Biology and Biological Control of Knapweed

Wilson, L. M. and C. B. Randall. 2003. Biology and Biological Control of Knapweed. USDA-Forest Service FHTET-2001-07. 2nd Edition.

Chapter 2: Biology of Knapweed Biocontrol Agents

Biological control of knapweeds is one of the oldest classical biocontrol programs in the United States and Canada. It began in the 1960’s with the importation of two seedhead flies: the knapweed banded gall fly, Urophora affinis, and the UV knapweed seedhead fly, U. quadrifasciata. In all, 16 agents have been introduced; of these, 13 are insect species, two are fungi, and one is a mite (not released). Only the insects are emphasized in this manual because they are by far the most widespread, readily available and easy to work with. This chapter is organized into two sections: seedhead-feeding (seedhead feeders) and root-boring (root borers) insects.

Basic Insect Biology

Insects are a diverse and complicated group of animals. Basic knowledge of insect anatomy and lifecycles will help a great deal in recognizing knapweed bioagents in the field and understanding their impact on the weed. Adult insects possess unique characteristics: 1) an exoskeleton, 2) a segmented body consisting of three regions (head, thorax and abdomen), and 3) three pairs of legs (Fig. 10).

Insects grow and develop through a series of stages. The transformation from egg through juvenile stages to adult is called metamorphosis. This process can be incomplete or complete. All the insects used in biocontrol of knapweed undergo complete metamorphosis (having four distinct life stages): egg, larva (of which there can be three or more instars), pupa, and finally, adult.

The insect bioagent’s lifecycle (Fig. 11) is closely matched, or synchronized, with knapweed’s. In fact, in order to qualify as an acceptable biological control agent, the insect must show that it eats and develops only on knapweed and no other plants. Without knapweed, or a specific complex of knapweed, the insect will die. This highly specific, tightly regulated insect-plant relationship is the most critical issue in classical biological control of knapweeds.

Figure 10. Diagram of insect body parts.

Figure 11. Example of an insect lifecycle showing complete metamorphosis.

Insects and Knapweed

Three types of insects are used in biocontrol of knapweed: flies, moths and beetles. In all, 13 species of insects, occurring in the seedheads or roots, are discussed in this manual (Table 1). All four fly species are ‘fruit flies’ (Family Tephritidae), in that they occur in the seedheads where the larvae eat developing flowers and seeds. One moth species and three weevil species complete the complex of eight seed-feeding bioagents on knapweed (Fig. 12). Among the root borers are three moth species and two beetle species. One beetle, Cyphocleonus achates, is a weevil, and the other beetle, Sphenoptera jugoslavica, is a metallic wood borer.

All of the insect bioagents damage knapweed plants as larvae by feeding internally in the seedheads or roots. In general, adults have little impact on the plant except for two of the seedhead weevils, Larinus minutus and L. obtusus. Adults of these weevils can significantly defoliate knapweed stems, further weakening the plant.

Table 1 lists the natural enemies of knapweeds in the United States and the species of knapweed they attack.

It is unlikely that any one of these species alone could successfully control knapweed. Most knapweed biocontrol programs use a combination of bioagents which together create multiple stresses on the plant and have a greater chance of contributing to the suppression of knapweed.

Table 1.  Knapweed bioagents established in the United States and the species of knapweeds they attack.

Figure 12. Distribution of knapweed biocontrol agents in a knapweed plant.

Identifying Insects

An important part of any successful biocontrol program is the ability to identify bioagents in the field. As adults, bioagents are relatively easy to identify with their variable size, color, and habits. Identifying the larvae is more challenging than the adults – and yet probably more important to know because it is in the larval stage that the bioagents: 1) do the most damage, 2) are often monitored in the field, and 3) provide conclusive evidence that the insects are established in the field.

Figure 13 is a key for identifying, in three easy steps, the larva of a fly, a moth and a beetle. This key is specific to knapweed insects, not insect larvae in general.

Figure 13. Key for identifying fly, moth and beetle larvae.

Fly larvae have no head capsules whereas beetle and moth larvae do. Fly larvae are sometimes confused with other larvae because they appear to have a broad, dark head. This is actually a dark, hardened anal plate anchoring the spiracles (breathing orifices).

Moth larvae have both head capsules and prolegs.

