Synchronizing the developmental cycle of the 
host to the mechanisms of insect diapause, 
aestivation, odcyte development, and to the 
temperature threshold of insect activity con- 
tributes to host specificity, permitting the 
insect to conserve its reproductive energies 
to specific plants and plant parts. The seed 
fly Hylemya seneciella (Meade) on tansy 
ragwort enters a pupal diapause after it exits 
from the flower head in July or August. Its 
emergence as an adult the following spring is 
timed to the appearance of flowers on Senecio 
jacobaea L, The temperature threshold of 
activity of the Scotch broom seed weevil 
(Apion fuscirostre (F.)) controls the time that 
the adult emerges from hibernation to when the 
buds of Cytisus scoparius (L.) Link develop. 
Oocyte formation in this insect takes place 
only in the presence of developing buds and 
flowers, and the insect will oviposit only on 
the broom pod. The adult weevils of Phrydiuchus 
spp. on Salvia spp. emerge and feed at the time 
of spring plant growth, aestivate during the 
summer dormancy, and feed and oviposit in 
the fall with the resumption of Salvia growth. 
Although host specificity assures that the 
control agent will not attack economic plants, 
it does not guarantee successful control. Past 
parsite introductions and their success led to 
attempts to correlate the type of insect with 
the degree of control obtained. Dodd (1960) 
considered the insect and the part of the host 
attacked, noting that the leaf-feeding and 
plant-sucking species, most of whichhad more 
than one generation per year, were well to the 
fore. The diversity of the agents and their 
modes of attack led Wilson (1964) to conclude 
that the insect parasites giving control will 
eventually include phytophagous species of all 
types and that their ability to control a plant 
will not be readily evident. 
It is unlikely that we will ever be able to 
predict with all accuracy the success of an 
introduction, although failure would imply that 
we lacked an understanding of the critical 
factors involved, which, in turn, would reduce 
the possibility that we could manipulate the 
environment to suit our purposes. It should be 
kept in mind that a parasite's objective is not 
to kill the host but to obtain energy to survive 
through the host with the maximum amount of 
parasite survival. 

79 
The growing weed passes through a series 
of physiological and chemical changes, periods 
of active food production and storage, periods of 
dormancy, flowering, etc. Throughout the 
growth cycle the plant is also subjected to 
periods of stress, at which time the environ- 
mental factors limiting its distribution and 
abundance approach, or exceed, its inherent 
tolerances. Much research on the tolerance of 
plants to feeding damage was conducted during 
range management studies, but may be con- 
sidered applicable here. 
The following generalizations from the text 
on ''Range Management" by Stoddart and Smith 
(1955) illustrate briefly some of this work. 
In the spring a plant utilizes 75 percent of its 
winter root reserves to produce 10 percent 
of its spring growth. Also "...food storage 
reserves are at a minimum during periods of 
greatest growth, reaching a maximum in late 
autumn,"' ''Where food supplies are low, forage 
plants are most subject to damage by grazing 
and undesirable plants are most easily killed 
at this time by mowing or spraying." In addi- 
tion to the periods of physiological stress 
" ,.it must be kept in mind that grazing 
injury may be associated more with soil 
moisture and opportunities for regrowth than 
with physiological stage of development." 
Thus, one of the first steps in selecting an 
agent for biological controlis an understanding 
of the weed's developmental cycle in relation 
to weather and growth rate. Selection of a biotic 
control agent is a matter of energy relation- 
ships and knowing which species will remove 
the most energy from the plant at the critical 
periods of development. The removal of plant 
biomass is impressive, but if done at the wrong 
time, it will not affect plant survival. Syn- 
chronization to the host plant becomes of 
prime importance, with climatic adaptation of 
the agent to the weed environment being one of 
the first considerations. 
The beetles of Chrysolina sp., used to control 
Hypericum perforatum L., were synchronized 
to feed on the spring and fall growth of their 
host in the United States. Removal of new 
growth just prior to Summer and winter dor- 
mancy prevented manufacture and storage of 
food reserves, which resulted in reduced root 
development and eventual death of the plant. 
That these beetles failed to control the weedin 
