Ecology of Tiger Beetles

by Gary A. Dunn

Adult tiger beetles are heliophilic insects, remaining inactive on cloudy, cool days. Activity of the adult beetles is regulated by air temperature, humidity (actual evaporation), light intensity, and wind. Most species do not become fully active until the air temperature reaches about 15 degrees C.

Contrary to first impression, the bright coloration and bold markings found on many of these beetles does not lead to their endangerment. Instead the colorful markings serve to disrupt the overall outline of the beetle and make it virtually unrecognizable against the substrate. It is usually the beetles' movement which reveals its position. There is undoubtedly considerable selective pressure to assure that individuals blend into the background. Those which do not are eliminated by predators (Willis 1967).

The dorsal surface of the abdomen is generally brightly colored in metallic or orange-red hues, although these colors are not visible because they are concealed by the folded elytra. However, when the beetle is disturbed and takes flight these bright colors are suddenly displayed and this serves to startle would-be predators.

Adults prefer to run, doing so in short zig-zag bursts. They usually fly only when disturbed by a larger animal or other moving object. Apparently the form and color of the threat is unimportant, for it is the size and motion that trigger the reaction to flight (Shelford 1911).

To become airborne the beetle squats, leaps into the air, and then begins to fly. Most species fly in a relatively low (1 to 3 feet), level, straight path and land 5 to 20 feet away from the source of the disturbance. Some species are more "shy" than others and habitually fly further. On windy days beetles may be carried higher and farther than normal. It may appear that the beetles land and face the direction of the approaching danger, but Moore (1906) proved that they land facing into the wind.

In cases where the habitat and "landing zone" is limited, I have observed beetles flying in a semicircular pattern and landing behind me. This is particularly common in areas of narrow habitats such as trails, roads, and beaches. I have also seen tiger beetles land on vegetation and open water, and crawl into crevices to escape danger.

The adults of several species have been noted to produce defensive odors. For example, Megacephala carolina (L.) produces "an odor which remains a long while" (Fairemaire 1878); Cicindela punctulata produces "an apple-like odor", C. sexguttata "a fragrant odor"and C. longilabris oslari produces "a strong, musky odor" (Leng 1902); C. duodecimguttata has a "musky" odor, C. tranquebarica a "faint apple-like" odor (Willis 1967), and C. ocellata a "fruity odor similar to punctulata but weaker"; C. nigrocoerulea produces a "fruity" odor (Willis and Stamatov 1971).

These odors are produced by the anal, or pygidial, gland which is poorly developed. Brandt (1888) surmised these glands are rather poorly developed in comparison to the closely-related Carabidae because the tiger beetles have more advanced escape methods such as taking running, flight, and use of sharp mandibles.

Adult tiger beetles also exude an unpleasantly scented, brownish fluid from the hypopharynx when captured. The function of this fluid is probably to predigest the prey when feeding. It was noted "that where it comes in contact with the net, holes will appear sooner than elsewhere" (Townsend 1884).

Adult tiger beetles are not completely free from predators. Graves (1962) observed a dragonfly, later identified as Aeshna interupta interupta Walker, capture an adult of Cicindela repanda while both were on the wing. Lavigne (1977) reported on the species of Cicindelidae that are prey of robber flies (Diptera: Asilidae). Wallis (1961) noted that these flies attack beetles while they are in flight and the softer parts of the thorax and abdomen are exposed.

LaRochelle (1974) gave accounts of 15 species of amphibians and reptiles known to have fed on tiger beetles. He also listed mammals that feed on tiger beetles: badger, northern plains fox, eastern mole, grasshopper mouse, opossum, raccoon, eastern striped skunk, eastern spotted skunk, and thirteen-lined ground squirrel (LaRochelle 1975b). Many birds, including burrowing owls, kestrels, waterfowl, raptors, gamebirds, shorebirds, woodpeckers, and songbirds are known to feed on tiger beetles (LaRochelle 1975a; Maser 1973; Smith 1976).

