/ícS'f'flk ^ y, 7 5 >"'í/- Department of Agriculture Collecting, Handling ^ Agricultural <^ ^_ Y Research \j '^ Service and Releasing Agriculture Handbook Number 579 Rhinocyllus conicus, a Biological Control Agent of Musk Thistle en CT- '.■'"Z Z^J ;.'. ^X'i ' rn " ¡^—, i v.—. c:::3 > ' - . C/ • 0 f ■} • o r ---^ ;; T_;> -:<; • ^ r t. CD o' i-N^_ ; o~^ :x5i c?:* (¿o United States Department of Agriculture Collecting, Handling Agricultural Research Service and Releasing Agriculture Handbook Rhinocyllus conicus, Number 579 a Biological Control Agent of Musk Thistle Norman E. Rees Abstract Contents Rees, Norman E. 1982. Collecting, handling, and releasing Introduction i Rhinocyllus conicus, a biological control agent of musk thistle. Musk thistle life cycle 1 U.S. Department of Agriculture, Agriculture Handbook Life history and habits of Rhinocyllus conicus 1 No. 579,7 p. Selecting a release site 4 Collections 5 This publication briefly describes the life cycle of musk thistle and the Releases 6 life history and habits of the biocontrol agent Rhinocyllus conicus Monitoring 6 Froel. It presents information on selecting new release sites and how Selected references 7 to collect, handle, release, and monitor this weevil. Important points to remember are ( 1 ) adult weevils must be collected in the spring when the females are gravid, (2) each release should contain at least 500 adults, (3) release fields should not be mowed or sprayed for at least 2 years following release, (4) cattle and other livestock should be excluded from the release sites during the weevils' egg-laying period, and (5) infested plants showing water stress should be watered immediately to assure survival of the weevil. Keywords: Rhinocyllus conicus, biological control, musk thistle, collecting, releasing, monitoring. Issued July 1982 Collecting, Handling, and Releasing Rhinocyllus conicus, a Biological Control Agent of Musk Thistle By Norman E. Rees* Introduction problem in many of these counties (fig. 1 ).^ It inhabits all types of land except deserts, dense forests, high mountains, and newly cultivated Biological control utilizes selected factors in the life history and habits lands. of both the target (pest) organism and one or more of its biological control agents to suppress the population of the target organism to a Reproduction of musk thistle is totally by seeds, which are produced tolerable level. Researching these factors, putting them into effect, from June and July until death of the plant from natural causes (fig. 2). and obtaining initial control often take considerable time; however, Only 2 percent of these seeds are attached to the plumeless pappus and once it is established, an efficient biological control system tends to be are borne on the wind; the rest fall to the ground close to the parent self-perpetuating and will continue to suppress the population of the plant and are distributed by birds, small animals, and-running water. target organism from that time forward without requiring additional expense to maintain. Musk thistle plants can act either as a biennial, a winter annual, or an annual. Seeds can germinate after 6- to 8-week resting period or can When a plant species, such as musk thistle, has been introduced into a remain dormant in the soil for several years. About 69 percent new land and is not held in check by local or accompanying natural germinate during the first year, 20 percent germinate the second year, biological controlling agents, it often becomes a problem and is and the remaining 11 percent may germinate later (fig. 3). labeled as a "weed" or a "noxious weed," depending on its impact on the rancher or farmer. Chemicals can often provide temporary Seedlings that germinate in the fall attain a rosette stage before winter. control, but permanent suppression, if possible, can only be obtained In the spring, the diameter of the rosette may reach from 1 to 3 feet from the use of one or more biocontrol agents that can suppress before the plant bolts. Although each plant typically sends up one populations of the target organism year after year. Generally, these main stem, damage to the terminal meristem, either mechanically or biological agents are found in the native habitat on the target organism. by frost, will generally cause groups of stems to develop, usually in After testing to determine that these agents can not reproduce on, or multiples of five. Plants attain heights of from 1 foot to 8 feet and pose a threat to, plants of recognized value, they can be introduced sometimes exhibit considerable variation in height, form, and color into the problem area. between years, locations, and individual plants (fig. 4). One such agent, the weevil Rhinocyllus conicus Froel., was introduced Flowers of musk thistle are reddish purple. Each terminal flower into Canada in 1968, and into Montana, Nebraska, and Virginia in produces about 1,0(X) seeds; each