PYRETHRUM
Chrysanthemum cinerariifolium (Trevir.) Vis., family CompositaePyrethrum is grown for the insecticidal material, pyrethrins, that is found primarily in the flower head. It is grown in numerous countries but Kenya, Tanzania, Ecuador, and Rwanda produced an average of 34.2 million of the 35 million pounds produced per year during 1966-70 (Fowler and Mahan 1972). Pyrethrum has been tested experimentally but has not been grown commercially in the United States (Drain and Shuey 1934, McClintock 1929) although, if mechanical harvesting of the flower heads could be perfected and the hand labor reduced, its growth in certain areas might be feasible.
Plant:
Pyrethrum is a tufted, slender, pubescent perennial 12 to 30 inches high with daisylike flower heads 1 1/2 inches across, on long slender stems. It is adapted to a temperate climate with 45 to 50 inches of rainfall. The seeds are sown in special beds. Four months later the 4 to 5- inch-high plants are transplanted into the field (fig. 163).
In another 4 months, harvest of the just-opening flower heads begins and is repeated every 2 or 3 weeks for several months. After three annual harvests, the plants decrease in productivity and are plowed under and another crop planted. Maximum productivity (800 to 1,000 pounds dried seed heads per acre) may be obtained the second season (Purseglove 1968*).
[gfx] FIGURE 163. - Pyrethrum plants cut with a grain binder and curing in shocks. Experimental production at Glenn Dale, Md.
Inflorescence:
The flower head consists of 18 to 22 white, pistillate ray florets almost an inch long and a tightly packed cluster of 40 to 100 yellow, short, bisexual disk florets (fig. 164).
The flower is not considered highly attractive to honey bees, which seem to collect pollen primarily and only at certain times. The main insect visitors were reported by Kroll (1961) to be adult coleoptera and diptera, and their presence was seasonal. Kroll (1961) and Smith (1958), however, indicated that bees increased production of pyrethrum, so presumably the flowers were visited by these insects.
Harvest begins when the florets on a head are about three-fourths open (Hartzell 1943). The pyrethrin content of the flower increases as the flower stage increases: Buds unopened, 0.84 percent; one row of disk flowers open, 1.83 percent; and overblown and ripening, 1.21 percent (Kroll 1964). A similar variation from 0.23 to 1.36 percent pyrethrin was also obtained from different plant sources by Hoyer and Leonard (1936).
[gfx] FIGURE 164. - Pyrethrum flowers in different stages of development.
Pollination Requirements:
The pollination requirements of pyrethrum are not too clear, probably because of differences obtained in tests with different cultivars or under different environmental or ecological conditions. Culbertson (1940) stated that seed formation seemed to be the result of self- fertilization or apomyxis because flower heads bagged and with the anthers removed set seed. Delhaye (1956) stated that pyrethrum is highly self-fertile although a higher set of seed, and seed with higher viability, are obtained when the pollen comes from another clone. Kroll (1961) discussed a test comparing production of plants in cages with bees present, with bees excluded, and open plots. He reported that the analysis of the data was not quite conclusive but gave strong indications that production of pyrethrum is increased by insect pollination, and that fertilized embryos contain more pyrethrin than unfertilized embryos.
Purseglove (1968*) stated, without supporting data, that pyrethrum is self-sterile and must be cross-pollinated to produce viable seeds. He stated that it is insect-pollinated mainly by coleoptera and diptera. Kroll (1961) stated that the percentage of unfertilized and nonviable seeds in the field is very high. This, he concluded, seemed to indicate that the number of insect visitors was never large enough to effect satisfactory fertilization, and, at the same time, it provided a strong argument in favor of the predominance of cross- as opposed to self- fertilization.
The fertile achene was shown by Chandler (1956) to contain 1.05 percent pyrethrin compared to only 0.71 percent of barren achenes, which shows the value of having pollinated flowers for highest pyrethrin production.
Parlevliet and Contant (1970) stated that most clones are highly self-incompatible. Smith (1958) reviewed a test by L. A. Notcutt which showed that the yield of seeds was greatest from cages with bees, least from cages excluding pollinating insects, and intermediate in open plots.
A United Nations (FAO) (1961) report stated that pyrethrum is a cross-fertilized plant that requires insects for cross-pollination, the main pollinators being bees and other hymenoptera.
Brewer (1968) stated that the floret's own pollen cannot reach the receptive surfaces of the style (the stigma) because the styler lobes are closed when they extrude through the anther tube. He concluded that by the time the style becomes receptive, the germination of the floret's own pollen is about past. Delhaye (1956) tested the effect of selfing and crossing on the germination of pyrethrum seed. He found the following: Selfed without bees, 0.0 to 1.0 percent; selfed with bees present, 1.7 to 22.7 percent; crossed without bees, 5.2 to 8.3 percent; and crossed by bees, 17.7 to 27.7 percent. Brewer (1968) concluded:
The flower morphology and the flower morphogenesis of Pyrethrum resemhles closely the classical concept known in the Compositae. The flowering rhythm of the inflorescences encourages crosspollination through:
(1) The individual floret discharges the ripe pollen before it unfolds the receptive surfaces of the style.
