Insects as predators and parasitoids D. L. A. Underwood Biology 316 - General Entomology A. Introduction 1. Definitions a. Predators kill and consume more than one prey organism to reach maturity. b. Parasitoids require only one host to reach maturity, but ultimately kill the host. c. True parasites feed on one or more hosts, but do not normally kill the host. d. The term "predatory" will refer to both predation and parasitoidism. 2. Prevalence a. About 25% of the insect species are predators or parasitoids (See pages 6-8 of the outlines for the tables). b. Nearly every order of insect contains predatory species. c. Some species are only predatory as immatures, others as adults, and still others are predatory throughout their life span. d. Some orders are nearly entirely predatory like the Odonata, the Mantodea and the Neuroptera. 3. Team Project - A takes notes, B speaks a. Identify three ways predators would differ from herbivores. B. Host/prey location and acceptance 1. Random searchers roam in the appropriate microhabitat and capture prey upon contact. Examples of microhabitats include leaf edges, veins, stems, flowers, etc. a. After an initial contact or meal, further searching involves more frequent turns resulting in the predator remaining in the same general location. b. Random searchers are the most common type of insect predator. c. Examples include most coleopteran predators, coccinellids, syrphid larvae, and lacewings. 2. Hunting, or directional foraging differs from random searching by using sight or other stimuli to orient to prey from a distance. a. Many hunters initially identify prey items via vision. i. A sphecid wasp that preys upon bees will attack any flying object that is approximately the same size as a bee but will only sting an actual bee suggesting that tactile stimuli are used at close range. ii. Robber flies and dragonflies have enormous eyes for detecting prey items in flight. b. Initial host location may also occur via chemical cues. i. Many predators use chemical signals produced by the prey for intraspecific communication. Insects as predators and parasitoids - Page 1 of 7
Parasitic tachinid flies and braconid wasps locate their respective stink bug or coccoid host by following their hosts' long-distance sex attractant pheromones. Parasitoid hymenopterans follow the aggregation pheromones of their bark and timber beetle hosts. ii. Other predators locate hosts by detecting chemicals emitted from stressed or insect damaged plants. Parasitoids of the scolytid timber beetles cue into the terpenes that are released from the infested pine trees. iii. Just as phytophages find their host plants by volatile chemicals, predators of these phytophages also are attracted by these chemicals. iv. Some parasitoids can even detect frass odors of their insect larval hosts. c. Host location is also accomplished by detecting sound. i. Blood-sucking flies of hylid treefrogs cue into the frogs' calls. ii. Tachinid parasitoids of some crickets find their hosts by listening to the cricket's calls (see attached description). iii. Sarcophagid parasitoids of cicadas do likewise. iv. Ceratopogonid biting midges find swarms of prey midges by detecting the sounds produced by wing-beat frequencies. d. Phoresy is a unique method of host location that involves "hitchhiking." i. A predator or parasitoid jumps onto another insect that carries it to a new location. ii. The new location may place the predatory insect in close proximity to a new host or prey. One species of blood-sucking ectoparasitic fly of some birds transport avian lice (Phthiraptera) from one bird nest to another. Many egg parasitoids attach themselves to adult females of the host species, thereby gaining instant access to her eggs. iii. Other phoretic species attach to and are carried back to the nest of the transport species. Some chalcid hymenopterans have larvae that actively seek out worker ants, to which they attach and eventually burrow into ant grubs or pupae within the nest. Female bot flies capture mosquitoes, hold them in such way to prevent injury, then place around 30 eggs on the undersurface, which will not interfere with flight. The mosquito then carries the bot fly eggs to a vertebrate host. The elevation of temperature associated with the vertebrate host induces the eggs to rapidly hatch and fall onto the host. Insects as predators and parasitoids - Page 2 of 7
3. Ambush, or 'lie-n-wait' predators remain in one place and strike at acceptable prey when they come within range. a. Many insects' vision is limited to recognition of movement so that a non-moving predator is undetectable to a potential prey organism. b. However many ambush predators are also cryptic, resembling things like bark, lichen, or leaves. c. Other ambush predators mimic an object that prey are attracted to like flowers. d. Odonate larvae rest concealed in submerged vegetation or on the substrate waiting for prey to pass by. e. One form of ambush also involves hunting tactics. Dragonfly adults often sit perched on vegetation, then dart out and capture prey on the wing much like the flycatchers of the bird world. f. Hypermetamorphic larvae are common within the parasitoid taxa. These larvae are active as first instars, seek out and find hosts, then metamorphose into relatively inactive, non-motile larvae. These larvae will metamorphose again at pupation. i. Meliod beetle larvae are hypermetamorphoic and active. They attach to desert bees that then transport them to their nests. ii. Many tachinids eggs hatch into hypermetamorphic larvae. 