Chapter 2: Bilogy of Yellow Starthistle Biocontrol Agents
Wilson, L.M.; Jette, C.; Connett, J.;McCaffrey, J.; Randall, C.B.; Kuykendall, C.; Lake, L. Biology and Biological Control of Yellow Starthistle. USDA Forest Service FHTET-1998-17 2nd Ed. July 2003. 60p.
Biological control of yellow starthistle began in North America in 1985. Since then, six biocontrol insects have been released: three beetles and three flies. All are seed feeding. All the beetles are weevils and include the bud weevil, Bangasternus orientalis, the hairy weevil, Eustenopus villosus, and the flower weevil, Larinus curtus. All of the flies are fruit flies and include the peacock fly, Chaetorellia australis, the closely-related false-peacock fly, C. succinea, and the banded fly, Urophora sirunaseva (Table 1, page 12).
|Type||Scientific Name||Common Name|
|Beetle||Bangasternus orientalis (Capiomont)||Yellow Starthistle bud weevil|
|Beetle||Eustenopus villosus (Boheman)||Yellow Starthistle hairy weevil|
|Beetle||Larinus curtus (Hochhut)||Yellow Starthistle flower weevil|
|Fly||Chaetorellia australis (Hering)||Yellow Starthistle peacock fly|
|Fly||Chaetorellia succinea (Hering)||Yellow Starthistle false peacock fly|
|Fly||Urophora sirunaseva (Hering)||Banded yellow Starthistle fly|
All of the agents are widely distributed in starthistle-infested areas of the western United States, particularly California, Oregon, Washington and Idaho. Though the degree to which each is prevalent may differ, most areas have one or more species well established.
This section describes, in detail, the life history, biology and impact of each of the agents and is organized into two subsections: seed-feeding weevils and seed-feeding flies.
Basic Insect Biology
Insects are a very large, diverse, complicated group of animals. In order to optimize using insects in a biocontrol program, it is useful to know something about insects. Basic knowledge of their anatomy and lifecycle will help a great deal in understanding and recognizing insects in the field. Adult insects share several characteristics, including: an exoskeleton; a segmented body in three parts, the head, thorax and abdomen; and three pairs of legs (Fig. 6).
Insects grow and develop through a series of molts. The transformation from juvenile to adult stage is called metamorphosis. This process can be incomplete or complete. Insects used in biocontrol of yellow starthistle all undergo complete metamorphosis, meaning they have an egg stage, a larva stage (of which there can be three or more instars), a pupal stage, and an adult stage (Fig. 7).
What is This Insect?
An important part of any successful biocontrol program is to be able to identify the insects in the field. As adults, the insects are relatively easy to identify, with their variable size, form, color, and habits. The larvae are more challenging than the adults – and yet are probably more important to know as this is the stage that 1) does the damage, 2) is monitored in the field, and 3) provides the best evidence that the insects are established in the field.
Using the key in Figure 8, you can tell in three easy steps if you are looking at a starthistle fly or a beetle larva. Beetle larvae are as variable as adult beetles are, but weevil larvae are distinctly white, C-shaped grubs with a brown head capsule and lacking prolegs. Fly larvae have no head capsule. They are sometimes confused with other larvae because they appear to have a broad, dark head. However, this is actually a dark, hardened anal plate that is used to anchor the larva to its host.
Using the key in Figure 9, you can tell weevil pupae from fly pupae that are located inside the starthistle head. Beetle pupae have well-developed appendages that are obviously not fused to the body. Fly pupae are contained inside a puparium.
Yellow Starthistle Weevils
Weevils are phytophagous (plant-eating) beetles that generally have long, well-developed snouts with chewing mouthparts at the tip. They use the snout to chew deep into a host plant. Antennae are elbowed and attached to the snout about halfway along its length. Weevils are hard-bodied insects with tough, thick exoskeletons. They possess two pairs of wings; the front pair is thickened to form a hard covering called elytra. When weevils are not flying, the elytra are held over the back to form a protective covering. The membranous hind wings are used for flight and fold under the elytra when not in use.
