of our blood by sucking it from the outside of the body.

[8] Here are a few insect examples of endo and ectoparasites. First, endoparasites. An example is the tree squirrel bot fly. Bot flies often lay their eggs on habitat substrates where mammals live. When these eggs hatch into larvae, the larvae enter a body orifice, such as eyes, mouth, nose, anus, or a wound. They migrate for about a week through the host’s body before choosing a site to settle underneath the skin. There they cut a warble pore through the host’s skin from the inside out. Rodent and rabbit-bot flies rarely infect humans, but when they do, they may bore directly through a person’s skin from outside in. At pupation time, the larvae crawl out of their mammalian host and drop to the ground to pupate. And for those of you who are curious, or totally want to be grossed out, there is a human bot fly. To view more photos of the squirrel bot fly, please visit the bot fly website at botfly.ifas.ufl.edu.

[9] Now let’s take a look at another bot fly, the horse bot fly. The horse boy fly, in the genus Gastrophilus, has a slightly different life cycle than that of the squirrel bot fly. Adults bot flies deposit the eggs on the forelimbs and head regions of a horse. As the horse licks its coat, it stimulates the eggs to hatch. The larvae then burrow into the mucosal membranes of the horse’s lips and gums and migrate to the stomach area. Here the larvae feed and may cause ulcers which can be fatal to the horse. Eventually, the horse will pass the larvae through fecal matter. Once on the ground, they pupate for about 1-2 months, before emerging as adult flies, which will then lay its eggs on the horse’s hair or skin for another generation of bot flies to hatch. Remember how I said in the life cycle of a horse bot fly, the larvae migrates from the mouth to the stomach area? If you take a look at the photo below, you’ll see the maggots in the stomach that killed a horse.

[10] Now let’s take a look at some ectoparasite examples. Bedbugs: “Don’t let the bedbugs bite!” Did your parents ever say this to you when tucking you in at night? Bedbugs are an insect that feed on the blood of vertebrates, particularly humans. They like to feed at night and will crawl out of the seams of mattresses or from cracks and crevices where they stay during the daylight hours. When they bite, bedbugs have a welt similar to a flea or mosquito bite. Some people have a more severe reaction than others. Some people may have no reaction at all. Bedbugs are again becoming a problem in the United States. Pest control companies are recording large numbers of calls about bedbugs in homes from people who have recently stayed in a hotel or gone on a cruise. Bedbugs have a sweet odor and leave behind blood spots when they feed. If you suspect bedbugs in a place, pull back the bedding and check the seams of the mattress and box springs for small brownish spots. Bedbugs live in cracks and crevices during the day, so can be hard to spot. They are also very quick moving, so it can be hard to catch once you see one. To view a bedbug feeding, take a minute and watch the bedbug video. A bedbug colony was discovered in a student’s apartment near the University of Florida’s campus. The apartment tenant had purchased a used mattress and had a few unwanted inhabitants. See the piercing-sucking mouthparts as they probe for a blood vessel. Watch how the bedbug takes its blood meal, and then defecates out excess water at the conclusion of the meal.

[11] (Bedbugs Feeding video)

[12] Another example of an ectoparasite is a flea, and these guys like to feed on mammals and some birds. They are flattened side to side, which helps them navigate in between the host’s hairs. Flea larvae do not live on the host, but on the ground, bedding, or on other furniture. After pupating, the emerged adult flea jumps onto a host to feed on its blood. Some people are very allergic to flea bites and others may not react at all.

[13] Remember we said in the chart before 50% of Hymenopterans are parasitic? Well, some wasps are able to find and parasitize beetle larvae that live in the soil. The picture on the left shows a wasp larva feeding on the side of a beetle grub. Remember, the beetle grub is the larger of the two in the left hand picture. On the right hand side, it shows the same wasp larva after it has consumed the beetle and, in the process, has grown much larger.

[14] Many parasites do not kill their hosts. Fleas may weaken the host by sucking too much blood, but this action does not kill the animal they’re living on. However, there is a type of parasite that kills its host. It’s called a parasitoid. Only the larvae of holometabolous insects are parasitoids, especially in the Diptera and Hymenoptera orders. Because they kill their hosts, parasitoids are often used to kill insect agricultural pests like aphids, cabbageworms and European corn borers. Parasitoids have also been used to control house and stable flies. Some parasitoid examples include the tarnished plant bug parasitoid. This wasp, from the family Braconidae, is laying an egg on a tarnished plant bug. The egg will hatch and the larvae will live inside its host for 7-10 days. It then leaves its dying host to pupate on the ground. The tarnished plant bug is a pest of alfalfa. The parasitoid originally lived in France, and was discovered by the USDA. The US government allowed it to be released in New Jersey to helps control the tarnished plant bug that was causing problems with alfalfa crops. The parasitoid has now become well established in many northeastern states.

[15] Another example is an aphid parasitoid. Aphids can suck a plant dry of its juices in a short amount of time. The adult aphidiid wasps lay eggs on the aphids where the larvae will live for two weeks. Different than the tarnished plant bug parasitoid, this parasitoid pupates within the host. Upon emergence, it leaves behind just the aphid exoskeleton, or an aphid mummy, as pictured here. This parasitoid is commercially available, so that one can buy them and release the parasitoid to help control the aphid populations in their yard or landscape.