4. Trappers are predators that construct traps in which to capture prey. a. This could be considered a form of ambush the predator waits near the trap then pounces on the prey once it is ensnared in the trap. b. Ant lion larvae are the classic example. c. Larvae of the glowworm, a type of gnat, live in moist caves. They spin slimy webs that dangle from the ceiling. The larvae rest behind the web and are bioluminescent, attracting prey into the web. Some forms of this gnat produce a web that is not only sticky but contains toxic chemicals. d. Hydropsychid caddisfly larvae also construct nets in which to trap prey. e. Some smaller assassin bugs of the subfamily Emesinae (Fig 24-22E in the lab text) wait near spider webs and steal the trapped prey from the spider. 5. Team Project - B takes notes, C speaks a. Rank the four general types of predator (random searchers, hunters, ambush predators, and trappers) with respect to energy requirements. b. Rank them again with respect to the average amount of food caught per unit time. Insects as predators and parasitoids - Page 3 of 7
C. Host/prey acceptance and manipulation 1. Prey acceptance and manipulation by predators a. A step-by-step sequence of stimuli is usually necessary for prey location and acceptance and may involve the shape of the prey, chemical coatings on the prey, and volatile chemicals produced by a host plant of the prey. b. Often size and/or the age of the potential prey/host are assessed. c. Chemical and tactile stimuli are involved in species identification and may include the predator biting or piercing the potential prey. d. Predators must be able to restrain their prey while killing it. This is often accomplished using raptorial forelegs as in mantids, modified hind legs as seen in some scorpionflies, or the labial mask of odonate nymphs. e. Mouthparts of predators are of two general types. i. Modification of various elements (labrum, labial palps, labium, maxillae, hypopharnyx) of the head and mouthparts forms piercing-sucking mouthparts with stylets that are either slender and needle like or knifelike. ii. The mandibles are strengthened and elongate. f. Many predators with piercing-sucking mouthparts also inject enzymes into the prey then suck up the predigested liquid (extraoral digestion). Only material that is digestible is taken into the body. This is obviously more efficient than eating the organism whole. 2. Host acceptance and manipulation by parasitoids a. Initially chemoreceptors in the antennae signal information as to the appropriateness of the potential host. i. Many parasitoid Hymenoptera sport relatively long antennae. ii. Species identification is confirmed by chemoreceptors located on the ovipositor of the female parasitoid. iii. Parasitoids of wood-boring insects must find their host strictly using chemoreceptors on the ovipositor (ichs on siricids). b. Size and/or the age of the potential host are also assessed. i. Many parasitoids reject old larvae that are close to pupation, while others seek out the biggest, oldest larvae. 3. Host immune responses a. The usual host reaction to a parasitoid egg or larva is encapuslation (see page 336) by aggregating haematocytes. Phagocytosis usually follows. b. Parasitoids in their normal host have evolved ways to avoid encapusulation. i. Avoidance: The parasitoid resides in a particular organ like the brain, a ganglion, a salivary gland or the gut and thus escape the immune reaction of the host haemolymph. ii. Evasion: The parasitoid is a molecular mimic to the host proteins and is not recognized by the host as being non-self, or the parasitoid insulates itself in a membrane or capsule, or development is so rapid that the host cannot respond in time to encapsulate. Insects as predators and parasitoids - Page 4 of 7
iii. Suppression: The host cellular immune responses may be suppressed by viruses associated with the parasitoid. iv. Subversion: The parasitoid is resistant to encapsulation and/or the capsule is subverted for use as a sheath. D. Parasitoidism 1. Types a. Most parasitoids are host discriminate and will not oviposit into or onto a host that is already harboring a parasitoid. b. Some parasitoids practice superparasitism, laying their eggs in parasitized hosts. All the eggs received by the host are from the same species of parasitoid. c. In multiparasitism the host receives eggs from one more than one species of parasitoid. d. Hyperparasitism involves a parasitoid parasitizing the larva of another parasitoid. Some species are obligate and others are facultative hyperparasitoids. 2. Host specificity a. Most parasitoids are restricted to a single species of host. b. This type of specificity is analogous to that seen in monophagous phytophages. E. Reproductive oddities 1. Polyembryony is found only among parasitoids. a. A single egg may divide to form new "eggs" which all develop into larvae. b. As many as 1500 embryos in a single caterpillar have been counted after oviposition by female Encyrtidae (Hymenoptera). c. Some encyrtids lay two types of eggs. The defender morphs hatch first and seek out and destroy any larvae already present in the host. These larvae fail to pupate and die. The second type of egg hatches later and the larvae reproductive morphs, completing development. 2. Skewed sex ratios are exceedingly common in parasitoid Hymenoptera. Insects as predators and parasitoids - Page 5 of 7
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