Adult beetles burrow in the soil to pass the night and to escape unfavorable weather, either hot and dry or cold and wet. They also burrow rather deeply in preparation for winter diapause.

Quite a few species of tiger beetles are attracted to lights. Most experts feel that it is only the more "advanced" species that fly to lights. Recently it has been suggested that the beetles actually come to the light to feed on other insects attracted to the lights. In either case it is interesting to note how the beetles' behavior differs between daylight hours and nighttime. Many species that cannot be approached within 20 feet during the daylight hours are easily picked up by hand as they scurry about a light!

Tiger beetles have very definite habitat preferences. Many species inhabit sandy areas such as sand dunes, sand pits, and sand "blowouts" (often far away from water); species encountered in these habitats include Cicindela formosa, C. scutellaris, C. limbalis, C. lengi and C. hirticollis. Other species frequent sand and gravel areas near water; examples include C. repanda, C. duodecimguttata, C. hirticollis and C. macra. In the western United States and along the coasts many species are associated with alkali flats; here you will find C. willistoni, C. nevadica, C. togata, C. circumpicta and C. marginata. Other species are found in various upland sites including steep hillsides (C. limbalis and C. splendida), rocky areas (C. rufiventris, C. olivacea and Amblychila baroni), roads, trails and forest openings (C. sexguttata, C. patruela, C. longilabris and C. unipunctata).

Several species of tiger beetles may inhabit the same area. Gause's Law of Competitive Displacement states that species with identical ecological niches cannot coexist in the same habitat for a long period of time (DeBach 1966). Within a given area of macrohabitat many species of tiger beetles do often coexist. However they are spared from direct competition in two ways. First, many of them actually occupy different microhabitats (e.g., soils with different moisture levels, textures, salinity, and vegetation/plant cover). Secondly, many species are separated seasonally; that is the various beetle species have different times of emergence and peak populations. The combination of these two types of segregation provides for nearly complete separation among most species, and considerably reduces the competition among the rest.

Selection of the breeding, or larval, habitat is a critical task left up to the adult female during oviposition. Because the larvae are relatively immobile and the habitat requirements are more circumscribed than that of adults the availability of larval habitat is often the limiting factor that controls the population levels of the beetles. When an area of larval habitat becomes endangered and disappears, so does the species it supports.

Copulation generally occurs on warm days with high humidity. After mating, the eggs are deposited singly in the soil, each in a small indentation created by the female's ovipositor. Part of the ninth and all of the tenth abdominal segments of the female are covered with hairs sensitive to conditions of soil moisture and structure. The female carefully selects the proper soil type before finally ovipositing. Survival of the larva is dependent upon this choice.

The larvae of the tiger beetles were first noted in the literature in 1798 (Hamilton 1925), but it was not until 1867 that the first work dealing with these beetle larvae was published by J.C. Schiodte. In the period between 1878 and 1911 George Horn, F.G. Schaupp, Victor E. Shelford, and Norman Criddle made many important contributions to our knowledge concerning the larvae of North American species. In 1925 Clyde C. Hamilton published a comprehensive study on the taxonomy of holarctic tiger beetle larvae. It remains the largest taxonomic work on North American Cicindelidae larvae to this day.

Tiger beetle larva

Tiger beetles have three larval instars. Upon hatching from the egg the first instar larva proceeds to enlarge the impression made by the female's ovipositor into a burrow. As the larva grows the burrow is enlarged.

Larval burrows of the tiger beetles are very characteristic. The entrance to the burrow is flush with the surface of the ground, and is clean and smooth. There is no "cone" of soil particles, as the larvae toss this material as far away from the burrow as possible. Most burrows are constructed so that they are perpendicular to the soil surface. However, there is considerable variation among individuals and species. The burrow depth varies greatly, although it does seem to be correlated to geographic location. Those in northern regions tend to be deeper. Most burrow construction takes place at night.

