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126

BIOLOGICAL WEED CONTROL-ClJRJBlll' S'rA1'US AND ·BROSffX:TS L.A. Andrea and R. B. Hawkes Entomology Research Division, Agr. Res. Serv. USDA, Albany, California 94706

Weeds are an ever-continuing problem to mankind. To effectively control--or perhaps a better term~to effectively manage a weed problem requires all the imagination and resources the weed scientist can muster. Plant physiologists and agronomists are needed to study the growth and development of weeds; systematists are needed to identify the weeds and other species in the plant co11111unity; plant ecologists are needed to study the weeds in relation to their environment; toxicologists are needed to develop and stuiy the mode or action ot herbicides; plant pathologists and and entomologists are needed to study and develop the weed's natural enemies. We could continue this list. However, the point we want to make is that be­ fore mankind can begin to reduce weed losses on a continuing and ecologically sound basis, a broad knowledge of weedy plants and their environment is es­ sential. Only by pooling the talents of a wide range of specialists can we hope to understand am eventually manipulate the complex of factors determining plant abundance. We would like to discuss but one small facet of this complex, the weed-feeding insect and its role in weed management.

Almost all plants have insect enemies, some mre than others. Their ability to regulate weed abundance has been amply demonstrated, although their influence will vary with the habitat, the weather and other environ­ mental factors. Man-induced changes in the environ.'118nt, including such cultural practices as irrigation, fertilization, grazing, pest control and so on, all have their effect on the weed insect. The fact that we d:> have weeds indicates that insects are not able to solve every problem. Two reasons wey insects often fail ares (1) potentially effective insects are not present in the weed area, or (2) certain cultural practices hove ren­ dered effective species ineffective. If insects are absent, we can seek out new ones in other areas and introduce them to the problem site. On the other hand, if potentially useful insect enemies are present, we can study and develop techniques to i~prove their effectiveness. The introduction of new insects has received the greatest attention and has been used pri'.DB.l'ily against introduced weedy plants. The technique has been one of searching out new insects in areas where the weed is considered native. Since we are dealing with plant-feeding insects, detailed biological and host specificity studies are conducted at overseas laboratories, and also in quarantine facilities in the United States, to assure that the insects will mt seri­ ously damage, or what is more important, reproduce on :,ther plants. No insect has yet altered its plant-host pattern to become a serious pest on a crop plant, in more than 50 recorded cases of the use of insects to control weeds around the world.

The availability of host-specific weed-feeding insects adds to the flexibility of the weed management program. Once a weed-feeding insect is established it becomes a part of the environ"ll8nt and exerts continual pressure on the weedy plant; insects will also seek out the weed over mch of its range, often in areas inaccessible to other methods of control; the 127 insects used are highly selective and do not disturb other plants. The use of weed-feeding insects is also governed by certain limitations. Un­ favorable temperature conditions, plant nutrition, and soil disturbance are but a few of the factors influencing their effectiveness. M>st insects are highly mobile and once they have been introduced to control a weed, they cannot be confined to any particular region of the plant's range. Thus, at the outset of each project we ~ust ascertain whether a weed has certain values that may partially offset or even outweigh its noxious qualities. If a conflict of interest exists, biological control is not considered until the conflict has bean resolved.

An example of a conflict of interest might be Johnsongrass. Sorghum halepense (L.) Pers. This is an extre~ely serious weed in California and much of the United States; but in limited areas of the southeast, it is cultivated for forage. If an insect is released against Johnsongrass in California, we can offer no assurance· that it would not eventually make its way to cultivated stands of the plant. In Israel, the senior author recently saw the larva of a small moth that attacks the rhizo~es of this plant and which apparently does not attack the roots of the cultivated sorghums. This insect may hold promise in managing Johnsongrass in the United States, but before introduction could be considered, the conflict of interest would have to be resolved. Despite their limitations, weed-feeding insects have controlled sufficient numbers of weeds so that the gains received offset the cost of imple!ll8ntation many fold. Continued studies are certainly warranted. California has derived certain benefits fro:n the introduction of weed-feeding insects, as in the case of Klamath weed, H,ypericum perforatum L., puncturevine, Tribulus terrestris L., tansy ragw,ort, Senecio .1 scobaea L., and the prickly pear cacti, Opuntia spp. The University of California has three entomologists working full or part time on biological control of weeds. In addition, several projects of the u. S. Department of Agriculture, Bio­ logical Control of Weeds Investigations Laboratory at Albany, California, will also prove of value to the agriculture in this state. In the time remaining we would like to review briefly several of the weed projects underway in California.