Beetle larvae are more variable. Weevil larvae (called grubs) are white, C-shaped, and have head capsules but no abdominal prolegs. Metallic wood boring larvae are narrow and tapering, with wide, somewhat flattened heads.

Figure 14 is a key for identifying the pupa of a beetle, a moth and a fly.

Beetle pupae have well-developed appendages that are obviously not fused to the pupal body.

Moth pupae have moderately well-developed appendages fused to the body.

Fly pupae are contained inside a barrel-shaped puparium.

Figure 14. Key for identifying fly, moth and beetle pupae.

Seedhead Feeders

There are eight different seedhead-feeding insect species for controlling knapweeds that are established in the United States and Canada (Table 2). Among the seedhead-feeding insects are four fly, one moth, and three beetle species. The fly, Urophora affinis, was the first insect to be introduced into the United States and Canada for the biological control of diffuse and spotted knapweed. The second Urophora species, U. quadrifasciata, was not approved for release in the United States because of taxonomic concerns, but nevertheless migrated to the United States after being released in Canada. Two other flies are Chaetorellia acrolophi and Terellia virens (Fig. 15). Another seedhead feeder is the seedhead moth, Metzneria paucipunctella. Among the beetles are two closely related weevils, Larinus minutus and L. obtusus. The other seedhead weevil is Bangasternus fausti.

All of the seedhead-feeding insects damage the plant when larvae consume immature seeds and other tissues in the flower head, or capitulum. Feeding by the insects sometimes causes the plant to encase the insect larva in a hard or soft gall-like structure. In forming these galls, the insect is draining valuable nutrients away from normal plant growth (referred to as a metabolic sink), further depleting the plant’s limited resources. Gall-forming insects are well adapted to plants like the knapweeds that produce a large number of small seedheads throughout the growing season.

Gall formers (the two Urophora flies) feed on actively dividing cells so they attack at the early stages of seedhead bud formation. The maximum number of gall-forming insect larvae in a seedhead is limited by the size of the seedhead, not the amount of food.

The impact that gall formers have on a plant is dictated by:

• Abundance of galls
• Power of galls as a metabolic sink
• Favorable weather conditions (i.e. drought, cold)

The other seedhead-feeding species either do not form a gall or construct a chamber in which to feed (Fig. 16). They inflict direct damage on developing seeds but do not create a metabolic sink.

Seedhead feeders are separated in time and space by such factors as:

• Type of knapweed patches insects will infest (isolated plants vs. dense stands)
• Larval feeding habits (e.g., feeding in the receptacle, florets, and seeds)
• Number of generations per year
• Number of larvae in the head
• Overwintering site (in or out of the seedhead)

Timeline of Attack

Knapweeds produce flower heads throughout the spring and summer, creating a constant supply of seedheads of different sizes and stages of development for the seedhead feeding insects to utilize.

Figure 15 is a comparison of adult U. affinis, U. quadrifasciata, C. acrolophi and T. virens.

Figure 15. Comparison of knapweed seedhead files.

Each seedhead-feeding insect prefers certain seedhead characteristics for oviposition (see Table 3). Figure 16 shows U. affinis ovipositing into knapweed flower bud and the position of its eggs. Figure 17 compares the galls of U. affinis and U. quadrifasciata. Figures 18 and 19 depict the position of fly, beetle and moth larvae inside the knapweed seedhead.

More than one bioagent can occupy a seedhead at one time. This coexistence is possible because of specialized adaptations.

Bioagents with a short adult life span attack fewer seedheads. Long-lived adults can attack many seedheads during their life span. Agents with more than one generation per year can attack seedheads during two distinct periods in the growing season.

Table 4 is a summary description of knapweed seedhead feeders. Table 5 compares the lifecycle of seedhead flies with the lifecycle of knapweed. Table 6 compares the lifecycle of seedhead moths and beetles with the growth stages of knapweeds.

Table 2.  List of seedhead feeding knapweed biocontrol agents.

Table 3. Knapweed seedhead size and stage of development preferred by each of the eight seedhead feeding biocontrol agents.

Table 5. Comparison of seedhead fly lifecycles by knapweed growth stage.

Table 6.  Comparison of seedhead moth and weevil lifecycles by knapweed growth stage.

