Predation/Predatory Insects
Predators are animals that must consume more than one individual of another animal to complete their life cycle. Under JL this definition, the predatory habit occurs in a wide range of insect groups. Some orders, such as Odonata and Neuroptera, are wholly or predominately predaceous. Other orders contain large numbers of species that are predatory, such as Hemiptera, Coleoptera, Mecoptera, Diptera, and Hymenoptera. Even a few species in the orders Ephemeroptera, Orthoptera, Plecoptera, Thysanoptera, Trichoptera, and Lepidoptera are predaceous. This article emphasizes the diverse ways in which predaceous insects live.
MODES OF PREDATION
Hunting
Many insect predators carry out active hunting. The adults of tiger beetles (Cicindellidae), ground beetles (Carabidae), and many ants (Formicidae) actively run over the ground to capture prey. Many walking or crawling predators, including ladybird beetles (Coccinellidae), lacewing larvae (Chrysopidae), syrphid fly larvae (Syrphidae), and predatory bugs (Hemiptera), feed on sedentary insects such as aphids and scale insects or on eggs and young larvae of more mobile species. Other insect predators are agile flyers and actively snatch insects out of the air. Dragonflies and damselflies (Odonata) have large eyes and strong wings and feed mainly on mosquitoes and other small flying insects. Robber flies (Asilidae) also catch their prey on the wing, but will often tackle and subdue insects at least their own size, even bees and wasps. Their legs are very strong and they have piercing mouth-parts to suck up prey juices. Other insects, such as many social wasps (Vespidae), pluck insects from the ground or vegetation while flying.
Many predaceous insects live in fresh water and pursue prey by swimming. The giant water bugs (Belostomatidae) are excellent swimmers and often capture and subdue small fish and tadpoles. Backswimmers (Notonectidae) hang ventral side up at the water surface and dart after aquatic organisms using powerful, oar-like hind legs. The diving water beetles (Dytiscidae) are also voracious predators that actively capture prey under water. Water striders (Gerridae) are true bugs that skim over the water surface and prey on small organisms that fall from above. Whirligig beetles (Gyrinidae) also hunt on the water film, but sometimes dive below the surface. Each eye is a double structure, with half adapted to seeing above the surface and the other half below.
Other active hunters are less visible. A large number of beetle and fly larvae, as well as immature insects from other orders, prey on small organisms by burrowing through soil, wet ground, rotting logs, or other vegetation. The immatures of many horse flies and deer flies (Tabanidae), for instance, are predaceous on organisms in moist soil and water. The larvae of some common midges (Chironomidae) feed on other organisms in the mud at the bottom of ponds or along its margins, as do the larvae of some crane flies (Tipulidae). Click beetle larvae (wireworms, Elateridae) are generally plant or detritus feeders, but some are predatory in soil or rotting logs.
Stalking and Ambush
Many predaceous insects subdue active prey, but not by hunting. These are the ambush predators or slow-moving stalkers. Many have special adaptations to efficiently capture prey. Praying mantids (Mantidae) have large, grasping forelegs, superbly fitted for grasping and holding prey, and remain motionless on vegetation until an insect comes near enough that they can strike out with their forelegs. A number of the true bugs also capture prey this way, including assassin bugs (Reduviidae), Nabidae, and ambush bugs (Phymatidae). Dragonfly and damselfly nymphs are successful ambushers that live in water. Their labium, or lower lip, is modified as a grasping tool that can be rapidly shot out to impale an insect, tadpole, or small fish. Tiger beetle adults are hunters, but their larvae dig vertical burrows in the ground and wait with their heads plugging the entrance. Such larvae grab small organisms walking near the burrow. They have back spines that help anchor them so they are not pulled out. After capture, the prey is dragged to the bottom of the burrow and devoured.
