The usual method of use against insects – flooding the population with sterile males- is dependent on the females mating only once. That is common behavior amongst
insects that live for only 1 year, but is rare amongst vertebrates.
Most attempts at control by contraception or sterilization have utilized chemicals
such as bromocriptine, quinestrol, mestranol, and cyprosterone. Table 20.3 gives Marsh’s
(1988) criteria for an ideal rodent chemosterilant.
The effect of a contraceptive or sterilizing agent upon the population’s dynamics
depends on the breeding system of the species and particularly upon the form of dom-
inance. In general, a vertebrate population will seldom be controlled adequately by
a contraceptive or sterilant specific to males (Bomford 1990) and so the target should
be either the female segment of the population or both sexes.
Caughley et al. (1992) explored the theoretical effect on productivity of three forms
of behavioral dominance, two effects of sterilization on dominance, and four modes
of transmission. Seventeen of the 24 combinations are feasible but lead to only four
possible outcomes. Three of these result in lowered productivity. The fourth, where
the breeding of a dominant female suppresses breeding in the subordinate females
of her social group, leads to a perverse outcome. Productivity increaseswith steril-
ization unless the proportion of females sterilized exceeds (n−2)/(n−1), where n
is the average number of females in the social group (Fig. 20.1). Hence, a know-
ledge of social structure and mating system is desirable before population control by
suppressing female fertility is attempted. Experimental tests of this dominant female
model using artificially sterilized female red foxes has shown that dominance is not
an important social suppressor of subordinate female reproduction. Thus, greater female
sterility led to lower juvenile recruitment (Saunders and McIlroy 2001).
The theoretically derived examples of reducing litter production exclude the effect
of increased fertility consequent upon lowered density. It cannot be modeled from
first principles in the same way as the expected reduction in litter production
WILDLIFE CONTROL 361
1 Orally effective, preferably in a single feeding.
2 Effective in very low doses (not exceeding 10 mg/kg).
3 Permanent or long-lasting sterility (preferably) lasting 6 months of longer, or at least through the major
breeding period of the pest species.
4 Effective for both sexes or preferably for females if only one sex.
5 Rodent-specific or genus-specific.
6 Relatively inexpensive.
7 There should be a wide margin between the chemosterilant effects and lethal doses. (If high in specificity,
this may be unimportant or the narrow margin be of value.)
8 Well accepted (i.e. highly palatable) in baits at effective concentrations.
9 Biodegradable after a few days in the environment.
10 If not highly specific, rapid elimination from the body of the primary target to avoid secondary effects.
11 No acquired tolerance or genetic or behavioral resistance.
12 Free of behavioral modification (such as altering libido, aggression, or territoriality).
13 Free from producing discomfort or ill feelings that could suppress consumption (i.e. bait shyness) on
repeat or subsequent feedings.
14 Humane (i.e. produces no stressful symptoms).
15 Easy to formulate into various kinds of baits.
16 Sufficiently stable when prepared in baits (i.e. adequate shelf life).
17 Not translocated into plants (or at a very low level), thus permitting use on crops.From Marsh (1988).Table 20.3
Characteristics of an
ideal chemosterilant
for rodents.