Urophora affinis

Order: Diptera
Family: Tephritidae
Common name: Banded knapweed gall fly

Impact

Larvae directly destroy seeds within the gall. Galls drain nutrients from other parts of the plant resulting in fewer seedheads and reduced vegetative growth. Between two and four galls in a single seedhead are common. The maximum number of galls that can develop in a seedhead is a function of receptacle disc area. For example, more galls are generally produced in spotted knapweed versus diffuse knapweed which has a smaller diameter disc area. In spotted knapweed, the metabolic sinks created by U. affinis galls compete with root reserves so that fewer and smaller flowering stems are produced the following year. In diffuse knapweed, each U. affinis gall reduced seed production by approximately 13.7 seeds and an average of 1.1 galls per seedhead reduced the above ground dry weight of the plant by 71 percent as well as average seed weight.

The corolla (flower petals) is suppressed or absent in heavily-galled seedheads. Woody galls can be felt when heads are rolled between the fingertips.

Comments

This was the first insect introduced (1973) into the United States for knapweed control. Urophora affinis does not disperse as well as U. quadrifasciata and other seedhead-feeding agents. On sites with both U. affinis and U. quadrifasciata infesting knapweed, U. affinis tends to dominate.

In some areas the combination of U. affinis and U. quadrifasciata have reduced seed production by 95 percent in spotted knapweed. U. affinis has been found to compliment the biological control activities of U. quadrifasciata, Metzneria paucipunctella, and Larinus minutus and other seedhead-feeding agents. Studies in Canada have shown that a combination of both Urophora flies and the root borer Sphenoptera jugoslavica can reduce diffuse knapweed seed production by 98 percent. Fly larvae are sometimes eaten by larvae of Metzneria and Larinus.

Urophora quadrifiasciata

Order: Diptera
Family: Tephritidae
Common name: UV knapweed seedhead fly

Weeds Attacked

Spotted, diffuse, squarrose, meadow, black, and brown knapweeds

Description

Pupation lasts about 14 days. First generation flies pupate 20 to 25 days after oviposition, about the time that seed development is complete. Emerging second generation adults (August) attack later forming seedheads and emerge the following spring with the onset of knapweed seedhead buds. Otherwise, first generation overwinters in head.

Impact

Florets damaged by U. quadrifasciata are destroyed and adjacent florets abort (approximately two seeds destroyed for each U. quadrifasciata). There does not appear to be a decrease in the number of seedheads on plants attacked by U. quadrifasciata. The fly spreads rapidly, more so than U. affinis. The presence of U. affinis in the seedhead tends to discourage U. quadrifasciata attack, but the combination of both fly species enhances seed reduction.

Comments

Urophora quadrifasciata entered the US in 1980. This is the most widely distributed knapweed biocontrol agent. The importance of U. quadrifasciata will increase as knapweed densities decline because it is less dependent on dense populations of knapweed than U. affinis. U. quadrifasciata has been found to compliment the biological control activities of U. affinis, Metzneria paucipunctella, and Larinus minutus, and other seed head feeding agents.

On sites with both U. affinis and U. quadrifasciata infesting knapweed, U. affinis tends to dominate. In many areas the combination of U. affinis and U. quadrifasciata have reduced seed production by up to 95 percent in spotted knapweed. Studies in Canada have shown that a combination of both Urophora flies and the root borer Sphenoptera jugoslavica can reduce diffuse knapweed seed production by 98 percent. U. quadrifasciata larvae are eaten by Metzneria and Larinus larvae.

Terellia virens

Order: Diptera
Family: Tephritidae
Common name: Green knapweed clearwing fly, verdant knapweed seed fly

Lifecycle

Weather conditions determine the number of generations (one or two). If there is only a single generation, flies spend the winter as pupae in the seed head oriented vertically above the receptacle in a loose cocoon of plant hairs. With two generations, flies spend the winter as mature larvae in cocoons partially embedded in the flower base (receptacle).

Adult T. virens begin to emerge in late May, about 4 weeks before spotted knapweed flowers. Mating and oviposition begin with the onset of warm weather and continues for the length of the adult’s 48-day lifespan.