Trapping
Insects that trap their food are similar to ambush predators, but go a step further and use inanimate materials to help snare their prey. The ant lions (Myrmeleontidae) are the larvae of insects in the order Neuroptera that as adults look somewhat like damselflies. Larvae burrow into sand and form conical pits. Small, walking organisms, such as ants, fall into these pits and have difficulty escaping because of the loose material of the walls. In addition, the ant lion at the bottom actively tosses its head to remove sand dislodged by the victim. When the prey falls to the bottom, it is grabbed in the long mandibles of the predator, dragged under, and sucked dry. Some snipe fly larvae (Rhagionidae) build similar pits and capture prey the same way.
A few larvae of fungus gnats (Mycetophilidae) capture prey by secreting webs of mucilaginous materials in moist locations, such as beneath rocks, under bark, or in caves. In New Zealand caves, individuals of Arachnocampa luminosa not only spin webs that hang from cave ceilings, but also glow to attract adults of chironomid midges that breed in the water below. The colonies of the gnats are tourist attractions, and the ceilings of the caves are covered with pinpricks of light reminiscent of starry nights.
Provisioning
Solitary wasps in the families Vespidae, Sphecidae, and related families are often provisioners—the adult females capture prey that they paralyze with a sting and place the prey in a closed cell along with an egg. The larva that hatches feeds on the enclosed prey until fully developed. Different provisioner species specialize on different prey and construct different kinds of cells. For instance, the mud-dauber sphecid wasp, Sceliphron caementarium, constructs cells out of mud and provisions these with spiders. Ammophila, another sphe-cid, constructs cells by burrowing into the ground and stocks them with caterpillars. Others may burrow into wood or plant stems. Prey species vary from aphids to grasshoppers, true bugs, caterpillars, or spiders. Such provisioning wasps as the cicada killer, Sphecius spe-ciosus . stretch the definition of predator. The female usually places one paralyzed cicada per underground cell, but sometimes she puts in two. Thus, the resulting progeny are sometimes parasitoids that can complete development on one host individual and sometimes predators that consume more than one prey.
Host Feeding
This is another behavior that blurs the line between parasitoids and predators. Host-feeders are female adult hymenopterous parasitoids, especially of the family Pteromalidae, that feed on host fluids oozing out of oviposition wounds. Such females host-feed to obtain protein for egg maturation. In many cases the host is parasitized as well. But some parasitoids have developed a habit of host-feeding without oviposition, and in some cases the host (which in this case becomes the prey) may die from excessive feeding. Some parasitoids even use fast-drying fluids secreted from the base of their ovipositors to construct “feeding tubes” from a concealed prey to the surface through which host fluids can be drawn up.
Mass Foraging
There are many social insects, primarily wasps and ants, that actively hunt prey for food. Most of these do so as individuals, capturing small prey, and returning directly to the nest. But some ants are able to overcome vertebrate and large invertebrate prey. These are the mass foragers, the army ants and driver ants. These ants move over the ground in large swarms or columns and subdue prey, such as large scorpions and lizards, that other ants cannot. They also devour many small arthropods and deplete an area of prey so rapidly that they must frequently move from one nest site to another. The army ants of the New World tropics and the driver ants of Africa have been depicted in fiction as very dangerous predators that may even threaten humans. While it is possible that penned animals may be killed, the ants are not generally dangerous. The swarm front or hunting column moves forward only relatively slowly, and it is easy to step out of the way. In fact, a swarm of army ants removes vermin as they raid through a house and may be beneficial, so long as the house is unoccupied while the ants are there.
Deception and Predation
Some insect predators use deceptive practices. Firefly (Lampyridae) adults use bioluminescence to attract mates. Each species has a set of recognition flashes that are exchanged between flying males and the sedentary and often wingless females. If the sequence is correct, a male is guided to a receptive female. However, the females of some species mimic the recognition flashes of other species and so lure fooled males to their death.
Perhaps some of the most sophisticated deceptive predation practices occur among the insects that live in or near ant nests. These “ant guests” or “myrmecophiles” may survive by stealing food from ants by soliciting, but many are also predators. To be successful they must deceive the ants into accepting or at least tolerating them.
