vulnerability to parasitoids for the bottom egg. When parasitoids are
abundant, the average survival of a pair of eggs exceeds that of single eggs.
As yet, there is little evidence that marking pheromones of seed
parasites are exploited by the parasite’s natural enemies. In part, this
is because so little is known about the composition of the marking
pheromones themselves. A mixture of saturated hydrocarbons and
diacylglycerols applied to a legume seed stimulated the oviposition or
‘stinging’ behaviour of a parasitic wasp,Dinarmus basalis(Rondani), that
attacks the azuki seed beetle,Callosobruchus chinensis(L.) (Kumazaki
et al., 2000). The hydrocarbons are thought to be constituents of the
marking pheromone in this bruchid, but neither the diacylglycerol nor
other active glycerols are known to mediate avoidance of occupied seeds.
Because eggs from different geographical populations ofC. maculatus
appear to differ in their tendency to deter further egg laying (Messina
et al., 1991), it would be interesting to determine whether there are
parallel differences in prey location or oviposition stimulation of the
parasitoid.
At least two studies of seed beetles have detected positive density
dependence in the risk of mortality from parasitoids; eggs or larvae on
heavily infested seeds are more likely to be attacked than those on lightly
infested seeds. In a study ofCallosobruchus chinensisand its parasitoid,
positive density dependence was caused by a substantial increase in the
parasitoid’s searching efficiency when seeds bore four beetle eggs instead
of one (Ryoo and Chun, 1993). There is also strong competition among
C. chinensislarvae within seeds. The net reproductive rate of an emerging
female shows a negative linear relationship with the number of larvae
per seed. Thus, both risk of natural enemies and intraspecific com-
petition favour females that lay only one egg per seed and avoid seeds
that already bear eggs. Ryoo and Chun (1993) could not disentangle
the relative importance of competition vs. enemies in determining
C. chinensisbehaviour.
Siemens and Johnson (1992) suggested that positively density-
dependent mortality from natural enemies was more important than
intraspecific competition in explaining why females of the seed beetle
Stator limbatus(Horn) lay few eggs per seed. Highest beetle productivity
from a Palo Verde seed (the single-host maximum) occurred when 13–15
larvae shared a seed, but females typically laid one or a few eggs on an
individual seed. Siemens and Johnson (1992) argued that parasitoid-
induced mortality was responsible for this behaviour and that the action
of the parasitoid would itself reduce intraspecific competition, at least in
a large host seed. However, they also showed that competition decreased
the per capita fitness of larvae with each increase in larval density, so that
total productivity increased in a decelerating fashion, as depicted in
Fig. 4.2. For reasons discussed earlier, the combination of a dome-shaped
larval competition curve and high seed availability (low search costs)
favours small egg clutches, even without density-dependent attack80 F.J. Messina