1977] 
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 = 1 10.024, 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. 