Females lay eggs in young, opening flowers heads from early June to early October. After laying one to several eggs into the flower heads between the flowers, the female marks the bracts of the head and upper stem leaves with a substance to discourage oviposition by other females. Each female will lay an average of 80 eggs that hatch within 3 to 5 days.

Larval development to pupation takes about 14 days. The barrel-shaped larvae spend their first two instars inside a single seed, feeding on ripening seed. Two to several Terellia virens larvae may infest a seedhead.

Impact

Terellia virens larvae cause considerable destruction of seeds; partial feeding damage on other seeds can reduce viability of the remaining seeds by up to 90 percent.

Comments

Terellia virens was introduced into the United States in 1992. It is now established in many states and is most successful in areas without Larinus species. The fly can co-exist in seedheads infested by Chaetorellia acrolophi and Urophora species but is a poor competitor in heads infested by Larinus species. Also, it appears to be severely hindered by high densities of Urophora affinis. Terellia virens prefers plants on south-facing slopes and dry locations.

Chaetorellia acrolophi

Order: Diptera
Family: Tephritidae
Common name: Knapweed peacock fly

Lifecycle

Chaetorellia acrolophi generally has two generations a year; a third generation is possible but rare. Adults emerge in early June when knapweed plants are in the bud stage. Mating begins immediately and oviposition lasts for the remainder of the 17-day lifespan of the adult female. Eggs are laid singly or in batches of two to four underneath the bracts of unopened buds. A female will lay an average of 69 eggs in its lifetime. Larvae hatch 4 to 5 days later and migrate into the center of the flower buds where they feed on immature florets as they descend to the seeds. Second and third instar larvae feed on developing seeds, florets, and partially on the receptacle. Larvae pupate 10 to 15 days after hatching. First generation adults generally emerge in July, mate and lay eggs, which develop into the second generation.

First generation larvae and pupae are white and pupae are enclosed in a white pupal case covered in pappus hairs from the seeds. Second generation larvae and pupae are tan-colored, with pupae enclosed in a yellow puparium covered with pappus hairs from the seeds. Second generation larvae typically overwinter in the flower heads, then pupate the following spring.

Impact

This fly does not cause plants to form galls. Larval feeding can significantly reduce seed production; a single larva can destroy all of the seeds in a single seedhead.

Comments

Chaetorellia acrolophi prefers plants in moist habitats and is generally associated with scattered plants rather than in dense stands of spotted knapweed. This fly is not widely distributed but is established in Oregon and Montana. In Oregon, it is most successful in areas where Larinus species are not present. This fly should supplement the impact of U. affinis by attacking isolated knapweed plants; however, in Montana it appears to be hindered by high densities of U. affinis. It was introduced into the United States from Austria in 1992.

Metzneria paucipunctella

Order: Lepidoptera
Family: Gelechiidae
Common name: Knapweed seedhead moth

Larvae hatch in 10 to 12 days as the flower heads are opening. Larvae enter the opened flower heads; first instar larvae feed on the florets while the second-instar larvae feed on the seeds. Third instar larvae mine into the flower base, which reduces the viability of uneaten seeds. Several young larvae can occupy a seedhead early in the season but only one larva survives beyond the third instar (Fig. 27). In the fall the moth larva moves from the receptacle to overwinter in the base of the seedhead. Pupation occurs in the spring and lasts for 3 to 4 weeks.

Impact

Larvae feed on developing seeds. Each larva can destroy on average of eight seeds and reduce the viability of others. Older larvae web seeds together preventing seeds from dispersing over long distances. Older larvae will eat Urophora larvae.

Comments

Metzneria paucipunctella was introduced in 1980. M. paucipunctella can suffer severe mortality during cold winters. Moth feeding compliments the biological control caused by Urophora species. M. paucipunctella larvae are aggressive and will kill one another or other knapweed seedhead-infesting larvae. White-footed deer mice are known to eat many of the larvae during the winter months.

Larinus minutus

Order: Coleoptera
Family: Curculionidae
Common name: Lesser knapweed flower weevil

Impact

Adult feeding can severely defoliate plants (Fig. 29). Larval feeding reduces seed production; a single larva can destroy the contents of an entire diffuse knapweed seedhead.

Emerging adults make characteristic emergence holes in the center of affected seedheads similar to the emergence holes created by B. fausti and L. obtusus (Fig. 30).