(2) When insects visit the inflorescence their path follows the development of the flower, i.e., from the margin to the centre, in order to collect pollen and nectar. Thus they deposit the foreign pollen they carry on fully opened styles.
(3) By sticking together, the pollen mass encourages transport by insects.
(4) Pollen does not germinate on genotypically identical styles. Strong evidence exists that the incompatibility system is sporophytically determined.
(5) The limited life of the pollen after anthesis reduces the chance for own pollen to germinate on styles of the same floret.
Lower germination percentages of the pure seed (P.G.S.) are due to rainfall during the maturing period of the seed.
Pollinators:
The previous references indicate that honey bees are not overly attracted to pyrethrum flowers, as compared to beetles and flies. There has been no attempt to concentrate honey bee colonies near the crop. A test should be conducted to determine the practicality of supplying honey bee colonies to pyrethrum fields for pollination purposes. The use of leafcutter and other wild bees and different species of flies should also be investigated.
Pollination Recommendations and Practices:
There are no recommendations on the use of pollinating insects on pyrethrum, even though the evidence indicates they are beneficial.
LITERATURE CITED:
BREWER, J. G.
l968. FLOWERING AND SEEDSETTING IN PYRETHRUM (CHRYSANTHEMUM CINERARIAEFOLIUM VIS.) A REVIEW. Pyrethrum Post 9(4): 18-21.CHANDLER, S. E.
1956. BOTANICAL ASPECTS OF PYRETHRUM III. THE NATURAL HISTORY OF THE SECRETORY ORGANS; THE PYRETHRIN CONTENT OF THE FERTILE ACHENES. Pyrethrum Post 4(1): 10-15.CULBERTSON, R. E.
1940. AN ECOLOGICAL, PATHOLOGICAL AND GENETICAL STUDY OF PYRETHRUM (CHRYSANTHEMUM CINERARIAEFOLIUM VIS.) AS RELATED TO POSSIBLE COMMERCIAL PRODUCTION IN THE UNITED STATES. Diss. submitted to Pa. State Univ., Hort. Dept., as partial furfillment of requirements for Ph.D. degree, 289 pp.DELHAYE, R. J.
1956. [PRELIMINARY NOTE ON THE FLORAL BIOLOGY AND CONTROLLED FERTILIZATION OF PYRETHRUM, CHRYSANTHEMUM CINERARIAEFOLIUM (TREV.) BOCC.] Bul. Agr. Congo Belge 47: 1327-1343. [In French]DRAIN, B. D., and SHUEY, G. A.
1934. THE ISOLATION AND PROPAGATION OF HIGH PYRETHRIN STRAINS OF PYRETHRUM. Amer. Soc. Hort. Sci. Proc. 32: 19O-191.FOWLER, D. L., and MAHAN, J. N.
1972. THE PESTICIDE REVIEW 1971. U.S. Dept. Agr. Stabilization and Conserv. Serv., 56 pp.HARTZELL, A.
1943. PYRETHRUM CULTURE IN DALMATIA WITH SOME APPLICATIONS TO THE AMERICAS. Jour. Econ. Ent. 36: 320-325.HOYER, D. G., and LEONARD, M. D.
1936. PYRETHRIN CONTENT OF PYRETHRUM FLOWERS FROM VARIOUS SOURCES. Jour. Econ. Ent. 29: 605-606.KROLL, U.
1961. THE INFLUENCE OF FERTILIZATION ON THE PRODUCTION OF PYRETHRINS IN THE PYRETHRUM FLOWER. Pyrethrum Post 6(2): 19-21.____ 1964. PYRETHRUM IN KENYA. Outlook on Agr. 4(4): 177-181.
MCCLINTOCK, J. A.
1929. PYRETHRUM. Tenn. Agr. Expt. Sta. 42d Ann. Rpt.: 40.PARLEVLIET, J. E., and CONTANT R. B.
1970. SELECTION FOR COMBINING ABILITY IN PYRETHRUM (CHRYSANTHEMUM CINARIAEFOLIUM VIS.). Euphvtica 19: 4-11.SMITH. F. G.
1958. BEEKEEPING OPERATIONS IN TANGANYIKA, 1949-1957. Bee World 39: 29-36.UNITED NATIONS FOOD AND AGRICULTURE ORGANIZATION (FAO).
1961. AGRICULTURAL AND HORTICULTURAL SEEDS. FAO Agro-studies 55, 531 pp.