Puncturevine, Tribulus terrestris L. -- Two beetles, Microlarinus lypriformis (Wollaston), a stem-mining weevil, am kl• lareynii (Jacq. du Val), a seed feeding weevil, were introduced to California from Italy in 1961 to control puncturevine (Huffaker et al. 1961). Both species are established in most of the puncturevine areas of the state, although the seed weevil may be encountered more frequently. Dr. R. D. Goeden, University of Cali­ fornia, Riverside, and Dr. c. B. Huffaker, University of California, Berkeley, have been evaluating the impact of these insects on the plant. Dr. Goeden notes that the weevils are only partially effective ani that the seed weevil annually infests only about one-half of the seeds in the plot areas studied in southern California (Andres and Goeden 1971). Indigenous parasites and predators reduce the effectiveness of both weevils (Goeden and Ricker 1967, 1970) • Dr. Huffaker, who has followed the weevils in the San Joaquin and Sacramento Valleys notes a slight, but apparent over-all reduction in tlE 128 weed. This impression is substantiated to some extent by the fact that there is a reduced need tor roadside treatments of puncturevine in some areas of California and Arizona (E. L. Dietz, Jr., personal communication; May and Roney 1969). It is also Dr. Huffaker' s b1presaion that feeding by the adult weevils on the young puncturevine pods reduces the spininess of the burrs, making the plant leas noxious (Huffaker, personal comunication}.

Yellow atarthiatle, Oentaurea solstitialis L. - annual thistle is one ot the top weeds in the State and the University of California has been involved in the search tor natural enemies of this plant in Europe for a number of years. In 1969, a seed f'ly, Urophora sirunaseva (Hering) was released in Contra Costa County, followed by releases in ~ndocino, Sacra­ mento, and Butte Counties in 1970, am by further releases in 1971. AP-­ proximatecy 450 flies have been released in all, but as yet there has been no establishment. Studies on this plant are continuing.

Tansy ragwort, Senecio jacobaea L. - This is a pasture and range­ land weed along the Pacific coast in California, Oregon, Washington and British Columbia. The plant is toxic to cattle, but may be grazed to some extent by sheep. The cinnabar moth, Tyria jacobaeae (L.) was introduced from Europe arxi released against this plant at Fort Bragg, Mandoc_ino County, in 1959 (Frick and Holloway 1964). The larvae of the moth feed on the flowers and foliage of tansy ragwort, often completecy stripping the plants. Following the initial release, there was little increase in Tyrie populations until 196.3. At that ti:ne a rather dramatic increase occurred arxi the larvae totally defoliated tansy ragwort over an area of about .3 acres. Since tmn the populations have continued to be large in the Fort Bragg area and many new releases have been made from the Fort Bragg stock (Hawkes 1968). The cinnabar moth is now well-established and is giving partial control at several locations in California and Oregon. While this insect does not seem to be able to kill the plant outright under California climatic condi­ tions, it is retarding seed production and causing a general unthriftiness of the weed in some areas. Total defoliation of tansy ragwort by the ci~­ bar larvae seldom kills the plant. With the advent of autumn rains, new foliage is produced from the crown and root sprouting gives rise _to new. plants at a ti".118 when the insect is in pupation and inactive. Ho-wever, I• Jacobaeae can change a vigorous stand of plants averaging about 2 feet in height to practically a non-seeding, depauperate stand averaging perhaps .3-4 inches in height. Some tansy ragwort starxis which were producing 16-17 flowering stems per square meter at the time of cinnabar moth release, are now producing only 2-3 flowering stems per square meter and by and large these are being stripped of flowers and foliage each year.

Two other insects, a seed fly, H.ylem.ya seneciella (:.feada), and a flea beetle, Longitarsus Jacobaeae (Waterhouse), have also been released in an effort to improve control of tansy ragwort (Frick 1969, 1970). The adult fe!'B8les of .H• seneciella lay their eggs in the flower heads of the plant and the larvae feed on the developing seeds. The flies were first released in 1966 and field observations in 1968 and 1969 indicated good establishment had been obtained. Unfortunately, the flowering stalks at the release site were heavily grazed by sheep in 1970, and no evidence or the flies has been found since. 129