Tiger beetle larval burrow under construction (note coarse pellets on one side only)

Ant hole - note circular field of losse soil and debris around opening

When first sighted larval burrows appear to be unoccupied. Larvae are very wary and quickly drop to the bottom of the burrow. They are sensitive to motion, and possibly to vibration of the substrate, but are quite insensitive to sound (Macnamara 1922). Larva also periodically plug the entrance to their burrow with soil, especially after eating, during rainy weather, during droughts, before hibernation or aestivation, before molting, and before pupation.

The larvae are elongate, cylindrical, and somewhat grub-like. The mandibles are powerful and curve upwards. The head and prothorax are fused and rounded, forming a circular plate. The head is held at nearly a right angle to the axis of the body. The fifth abdominal segment has a tergal hump bearing anteriorly curved spines.

The larvae wait at the burrow entrance and ambush passing prey. The rounded cephalo-pronotal plate helps disguise the larva's presence and hide the opening to the burrow. The larva employs the thoracic legs, tergal spines, and terminal spines to support themselves in the burrow by assuming an "s" shape and bracing themselves against the walls of the burrow.

Tiger beetle larva feed on many types of small arthropods. LaRochelle (1977) gives a detailed account of the larval feeding for two species. Prey are generally consumed at the bottom of the burrow, unless too large. The tergal spines help keep the larva anchored in place when grabbing large prey. The first instar larva generally needs one good meal to store enough energy for its molt into the second instar. The second and third larval instars require at least several meals to survive and grow.

As you might guess, larvae may often have severe problems with competition for available food. However, long developmental periods and 24 hour feeding abilities are probably evolutionary attempts to overcome this problem (Willis 1967). Given the precarious existence of the larvae, it is no wonder that the larval stage comprises the longest portion of the tiger beetle life cycle.

Larvae have few natural enemies. Some bee flies (Diptera: Bombyliidae) are parasites of larval tiger beetles. By far the greatest enemy that larval tiger beetles face are humans who destroy precious larval habitats through soil disturbances (construction, flooding, offroad vehicles, etc.).

The third instar larva is responsible for construction of a special pupal chamber, usually off to one side of the larval burrow. The larvae plugs the burrow with soil obtained from construction of the pupal chamber. The larva becomes quiescent, resting upon its dorsal surface. Within a period of time (one to three weeks) the larva is no longer able to move its appendages. The abdomen thickens and becomes a translucent, creamy-white color. Gradually the cuticle splits and body contortions are used to free the pupa from the old larval exoskeleton. The pupa then contracts to a slightly smaller size.

Tiger beetle pupa

The first five abdominal segments of the pupa's abdomen bear a pair of dorsal tubercles, each with a ring of apical setae. These tubercles hold the pupa up off the substrate.

The pupa gradually darkens, and the developing mandibles, tibia, tarsi and wing pads of the adult can be distinguished easily. The adult beetle emerges from the pupa via a dorsal split in the integument; this process takes about two hours to complete. The pupal period generally last 18 to 24 days (unless overwintering in the pupal stage).

After emergence the cuticle of the new beetle begins to harden and darken. In an hour or two the elytral markings begin to show and the elytral trachae are visible. Within ten hours the beetle is able to stand, and within 48 fours may become fully active. Some adult beetles may continue to change color long after emergence, and it has been shown by several researchers that the coloration varies according to environmental influences during pupation and subsequent emergence. Also, lipid deposits may stain and darken light-colored markings as the beetle ages.

NATURAL ENEMIES

Tiger beetles have a number of natural enemies. The adults are preyed on by various insect-eating animals, mainly insectivorous birds and larger insects (robber flies, dragonflies, etc.). Larvae are parasitized by larvae of certain bee flies (Bombyliidae) and a type of wasp (Tiphiidae). Probably man is their worst enemy, since they are readily killed by insecticides and such disturbances as dune buggies and dam building.

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