Starthistle weevils are univoltine, meaning they complete one generation per year. The adult overwinters in protected areas on the ground and become active the following spring. Thus, the first weevils seen in the spring are from the overwintering generation and the new weevils that emerge mid- to late summer are from the new generation. As adults, weevils generally cause little damage to the plant. They may do some feeding of the foliage, but the amount is usually negligible. The exception is the hairy weevil (E. villosus) which feeds on starthistle buds, often killing the bud. For most of the insects, however, it is the internally-feeding juvenile stage, or larva, that causes damage. Larvae undergo three molts (or instars) during their development inside the starthistle head.
Table 2 provides details on three weevils available for starthistle biocontrol. Table 3 compares lifecycles of each of the weevils.
Table 2. Comparison of adult yellow starthistle weevils.
|Bangasternus orientalis||Eustenopus villosus||Larinus curtus|
|Emerges 1st (bolting)||Emerges 2nd (BU-I)||Emerges 3rd (BU-1)|
|Cylindrical body shape||Cylindrical-oblong body shape||Oblong body shape|
|0.16 – 0.24 inches||0.16 – 0.24 inches||0.2 – 0.24 inches|
|Brown with yellow/white hairs||Brown with gray/white hairs||Brown|
|Mottled appearance||Striped||Yellow, Spotted|
|Short snout||Long, slender snout||Medium-sized snout|
|Release at BU-1 to BU-3 stages||Release at BU-3 to BU-4 stages||Release at flowering stage|
Table 3. Comparison of weevil lifecycles by yellow starthistle growth stages.
|YST Stage||Bangasternus orientalis||Eustenopus villosus||Larinus curtus|
|Seedling||Adult overwinters in the ground litter.||Adult overwinters in the ground litter.||Adult overwinters in the ground litter.|
|Rosette||Adult overwinters in the ground litter.||Adult overwinters in the ground litter.||Adult overwinters in the ground litter.|
|Bolting||Adult emerges; visible on plants.||Adult overwinters in the ground litter.||Adult overwinters in the ground litter.|
|BU-1||Female oviposits; black egg cases visible.||Adults emerge, feed on buds; visible on plant.||Adults emerge; visible on plant.|
|BU-2||Larvae develop; black egg cases visible.||Adults emerge, feed on buds; visible on plant.||Adults emerge; visible on plant.|
|BU-3||Larvae develop; black egg cases visible.||Adults emerge, feed on buds; visible on plant.||Adults emerge; visible on plant.|
|BU-4||Larvae develop; find in seedhead.||Female oviposits. Feeding damage visible.||Female oviposits.|
|Flowering||Larvae develop; find in seedhead.||Larvae develop; find in seedhead.||Larvae develop; find in seedhead.|
|Seed Formation||Larvae develops; find in seedhead.||Larvae develops; find in seedhead.||Larvae develops; find in seedhead.|
|Mature||Larvae pupate.||Larvae pupate.||Larvae pupate.|
|Dissemination||Adults emerge; visible on plant.||Adults emerge; visible on plant.||Adults emerge; visible on plant.|
|Senescence||Adult overwinters in ground litter.||Adult overwinters in ground litter.||Adult overwinters in ground litter.|
Insect Order: Coleoptera
Insect Family: Curculionidae
Common Name: Yellow starthistle bud weevil
Description: Adults are 0.2 to 0.3 inch (4 to 6 mm) long. The weevil is dark reddish brown with pale colored hairs giving it a mottled appearance, and has a somewhat flattened, cylinder-shaped body and a short snout (Fig. 10). It has one generation per year.Adults emerge from overwintering sites early in the yellow starthistle bolting stage to mate and lay eggs. The adults are especially active during warm periods of the day. Although adults feed on foliage, they do not significantly damage the plant.
Oviposition begins within 1 or 2 weeks of emergence and continues for 4 to 8 weeks. Eggs are laid singly on terminal leaflets, on or at the bases of young flower buds and on or near BU-1 and BU-2 buds. They are covered with a dark mucilage to protect them from predators and desiccation (Fig. 11). At high weevil densities, dark-covered eggs can be seen on most plants. Presence and density of these conspicuous eggs are a good indicator of B. orientalis presence at the site.