lateral bud produces smaller flowers 1969 to control musk thistle; however, the Nebraska release did not with fewer seeds, averaging about 840 seeds per head. In the Gallatin become established. Populations at the other release sites increased in Valley, flowering continues until the plants are killed by adverse density and area, and by 1975, the reduction of musk thistle by R. weather conditions. conicus became apparent. Since that time, hundreds of new releases have been made throughout the United States, using progeny of the original releases. Life History and Habits oí Rhinocyllus conicus The increasing acceptance of biological control and the popularity of Rhinocyllus conicus weevils overwinter as adults. In the spring, they utilizing R. conicus to control musk thistle have created a need for a congregate on musk thistle plants prior to and during the bolting stage. publication to assist those interested in collecting the weevil for This occurs about mid- to late-May in the Gallatin Valley of Montana. release in new areas. This publication provides such information. Congregating adults feed, mate, and deposit eggs. Each female will produce from 100 to 150 eggs, which are laid individually or in clusters of two to five, generally on the bracts of the bud. The eggs are Musk Thistle Life Cycle then covered with masticated plant material, which becomes tan and appears as "warts" on the plant (fig. 5). Musk thistle^ is a common name for several species of plants of the large headed Carduus nutans L. group, including C. macrocephalus As the primary (terminal) buds become saturated with eggs, the Desf., C. nutans L., and C. thoermeri Weinmann, These species are weevils begin laying eggs on the stems and on the secondary (lateral) similar in appearance, have similar life cycles, and respond alike to/?. buds. Cool spring weather and a dense weevil population tend to conicus. Musk thistle, as such, was introduced into the United States extend the egg-laying period, which seems to result in an increase in over 100 years ago. It has increased, unchecked, until it now can be the number of eggs being deposited on the lateral buds and on the buds found in 12 percent of the counties of the United States and is a serious of certain other thistle species (table 1). 'Research entomologist, Rangeland Insect Laboratory, Western Region, Agricultural Research Service, U.S. Department of Agriculture, Bozeman, Mont. Also called nodding thistle in some countries. ^The year in italic, when it follows the author's name, refers to Selected References, p. 7. The eggs hatch after 6 to 8 days, and the cream-colored larvae burrow plant to concentrate nutrients and tissue in the affected area, which is through the bracts into the future seed-producing tissue of the bud, similar to the effect created by gall-producing insects. The mature which is known as the receptacle. Here they develop for the next 25 to larva eventually coats the inner cell wall with feces and masticated 40 days, feeding on the receptacle and maturing seed tissue, which plant material to produce a hard, protective chamber for the 8- to creates individual cells (fig. 6). The feeding appears to stimulate the 14-day pupal stage (fig. 7). Musk Thistle (Carduus Nutans) Key To Infestation Density ^^1 Economic täiiäässi Potentially Economic L¿;V) Occasional Plants ^^1 Eradicated Figure I.—Musk thistle distribution and density by county in mainland United States, 1974 (Dunn 1976). Figure 2.—Seeds of musk thistle. Figure 3. —Musk thistle seedlings. Larvae that tunnel into stems of musk thistle do not construct cells. cells for 2 weeks or longer, during which time their color turns from a Nevertheless, these larvae usually remain in the immediate vicinity cream or reddish tan to almost black. The body hair is a patchy where they initially began to feed. mixture of black and yellow, which gives the appearance that the weevils are covered with pollen. Although size is variable, most larger Adult weevils emerge from their pupal cases but remain within their weevils are no more than about one-quarter of an inch long (fig. 8). Figure 4. —Musk thistle flower. Table 1.—Thistle species hosts for Rhinocyllus conicus Found in Common name Scientific name United States Plumeless thistle Carduus acanthoides L. Yes. Welted thistle C. crispus L. Yes. Curled thistle (Unknown) C. nigrescens No. Musk thistle or C. macrocephalus Desf. Yes. nodding thistle. C. nutans L. Yes. C. theormeri Weinmann Yes. Great marsh thistle. C. personata (L.) Jacp. No. Italian thistle C. pycnocephalus L. Yes. (Unknown) C. sanctae balme No. Slender-flower thistle. C. tenuiflorus Curtis Yes. Canada thistle Cirsium arvense (L.) Yes. Creeping thistle Scop. Wavyleaf thistle Wavy-leaved C. undulatum (Nutt.) Yes. thistle. Spreng. Bull thistle C. vulgareiSavi) Yes. Tenore Milk thistle Silybum marianum L. Yes. Scotch thistle Onopodrum acanthium L. Yes. Figure 5.—Eggs of Ä.