J. Daniel Hare.
The Biology of Phaneta imbridana (Lepidoptera: Tortricidae), a Seed Predator of Xanthium strumartum (Compositae).
Psyche 84(2):179-182, 1977.

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THE BIOLOGY OF PHANETA IMBRZDANA
(LEPIDOPTERA : TORTRICIDAE), A SEED PREDATOR OF XANTHIUM STRUMARIUM (COMPOSITAE)
Department of Ecology and Evolution*
State University of New York
Stony Brook, New York 11794
Of the more than sixty North American species of Phaneta, host plants are known for less than one third. All of the known hosts are in the family Compositae, and most species feed only on the flowers or seeds of their host plant (Heinrich, 1923, Mackay. 1959). (Host plants are listed by these authors for the species of the genus, Thiodia, the North American members of which have been trans- ferred to Phaneta(0braztsov, 1952)). Although Phaneta imbridana (Fernald) has been known to taxonomists for years (Fernald, 1905, Miller, 1970). nothing is known of its biology or life history. I therefore report certain aspects of the ecology of P. imbridana and its relationship with a local host plant, Xanthium strumarium, unique among the Compositae by having relatively large fruits and seeds. This information was obtained as part of a larger study of the variation in susceptibility of populations of X. strumarium to seed predation by more than one species of seed predator along Long Island beaches.
A. Life Cycle
Adults emerge in late August and can be found until late Sep- tember, with oviposition occurring throughout the adult period. Females oviposit directly on the surface of the full-sized but im- mature burrs of X. strumarium. Eggs soon hatch and the larvae bore through the burr wall and begin to feed on one of two seeds of the burr. If one seed is insufficient for complete larval develop- ment, larvae will attack the other seed within the same burr, or rarely, seeds of another burr on the same plant. Full larval devel- opment is completed by late September or early October, at which *Present address: Department of Entomology, Connecticut Agricultural Experi- ment Station, Box 1106, New Haven, CT 06504. Manuscript received by the editor October 1, 1977.



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Psyche
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Table 1
Distribution of Phaneta imbridana among
Populations of Xanthium strumarium
Mean Proportion Seeds Attacked (1973-1975) Population Number
1 2 3 4 5 6 7 8 9 1 0
Upper Seed 03
.12
.05 .09 .O1 .05
.03 .07
.07 0.0
Lower Seed
07 -20
.24 .20
.07
.15 .I0
.22
.12 .05
time larvae leave the burr through a hole bored near its basal end. Since burrs reach full maturity and are easily dislodged and dis- persed before larvae leave the burr, passive long-range dispersal of P. imbridana may occur in the larval stage. Local populations of P. imbridana overwinter as last-instar larvae in the dry pithy stems of X. strumarium. It is unlikely that P. im- bridana is limited to X. strumarium for overwintering. however, the other common herbaceous species associated with X. strumar- ium do not contain overwintering larvae. Pupation occurs in the stem fragments in the following summer. Mating behavior was not observed.
B. Role as a Seed Predator
Levels of seed predation were measured for ten populations of X. strumarium over a three-year period. Consistent, significant differences in the abundance of P. imbridana were observed among plant populations (Table l), however, mean seed loss was less than 10%.
The two seeds within a burr of X. strumarium differ in size and germination requirements (e.g. Wareing and Foda, 1957). The lower seed is larger and germinates the spring following produc- tion, while the smaller, upper seed remains dormant for one year or more if its seed coat remains intact. Phaneta imbridana is more commonly found in the lower, non-dormant seed within a burr (p less than .001). Although one cannot exclude the possibility that larvae or ovipositing females may be choosing seeds on the basis of their dormancy properties, differential seed predation within burrs is best explained by burr asymmetry. Since the larger seed occupies more than half of the burr cavity, it is covered by more



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19771 Hare - Biology of Phanta imbridana 181 than half of the burr surface, and oviposition is more likely to occur on burr surface adjacent to lower than to upper seeds. C.
Interactions with Other Seed Predators
The tephritid fly, Euaresta aequalis Loew, is another common seed predator of X. strumarium. The abundance of E. aequalis also varies significantly among populations, and larvae are more frequent in lower than upper seeds. The oviposition periods of both insect species coincide. Most local populations of X. stru- marium are not attacked by both insect species, however, in those plant populations which experience at least 5% seed predation by both species, the abundance of the two species on individual plants is significantly negatively correlated ( r = -.42, p less than .01). An oviposition experiment was performed using plants from several populations grown under uniform conditions and then simulta- neously exposed to both insect species. The number of burrs at- tacked by both species was much less than expected assuming that their oviposition behaviors were independent (Table 2), and the number of burrs containing one larva of P. imbridana and one undamaged seed was much greater than expected. These results indicate that within populations, some plants may produce burrs more susceptible to one insect species than the other, Table 2
Frequency of Attack of
Seeds Within Burrs
Disposition
PP UP PE EE UE UU
Observed
16 70
43 110
46 87
Expected 14.1 56.5
60.2 64.2
130.5 56.5
Difference
1.9 13.5
-17.2
45.8 -74.5 30.5
G-test Statistic = 110.024, p less than .005 PP = Both seeds containing P. imbridana. UP = One seed containing P. imbridana and the other undamaged. PE = One seed containing P. imbridana and the other containing E. aequalis. EE = Both seeds containing E. aequalis.
UE = One seed containing E. aequalis and the other undamaged. UU = Both seeds undamaged.




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182 Psyche [June
and also that within plants, P. imbridana may avoid ovipositing on burrs previously attacked by others of its own or different species. Further investigations are in progress to determine which particu- lar aspects of burr morphology and chemistry most strongly influ- ence susceptibility of burrs to each insect species. I thank William E. Miller and a reviewer for their information concerning the taxonomy and ecology of the genus Phaneta. Spec- imens were kindly identified by D. R. Davis of the U. S. National Museum. Contribution #226 from the program of Ecology and Evolution at the State University of New York at Stony Brook. FERN~D, C. H.
1905. North American Tortricidae. Can. Ent. 37: 399-400. HEINRICH, C.
1923.
Revision of the North American Moths of the Subfamily Eucosminae of the Family Olethreutidae. Bull. U, S. Nat. Mus. #123. 298 pp. MACKAY, M. R.
1959. Larvae of the North American Olethreutidae (Lepidoptera). Can. Ent. Suppl. #lo. 338 pp.
MILLER. W. E.
1970.
Fernald Types of North American Olethreutinae (Lepidoptera : Tortri- cidae). Proc. Ent. Soc. Wash. 72: 288-294. OBRAZTSOV. N.
1952.
Thiodia Hb. as not a North American Genus (Lepidoptera, Tortrici- dae). Ent. News 63: 145-149,
WAREING, P. F.. AND H. A. FODA
1957.
Growth Inhibitors and Dormancy in Xanthium seed. Phys. Plant. 10: 266-280.




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