Eggs hatch within 1 week after oviposition and the larva tunnels superficially through the plant tissue, until it reaches the bud. Inside the bud, the larva develops and feeds on the receptacle tissue and developing seeds. Usually only one larva develops per bud. It will pupate in the flower head in a brown, thin-walled chamber (Fig. 12). The new generation adult emerges from pupation chamber in late summer to feed on foliage then overwinter in ground litter. A small percentage of weevils may overwinter in the head.
Larval feeding in the head can reduce the number of seeds by 40 to 60 percent.
Bangasternus orientalis was introduced in 1985. It is now widely established in California, Oregon, Washington and Idaho and is abundantly available for collection and redistribution. Purple starthistle (C. calcitrapa) are also a host for this weevil.
This is the first of the three weevil species to appear in the spring. B. orientalis is a good flier and disperses well. It is difficult to establish at sites where E. villosus is already established due to the predatory nature of E. villosus.
Insect Order: Coleoptera
Insect Family: Curculionidae
Common Name: Yellow starthistle hairy weevil
Description: Adult E. villosus is an oblong, cylindrical weevil 0.2 to 0.3 inch (4 to 6 mm) long with a long, slender snout. They are to, brown with gray to whitish longitudinal stripes and have long hairs on the back. (Fig. 13).
Adults emerge from overwintering sites in the soil litter during the starthistle BU-2 bud stage. Males typically emerge 1 to 2 weeks before females. Adults feed extensively on young buds, by chewing holes into the bud. Adults continue to feed and mate for about a month (Fig. 14).
Unlike other weevils, E. villosus eggs are laid inside the seedhead. A females chews a hole at the base of a bract in a BU-4 bud and lays a single egg in the hole that she then plugs with frass (Figs. 15 and 16). The egg hatches within 3 to 4 days.
Weevil larvae develop and feed on the receptacle tissue and developing seeds for 16 to 19 days. (Fig. 17). Pupation occurs within the head in a chamber and lasts 8 to 13 days (Fig. 18).
New generation adult weevils emerge during the seed dissemination stage and overwinter outside the seedhead in the ground litter (although a small percentage overwinter in the seedhead) (Fig. 19).
Eustenopus villosus has excellent biocontrol potential because of the dual impacts of adult and larval feeding. Adult feeding and oviposition can destroy young buds, which appear as brown, dry and tilted heads Fig. 20 and see also, Fig. 11, page 16). Larval feeding in the head can reduce the number of seeds by 90 to 100 percent. Eustenopus villosus is slower to disperse than B. orientalis.
Eustenopus villosus was introduced in 1990. It is now widely established in California, Oregon, Washington and Idaho and is abundantly available for collection and redistribution.
External indicators of the presence of E. villosus are oviposition scars that look like brown scabs on the side of the bud or at the base of a spiny bract.
The hairs on E. villosus tend to rub off late in the season, making the weevil look like L. curtus.
Insect Order: Coleoptera
Insect Family: Curculionidae
Common Name: Yellow Starthistle flower weevil
Description: L. curtus weevils are medium-brown, oval-shaped with a medium-sized snout and is 0.2 to 0.3 inch (5 to 6 mm) long (Fig. 21). They are often pollen-covered, giving them a yellow, spotted appearance.
Adult weevils appear from overwintering during the BU-3 bud stage. Adults feed and mate on open flower heads and are often seen face down into an open flower head (Fig. 22).
The female weevils must feed on yellow starthistle pollen to develop ovaries. Females prepare an opening among the florets and oviposits a single egg a few millimeters above the receptacle. Eggs hatch in about 4 days. The long oviposition period extends through the entire flowering period.
The weevil larva develops during the seed formationstage and feeds within the capitula on the receptacle and developing seeds. Larval development lasts from 17 to 20 days. Pupation occurs in a chamber within the mature head and takes 4 to 5 days to complete. The new generation of adults begins to appear during the seed dissemination stage to overwinter outside the seedhead in the ground litter.
Adult feeding on pollen and flowers does little to damage the plant. Seed reduction of 75 to 100 percent is common.