Comments

Larinus minutus larvae are aggressive and will kill one another or other insects in the same seedhead.

Larinus minutus is established on squarrose knapweed in California.

Population increases of L. minutus on spotted knapweed have been slow; however, it still appears to be a very promising agent. The insect can have a significant impact on the plant growth and density across a wide range of habitats.

A study in Minnesota found that reduction in spotted knapweed infestation increased by 26.5 percent with the addition of L. minutus to existing U. affinis and U. quadrifasciata populations. The number of seeds destroyed in individual seedheads increased. L. minutus and the two Urophora species were found to successfully cohabit in spotted knapweed seedheads.

In addition to seed destruction by larvae, adults can do extensive damage by feeding on growing plants in the spring, which often results in the near total destruction of all growing diffuse knapweed plants in the vicinity of the original insect release. Diffuse knapweed plants under attack by L. minutus typically turn a characteristic blue-green color, have few leaves and often have distorted growth. Adult L. minutus can also destroy diffuse knapweed seedlings, resulting in suppressed recruitment of new plants. The insects develop large populations within 3 to 5 years and disperse rapidly to new areas.

Larinus minutus was introduced in 1991.

Larinus obtusus

Order: Coleoptera
Family: Curculionidae
Common name: Blunt knapweed flower weevil

Weeds Attacked

Spotted is preferred and to a lesser extent diffuse knapweed.

Lifecycle

Larinus obtusus has one generation per year. Adults spend the winter in soil litter at or near the base of plants. Overwintering adults appear at the end of May and reach peak population levels during early July. Adults feed heavily on the foliage and flowers prior to mating and laying eggs. Females oviposit throughout their 5- to 6-month lifespan among the inner florets of newly opened flower heads. Occasionally adults may hibernate a second time and live a second season.

Eggs hatch in 3 to 6 days and larvae begin feeding on pappus hair and developing seeds. More than one larva can occupy a seedhead. Larvae develop through three instars over a 4- to 6-week period, pupating in chambers constructed from cemented seeds and pappus hairs. The pupal period generally lasts 9 days. Adults emerge late July and early August through holes chewed in the tops of the pupal chambers and vigorously feed on foliage before moving to overwintering sites in the soil.

Impact

One or two larvae can destroy most of the developing seeds in the head. Any seeds not eaten become part of the pupal chamber. Adult feeding on foliage can reduce photosynthetic capacity and plant vigor.

Emerging adults make characteristic holes in the center of affected seedheads, similar to the emergence holes created by B. fausti and L. minutus.

Comments

Larinus obtusus prefers moist sites in contrast to the other seedhead weevils for knapweed, which prefer and thrive in drier sites. It has not yet been established on knapweed species other than spotted in the United States. L. obtusus has been slow to build up significant populations in spotted knapweed in western Montana. L. obtusus is well established in Oregon, Idaho, Colorado, Washington and British Columbia. Larinus obtusus was introduced in 1993.

Bangasternus fausi

Order: Coleoptera
Family: Curculionidae
Common name: Broad-nosed knapweed seedhead weevil

Depending on the placement of the egg, the new larva mines directly into the bud or into the stem and then tunnels to the bud where it feeds within the seedhead. Pupation occurs in the damaged head within a cell constructed by the larva of frass and fused seeds. It takes approximately 32 days for B. fausti to go from egg to adult. Adult B. fausti feed on knapweed foliage in the spring and on flowers in the summer.

Impact

Bangasternus fausti feeds in the flower base and destroys the flowers and ovules before they produce seeds. Weevils can consume 95 to 100 percent of the seed. In the fall, attacked seedheads have a characteristic emergence hole similar to emergence holes of Larinus species.

Comments

Bangasternus fausti was introduced into the United States in 1992, and has become well established on spotted knapweed. It is not known how B. fausti will interact with other seedhead- infesting biological control agents. Early concerns about the potential of B. fausti to displace Urophora affinis have yet to be realized. Under favorable conditions weevil density can increase dramatically allowing the collection of large numbers of weevils for collection and redistribution (Fig. 33).

Bangasternus fausti prefers hot dry sites. It attacks early buds and often occurs with Larinus spp. However populations of B. fausti are slower to build than Larinus.

