180 
Psyche 
[June 
Table 1 
Distribution of Phaneta imbridana among 
Populations of Xanthium strumarium 
Mean Proportion Seeds Attacked (1973-1975) 
1 
2 
3 
Population Number 
4 5 6 7 
8 
9 
10 
Upper Seed 
.03 
.12 
.05 
.09 .01 
.05 
.03 
.07 
.07 
0.0 
Lower Seed 
.07 
.20 
.24 
.20 .07 
.15 
.10 
.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 herbacepus 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 1), 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 