[16] Now let’s review some terms. Have you ever wondered what a parasite of a parasite is called? Would you believe this is commonly seen among insects? Hyperparasites can be a problem if they attack a parasitoid that is being used to control an insect pest. For example, a pteromalid wasp parasitizes aphid parasitoids. This hyperparasite probes aphids until it finds one that’s infected with the braconid wasp. The pteromalid then lays an egg on the parasitized aphid and the larvae will eat the pupating braconid. Sounds like something from a science fiction movie, huh? For another example of hyperparasitism, watch the video clip “Silk Thread Disappearing Act”.

[17] (Silk Thread Disappearing Act video) Insects are never free from danger, and often an insect’s worst enemy is another insect. A cloverworm caterpillar seems helpless, defenseless to a hungry ladybug larva. But the caterpillar can escape—it drops out of sight on a line of fine silk. Its victim seems to have vanished, so the ladybug larva just gives up. Once the threat has gone, the caterpillar hauls itself up and keeps on eating. This wasp is also hunting the caterpillar, not to eat it, but to lay her eggs inside it, and the wasp is not so easily fooled. The caterpillar tries its vanishing act again, but the wasp just slides down the line. Injecting her eggs won’t kill the caterpillar right away, but it will die as her grubs hatch and eat the caterpillar from the inside. Its day isn’t over yet. Another kind of wasp needs a caterpillar incubator. This one reels in the lifeline and injects her eggs. When they hatch, they will eat the grubs of the first watch. Even the muggers get mugged. It seems that every defense is open to a counter-defense.

[18] Sometimes several larvae of the same species develop in a single host, as shown in this picture of a cabbageworm parasitoid. This is called gregarious parasitoidism. The worm has several parasitoid cocoons attached. The larvae kill the worm when they emerge from their cocoons. To view an example of gregarious parasitoidism, watch the video clip titled “Parasitism of Caterpillars”.

[19] (Parasitism of Caterpillars video) Wasps have turned exploitation into a fine art. They nourish their young at someone else’s expense. This hawk moth caterpillar doesn’t know it yet, but it’s about to become a walking incubator. The female wasp hijacks the caterpillar. Resistance is futile. As soon as she finds somewhere soft enough, she will inject her precious eggs. The inside of the caterpillar is mostly liquid, so her eggs will be cushioned and safe from the outside world. The mother can leave them; the caterpillar will take care of them from now on. It gorges itself, trying to stockpile food reserves for the time when it becomes a moth. But the wasp eggs have hatched into grubs and they absorb all the nourishment the caterpillar can provide. As fast as the caterpillar stores fat around its gut, the wasp grubs devour it. And then they take control. They release a chemical that imitates one of the caterpillar’s own hormones, one that stops it from beginning its transformation into a moth. Instead, it keep on eating and growing until it turns into a supergiant—a huge eating machine that just manages to keep up with the ravenous appetite of the parasites inside. Eventually, when they’ve taken all they need from the caterpillar, they release chemicals to paralyze it and they wriggle out through the body wall. Their first task in the outside world is to spin their silken cocoons to begin their transformations into wasps. The caterpillar will never complete its own metamorphosis, but from one caterpillar’s body, hundreds of wasps have been given life.

[20] Another term to be familiar with is multiple parasitoidism, when two or more parasitoid species attack one host individual. You probably have seen mud dauber wasp nests under the eaves of buildings. They look like a dirt clod clinging to the building. Inside these nests contain the wasp’s developing young. Other wasps, flies and other insects enter these nests and lay their eggs. These invaders will hatch and feed on the mud dauber young. If you were to open one of these nests, you would most likely find at least two different kinds of insects that have parasitized the nest. In the picture below, you can see a southern green stink bug with an attached parasite egg.

[21] Make sure you’ve completed your textbook readings and filled out your study guide. You’ve learned about various examples of endo and ectoparasites, and parsitoids in general. For your journal assignment, I would like you to write a one page essay in your journal discussing the following questions: First, what advantages might an endoparasitoid have over an ectoparasitoid? And second, what advantages might an ectoparasitoid have over an endoparasitoid?

[22] We are now nearing the end of the eleventh unit on insects. During these units, you’ve likely become aware that, though insects are small, they’re amazingly complex and are an important part of our world. For your second assignment of this unit, I would like you to ask yourself: Why are insects so small? There are some physical limitations that are associated with their external skeleton, but I want you to focus on the advantages of being small. While some insects are as large as a mouse, some of the most successful and advanced are smaller than the letter i on this screen. Please enter this short essay in your journal. What are the advantages of being small?

[23] Review Quiz

[24] Who would have thought that an organism so small could do so much damage? Well, that concludes this unit. You should now have a good grasp on various insect parasites, how they affect different animals, and their importance in controlling major agricultural pests. Don’t forget that there are two journal assignments due for this unit. This concludes unit 11.