Root Borers

There are five root boring insect species established in the United States and Canada for the control of diffuse, spotted and squarrose knapweeds. Three species are moths (sulfur knapweed root moth, Agapeta zoegana; gray-winged knapweed root moth, Pelochrista medullana, and brown-winged knapweed root moth, Pterolonche inspersa), and two are beetles (knapweed root weevil, Cyphocleonus achates, and bronze knapweed root borer, Sphenoptera jugoslavica). All these insects can be present in the root at the same time. Studies are underway to determine how these insects coexist and compete in knapweed roots.

All the insects are univoltine, which means they produce only one generation per year. Most larvae complete their development in a single root; however, larvae of the sulfur root moth, Agapeta zoegana, can migrate a short distance between roots during the growing season.

Agapeta zoegana

Order: Lepidoptera
Family: Cochylidae
Common name: Sulfur knapweed root moth

Pterolonche inspersa

Order: Lepidoptera
Family: Pterolonchidae
Common name: Brown-winged knapweed root moth

Weeds Attacked

Diffuse and spotted knapweed

Description

Impact

Infested diffuse knapweed plants can be recognized in the spring by the silken tubes around the crown of the rosette. P. inspersa larvae cause considerable root damage and as a result, plants attacked by the larvae are stunted and produce fewer flowers. The infested root becomes spongy and easy to pull from the ground. Feeding damage reduces root storage.

Comments

Pterolonche inspersa, a native moth of Europe, was released in 1988. P. inspersa larvae are known to eat the larvae of the bronze knapweed root beetle, Sphenoptera jugoslavica. This moth is established in British Columbia and Idaho - approximately 10 years following its release.

Pelochrista medullana

Order: Lepidoptera
Family: Tortricidae
Common name: Gray-winged knapweed root moth

Weeds Attacked

Spotted and diffuse knapweed

Description

Larvae hatch 7 to 9 days after oviposition and move to the center of the rosette and mine into the root crown. Larvae mine spiraling tunnels around the cortex of the root, just under the epidermis, similar to Agapeta zoegana. The tunnels are lined with a silken web. Larvae overwinter in the roots and complete development in the spring or early summer. Usually only one larva is found on an infested plant.

Impact

Damage to the roots is similar to that caused by Agapeta zoegana. Only third to sixth instar larvae cause measurable damage, reducing root storage capacity and exposing the plant to pathogens. Small plants, <0.4 inch (10 mm) root diameter, can be completely destroyed. Plants that survive insect attack are usually smaller and produce fewer flower heads than uninfested plants.

Comments

This moth was established in 1984. Limited numbers of P. medullana have been released in Idaho, Montana, Oregon and British Columbia. However, to date, there is no evidence of establishment of this agent in the United States or Canada.

Cyphocleonus achates

Order: Coleoptera
Family: Curculionidae
Common name: Knapweed root weevil

Weeds Attacked

Spotted knapweed preferentially, diffuse and squarrose knapweed secondarily.

Description

Impact

Small plants can be killed as a direct result of larval feeding. Most damage is done when multiple larvae occupy a root or when the attacked roots are small. Older larvae cause a gall to form in the root, which acts as a metabolic sink. Plants are stunted and some survive only one season after being infested with C. achates. Tunneling in the root also exposes the plant to bacterial and fungal infection that can cause additional secondary injury.

Comments

This root-boring weevil was first released in the United States in 1988 and is now established in several states and provinces. C. achates is not a strong flyer and consequently has been slow to establish and spread. In hot weather adults can be seen on the tops of the plants. Its habit is to sit perfectly still and when disturbed, to drop to the ground and play dead. Up to 25 larvae have been recorded in the same root. This is probably the best knapweed root-boring bioagent available today. Techniques have been developed to mass-rear this insect for greater production and more rapid distribution.

Sphenoptera jugoslavica

Order: Coleoptera
Family: Buprestidae
Common name: Bronze knapweed root borer

Comments

Sphenoptera jugoslavica was first released in the United States in 1979 and is now widely established in the western United States and Canada. It performs best in hot, dry diffuse knapweed sites with shallow, stony soil.

Table 7.  Summary description of knapweed root borers.

Table 8.  Comparison of knapweed root borer lifecycles by knapweed growth stage.

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