Some of these predators live on the periphery of nests and prey on foragers that they encounter. The reduviid Acanthaspis concinnula fools ants by placing the bodies of its prey on its back. Another redu-viid, Ptilocerus ochraceus, secretes “tranquilizing” attractive substances from trichomes on its abdomen that apparently subdue ants long enough for the bug to kill them.
Other insects, particularly beetles of the family Staphylinidae, live in the brood chambers of ants, where, among other things, they prey on ant larvae. Most of these predators apparently use chemical subterfuge to gain entry and stay in the brood chambers. Apparently, the chemicals mimic recognition substances produced by the ant brood. In fact, ants may pay more attention to the larvae of the beetles than they do to their own larvae. Some lycaenid butterfly larvae also have this habit. Early instars of these caterpillars live on plants and secrete attractive substances that ants collect. Larger caterpillars move to the brood chambers where they prey on ant larvae.
BEHAVIORAL ECOLOGY AND PREDATORS
Behavioral ecology is a field in which actions and movements of individual organisms are studied in order to understand interactions and impacts between organisms and their environment. Predaceous insects are often used as subjects in behavioral ecological studies because they are numerous, generally have a limited number of responses, and are small enough to be easily manipulated and yet large enough for easy observation. For instance, numerous studies have investigated how the number of prey eaten by a predator varies as prey numbers change. These functional responses usually show an increased tendency of predators to devour more prey as the number of prey individuals increases until the predators become satiated, so that the number of prey eaten increases rapidly at first as prey numbers increase and then becomes steady. However, backswimmers (Notonectidae), which feed on a variety of prey, may switch from one prey type to another as the latter become more abundant. In such a case, the functional response is S-shaped, at first starting out slowly and then increasing rapidly until slowing again at high prey numbers. Such S-shaped, or sigmoid, functional responses are usually more typical of vertebrate predators than insects.
An important aspect of behavioral ecology has to do with searching by organisms in areas that contain various amounts and quality of food. Many predators, such as ladybird beetles (Coccinellidae), aggregate in places containing many prey. Finding the most profitable way in which a predator can divide its time between encountering prey within an area of prey abundance and searching for other concentrations of prey is an exercise in optimal foraging theory. Predators such as backswimmers (Notonectidae) have been found to do this rather well. The actual time they spend foraging in an area varies with prey abundance in a way that is close to optimal. Like the better-studied parasitic Hymenoptera, some predators can improve their foraging efficiency over time by learning to associate particular chemical cues of their host’s environment with successful foraging events.
Ants have been used to investigate the kind of prey searching called center place foraging. In this situation, an organism departs and returns to a fixed location, often a nest, during a foraging session. Studies of ant foraging show that a center place forager often restricts its searching to only part of the available area, foraging in different areas at different times. Also, such foragers often do not search near the nest, but only at some distance from it. Both these characteristics may increase the efficiency of foraging by decreasing searching overlap with nest mates.
IMPACT OF PREDATORS ON POPULATIONS OF THEIR PREY
While most biological control work has involved parasitoids, some predators have been used as well. In fact, the first really successful classical biological control project was the control of the cottony cushion scale, Icerya purchasi, by the vedalia beetle (Coccinellidae), Rodolia cardinalis. Other coccinellids are important predators of aphids and mites. Ground beetles are important generalist predators, and at least one of these (Calosoma sycophanta) is an effective predator of the gypsy moth. Other predators used in biological control practice include the true bugs Orius (Anthocoridae) and Geocoris (Lygaeidae), clerid beetles, green lacewings, syrphid flies, and marsh flies (Sciomyzidae). On the other hand, some robber flies are voracious predators of bees and are considered to be economically harmful. Also, because of their propensity to attack nontarget prey, generalist predators are not considered good prospects for use in programs that introduce exotic natural enemies into new areas.
Insect predators may be more important in the regulation of their prey species than is commonly thought. Because they often leave no identifiable remains after devouring prey, the extent of predation is hard to quantify. It is probably true that insect predators help keep the majority of potential pest organisms under control. Without them, the world likely would be quite different.