Larinus curtus was introduced in 1992. It is now established in California, Oregon, Wahington, and Idaho. L. curtus is the last of the three starthistle weevils to emerge during the summer.
Though not as widely distributed as the other weevils, L. curtus is a strong flier and is expected to disperse well on its own. The weevil is parasitized by a protozoan (called Nosema) that kills larvae and can decimate some weevil populations.
Yellow Starthistle Flies
Flies used for starthistle biocontrol are fruit flies; larvae of the flies eat the developing starthistle seeds (fruit). They are small, light to dark bodied, with short antennae and patterned wings.
Adult flies are found on yellow starthistle plants and other flowers where they obtain nectar.
Flies, like weevils, undergo complete metamorphosis: egg, larva, pupa and adult. Yellow starthistle flies usually have two generations per year, although C. australis may have three generations per year if the site has a long growing season.
The overwintering generation emerges in the spring from old seedheads during the starthistle bolting stage to feed, mate, and lay eggs. The summer generation adults emerge from a seedhead during the BU-3 stage and feed, mate and oviposit. The larvae from this generation overwinter in the seedhead to finish development and emerge the following spring.
Female flies lay up to 250 eggs that hatch within 8 to 12 days. Fly eggs are generally elongated, white or pale yellow and are deposited on a yellow starthistle bud or in a flower head. The fly larva is white, turning yellowish with maturity, legless, cone-shaped without a head capsule, slightly thicker at one end, and about 0.13 inch (3 mm) in length. Larvae feed and develop through 3 instars inside the yellow starthistle seedhead.
Flies pupate inside the seedhead concealed within a barrel-shaped puparium, which is pale yellow with dark ends. Pupae are about 0.14 inch (3.5 mm long). The pupal stage lasts about 2 to 3 weeks.
Table 4 (page 21) compares the three yellow starthistle adult flies for use in identification. Table 5 compares the fly lifecycles at each growth stage of yellow starthistle for use in field activities.
Table 4. Comparison of adult yellow starthistle flies.
|Chaetorellia australis||Chaetorellia succinea||Urophora sirunaseva|
|Straw-colored body||Straw-colored body||Black body|
|4 black spots on thorax||6 black spots on thorax||Yellow spot on thorax|
|Stray-colored wing bands||Stray-colored wing bands||Black wing bands|
|2-3 generations per year||2-3 generations per year||2 generations per year|
|Female oviposits inner side lateral bracts||Female oviposits inner side lateral bracts||Female oviposits on top of buds|
|Release at BU-3 bud stage||Releasae at BU-3 bud stage||Release at BU-2 thru BU-3 bud stage|
Table 5. Comparison of fly lifecycles by yellow starthistle growth stages.
|YST Stage||Chaetorellia australis and C. succinea||Urophora sirunaseva|
|Seedling||Larvae in old seedheads.||Larvae within gall.|
|Rosette||Pupae in seedhead.||Dissect seedheads for galls.|
|Bolting||Adults emerge.||Adults emerge; find on plants.|
|BU-1||Females oviposit eggs.|
|BU-2||Larvae develop, feed on seeds. Pupate.||Females oviposit eggs. Eggs hatch in 2-4 days. New larvae tunnel, develop, feed on seeds. Larvae pupate. Find adults on plants.|
|BU-3||New generation adults emerge, oviposit.||New generation adult emerges, oviposits. Eggs hatch in 2-4 days. Larvae develop, feed on seeds.|
|BU-4||Eggs hatch in 2-4 days. Larvae tunnel, feed on developing seeds. Find adults on plants.||Larvae develop, feed on seeds.|
|Flowering||Larvae develop, feed on seeds.|
|Seed formation||Larvae found in seedheads.|
|Mature||Larvae found in seedheads.|
|Dissemination||Larvae found in seedheads.||Larvae overwinter. Dissect seedheads for woody galls.|
|Senescence||Larvae found in seedheads.||Larvae overwinter. Dissect seedheads for woody galls.|
Chaetorellia australis and C. succinea
Insect Order: Diptera
Insect Family: Tephritidae
Common Name: Yellow starthistle peacock fly and the false peacock fly
Two small fruit flies introduced from Greece in 1988. These flies can have 2 or 3 generations depending on the length of the growing season.