A.dults of the flea beetle, ~. jacobaeae, feed on the plant foliage to a limited extent. The eggs are laid around the plant crown at the soil line during October and Noveaber, and the ensuing larvae develop in the roots of tansy ragwort during the winter and spring months. This winter feeding in the roots should augment the damage caused by the summer de­ foliation by cinnabar moth larvae, thus placing greater stress on the plant. The beetles were released in 1968 and 1969 arrl have beco~ well­ established at a site 10 miles north of Fort Bragg. Observations in mid­ October, 1971, irxiicated a beetle population of about J-4 per square foot in the illll18diate vicinity of the original release site. Composite thistlesa milk thistle, Silybum marianum (L.) Gaertn.; Italian thistle, Carduus ptcnocephalus L.; slenderflower thistle, .Q. tenuiflorus Curt.; ~usk thistle, Q. nutans ~ The first 3 plants are wide­ spread in California. During May-July, 1971, a weevil, Rhinocyllus conicus (Frolich), was introduced from Europe and released against milk thistle in California. The first releases were m.ade near Moraga, Contra Costa County and at Santa Barbara and on Santa Cruz Is land, Santa Barbara County. This is a cooperative project between our laboratory and Dr. Goeden, University of California at Riverside. The adult weevils do a srnall amount of feeding on the spring foliage and oviposition occurs on the flower heads. The larvae bore into the heads and feed on the seeds. Observations later in the summer of 1971 showed a large number of eggs had been laid an::i many heads were being attacked at the Moraga release site. Samples taken in October and N~vember at Santa Barbara (U. W. Ricker, University of Cali­ fornia, Department of entomology, Riverside, personal communication) and Moraga indicated 9% and 13% of the heads to be infested, respectively, at these two release sites. Thus it appears that good populations of the insect had built up by the winter. Rhinocyllus conicus occasionally attacks Italian thistle and slender­ flower thistle, but probably offers little potential for control of these plants. Thus Dr. Goeden is currently in Italy trying to find insects more closely associated with them. This weevil also attacks the heads of musk thistle, Carduus nutans L. and has been released against that plant in ~ontana, Nebraska and Virginia as well as at various sites in Canada. Russian thistle, Salsola pestifer A. Nelson, (:§.!!,!!var. tenuifolia Tausch.). -- This plant, often called "tumbleweed," is a wide­ spread pest in the United states. Its most damaging quality in California is the .fact that it is the primary spring host for the beet l~bopper_,.---> Circulifer tenellus (Baker), vector of "~P" virus of s~gal" ~~ tomatoes_,.__l>eans, c~urbits; e~, the membership orthe Western . Pl-ant---lfoard moJ>~ed a resolution requesting our laboratory to initiate research t~ fin::i insects for the biological control of this plant. The importance of Russian thistle to California agriculture prompted the California Department of Agriculture to enter into a cooperative agree­ ment with our laboratory, under which the state partially funded a bio­ logical control research program. Dr. Goeden at the University o.f Cali­ fornia at Riverside is also cooperatively involved in this program and has conducted an extensive survey of the plant in southern California to determine which insects may already be associated with it there (Goeden 130 and Ricker 1968). An extensive literature survey showed approximately 275 species of insects recorded from Russian thistle throughout the world, many of which are general teeders 'am thus not suitable to be considered for biological control purposes. However, a number or insects live in close association with Russian thistle and appear host specific.

One such insect is a moth, Coleophora parthanica Meyrick, toum attacking the plant in Egypt, Turkey, Pakistan and southern Russia. The moths lay their eggs on the leaves and the larvae mine into the leaves and stems. The insect baa three generations per year in Egypt and Pakistan. Plants under moderate attack in those two countries have thickened, knarled stems and are generally much smaller than the robust "tumbleweeds" so tre­ qent ly seen in this country. In Pakistan, natural enemies are known to be responsible tor keeping the moth from reaching its .full population potential. We would expect Coleophora to be even more effective against the plant in the United States, since the effect of population suppression by natural enemies should be much less. Preliminary host specificity tests with Coleophora on a number of plants, including sugar beets, table beets, Swiss chard and spinach, were conducted in Egypt and Pakistan. The insect did not attack any or the plants tested and approval was granted to introduce it into our quarantine laboratory at Albany where it is undergoing more intensive teats on a wider range of plants.

In summary, the biological method offers the possibility .for SUP­ pressing weeds with a minimum of environmental disturbance. It is one of several avenues open to plant !Dan&geruent and should be exploited to the fullest extent, both as a separate entity and in combination with others. 131

LITERATURE CITED

Andres, L. A., and R. D.' Goeden. 1971. The biological control of weeds by introduced natural enemies, p. 143-164. l!! C. B. Huffaker (Ed.) Biological Control, PlenUlll Press, New York, 511 p. Frick, K. E. 1969. Attempt to establish the ragwort seed fly in the United States. J. Econ. Entomol. 62s 1135-8. ----· 1970. Ragwort flea beetle established for biological control of tansy ragwort in northern California. Calif. Agric. 24(4): 12-lJ.

Frick, K. E., and J. K. Holloway. 1964. Establishment of the cinnabar moth, Tyz:ia .iacobaeae, on tansy ragwort in the western United states. J. Econ. Entomol. 57s 152-4,. Goeden, R. D., arxi D. W. Ricker. 1967. Geocoris pallens found to be predaceous on M:l..crolarinus spp. introduced to California for the biological control of puncturevine, Tribulus terrestris. J. Econ. Entomol. 60s 726-729 • 1968. The phytophagous insect fauna of ____R_u_s_s_i-an_t_hi_s_t~l-e~(s-a'""'l,...s-o~l-a kali var. tenuifolia) in southern California. Ann. Entomol. Soc. Amer. r,r;-67-72 • • 1970. Parasitization of introduced ------puncturevine weevils by indigenous calcidoides in southern California. J. Econ. Entomol. 6J: 827-831.

Hawkes, R. B. 1968. The cinnabar moth, Tyrie jacobaeae, for control of tansy ragwort. J. Econ. Entomol. 61: 499-501. Huffaker, C. B., D. W. Ricker, arxi C. E. Kennett. 1961. Biological control of puncturevine with imported weevils. Calif. Agric. 15: 11-12.

May, J., and J. N. Roney. 1969. Arizona Cooperative Insect Survey. Report No. JJ. August.