Chaetorellia flies are straw-colored with several black spots on the thorax and light brown wing bands (Fig. 23). They are about 0.12 to 0.24 inch (3 to 5 mm) long; females are typically longer than the males and have an ovipositor. C. australis can be distinguished from C. succinea by the number of black spots in the thorax.
Chaetorellia overwinters in yellow starthistle seedheads as a mature larva and pupates when yellow starthistle is in the rosette stage. The new generation adults emerge in the spring during the yellow starthistle bolting stage and feed on plant nectar.
First generation adults develop on bachelor’s button (C. cyanus). Summer generation adults generally emerge at the BU- 3 stage. Female flies oviposit during the BU-3 or BU-4 bud stage. Eggs are laid singly at the lateral walls of the closed capitulas beneath an bract of a flower head. The eggs are white, spindle-shaped and have a long characteristic filament thickened at the distal end, which can extend beyond the margins of the bract. Eggs hatch within 2 to 4 days. The development from egg to adult takes about 4 weeks.
The fly larvae tunnel into the center of the head, where they feed on the ovaries and developing seeds during the BU-4 through flowering stages (Fig. 24).
Internal larval feeding reduces the number of developing seeds in the bud by 80 to 100 percent.
This agent disperses very well, thus it is widespread in most areas it was established.
Bachelor's button was thought to be necessary for optimum establishment of C. australis. Although the fly is established at most sites without bachelor's buttons, it is possible that preference for bachelor's button at emergence could be a limiting factor at some sites.
Chaetorellia succinea is very similar to C. australis. With experience, however, it can be distinguished from C. australis by two extra dots on the thorax. Chaetorellia succinea has six dots (three per side) on the thorax, C. australis has four dots (two per side) on the thorax (see Table 4). Chaetorellia succinea was unintentionally introduced into the United States. In Idaho, it was first released in 1997 and has since spread over 100 miles away.
Insect Order: Diptera
Insect Family: Tephritidae
Common Name: Banded yellow starthistle gall fly
Adult flies are black with a yellow triangle on the back of the thorax. Wings are marked with dark crossbands. The adults are approximately 0.2 to 0.24 inch (3 to 5 mm) long. The female is typically longer than the male and has an obvious ovipositor (Fig. 25).
Urophora sirunaseva overwinters in galls in the yellow starthistle seedhead as a mature larva and pupates inside the gall in the spring; pupation lasts 4 to 5 weeks.
First generation adult flies emerge from galls in the spring, about 2 to 4 weeks after C. australis, and begin mating within 3 to 4 days. Females then begin oviposition. Eggs are laid on the top of BU-2 or BU-3 buds where the points of the smaller bracts emerge. The eggs are white and spindle-shaped, and about 0.006 inch (0.15 mm) long, and hatch within 9 or 10 days.
After hatching, the larvae eat through the florets in the head, and can be found on and between the florets. When the larvae reach the receptacle, the gall begins to form. Tissues begin to grow and change consistency, forming a hard gall around the larva. There is one larva per gall and up to four galls per seed head. Galls of the summer generation larvae are thin and delicate compared with the thicker galls formed by the overwintering generation.
Larva and pupa can be found by dissecting the seedhead. Presence of a woody gall within a seedhead is an indication of presence of the agent (Fig. 26).
Pupation lasts about 2 to 3 weeks inside the gall. The summer generation adults emerge in late June and early July and begin oviposition within 3 days. An emergence hole in the gall can be detected (Fig. 27).
Larvae of this generation feed and develop for about 3 weeks. Once the larvae reach maturity, they enter diapause and overwinter in the gall formed in the yellow starthistle seedhead for approximately 7 months.
Internal larval feeding reduces the number of developing seeds in the head by approximately 50 percent. Heads infested with this gall fly produce fewer seeds than heads infested with Chaetorellia flies.
The presence of high densities of U. sirunaseva at a site does not appear to interfere with the survival of other seedhead infesting agents. The seedhead fly, U. quadrifasciata, is sometimes found in yellow starthistle seedheads.