FILE: <bc-14.htm> GENERAL INDEX [Navigate
to MAIN MENU ]
SPECIAL PROBLEMS In Biological
Pest Control (Contacts) Problem 1: The
cottony-cushion scale, Icerya purchasi, kills citrus
trees in the absence of its key predator the vedalia beetle, Rodolia cardinalis.
The presence of Rodolia reduces the scale density to a very
inconspicuous level. Cold weather is
detrimental to Rodolia, causing it to slow down its searching
and feeding activities on the scale. A number of coastal citrus orchards
in southern California experienced outbreaks of cottony-cushion scale during
an unusually cold spell in May, which did not result in a tree kill, but
interfered with fruit production. An entomologist recommended heating the
orchards to control the scale outbreak. He told the growers that the physical
factor "heat" was detrimental to the scale and reduced its density
below the economic threshold. a. What factors regulate the scale
population? b. What is the key factor responsible for
the observed scale density? c. What effect did heat have on the scale
population density? d. What are the limiting factors
determining the density at which the scale will exist? Problem 2: In the East
African grasslands (5,000 ft elevation) there occurs a community of mammals
consisting of zebras,antelopes and several predators (lions, leopards,
etc.). There are also other animals
such as pigs, rodents, etc. in the area. Zebras feed selectively on tall
rough grass, which allows sunlight to reach tender grass species lying
beneath, permitting their growth. Antelopes feed only on the tender grass
species. The antelope population densities
are highest when the zebras are present, but decline markedly when zebras are
removed by poaching.The predators in the area favor the antelopes and live
almost exclusively by feeding on them. The lion is the most voracious feeder,
especially when antelopes are very abundant. a. What regulates the antelope populations? b. Name apparent common limiting factors
for the antelope population, and list their hierarchy. c. What is the apparent key factor
responsible for the density of antelopes at any give time? d. Assuming that there are five antelope species present,
point out where the two types of competition exist. Problem 3: Assume the same situation as in
Problem 2, but where all predators have been poached from the area (no
antelope predators). a. What regulates the antelope populations? b. What seems to be a common limiting
factor for the antelope population? c. What key factor is primarily responsible
for the level of the antelope population densities? d. Assuming that there are five antelope species
present, point out where the two types of
competition exist. Problem 4: A lake in south
California is 3 miles long and 2 miles wide. It has an average depth of 4
feet. The encephalitis virus vector mosquito Culex tarsalis
breeds to enormous numbers in the lake. If one species of mosquito fish, Gambusia
affinis affinis, is introduced into the lake, the mosquito
population density drops by 90%. a. What factors regulate the mosquito
population with fish present? b. What key factor affects mosquito
population density? c. What limits the mosquito population? d. What limits the fish population? e. What regulates the fish population? f. What key factor affects the fish
population density? g. Where is competition (describe for all)? Problem 5: Assume the same
situation as in Problem 4, but where no mosquito predators of any kind exist.
a. What factors regulate the mosquito
population? b. What key factor affects mosquito
population density? c. What limits the mosquito population? d. Where is competition? Problem 6: Assume the same situation
as in Problem 4, but where two subspecies of mosquito fish are introduced, Gambusia
affinis affinis and Gambusia affinis holbrooki.
The newly introduced Gambusia is not able to tolerate high
water temperatures as well as the previous species. Culex tarsalis breeding is favored in
warmer water. a. What regulates the mosquito population? b. What key factor affects mosquito
population density? c. What limits the mosquito population? d. What limits the fish population? e. What key factor affects fish population
density? f. Where is competition? Problem 7: Assume the
situation in Problem 4 with only one predatory fish species present. Emergent
vegetation, in the form of sedges and other higher aquatic plants, protrudes
above the surface of the water, which furnishes protection for Culex
tarsalis from the ravages of Gambusia. The Culex
population density then soars to a level of public health importance. a. What regulates the mosquito population? b. What limits the mosquito population? c. What environmental factors determine the
vitality and activity of the mosquitoes? d. What is the role of the emergent
vegetation in the system? Problem 8: Assume the same
situation as in Problem 7, except that an herbivorous fish, Tilapia
zillii, is introduced in the lake. Tilapia browse
out all the emergent vegetation, and the Culex tarsalis
density drops to a very low level (under 0.25 larvae/400-ml dipper). a. What regulates the mosquito population? b. What limits the mosquito population? c. What is the role of Tilapia
in the system? d. What regulates the Tilapia
population? e. What controlled the Culex
outbreak? Problem 9: An alfalfa field
is attacked by a lepidopterous caterpillar that feeds only on alfalfa. Strip cropping
(cutting alternate rows at different times to harvest) reduces the incidence
of the caterpillars in the field and results in economic control. The
caterpillar is attacked by several hymenopterous parasitoids and its eggs are
devoured by several species of predators. The rationale behind the strip
cropping is to preserve these natural enemies in the field so that they can
be available for killing and lowering the caterpillar population density. a. What factors can be suspected as
involved in regulation of the caterpillar population? b. What is the limiting factor for the
caterpillar population? c. Could a key factor exist? d. What environmental factors
influence indirectly the vitality or activity of the individuals comprising
that population? e. Where is competition? f. Where is control? Problem 10: The codling moth
infests walnuts in California. In a hypothetical situation it infests 100% of
the nuts, with usually only one larva surviving per nut. The first larva to
enter the nut by chance drives out all subsequent larvae that try to get in,
thereby exhibiting a kind of territoriality. There is no effective
parasitoid, predator or pathogen. a. Where is there a regulating factor? b. Name some environmental limiting
factors. c. Is there a key factor that is primarily
responsible for the level of the codling moth density? d. Where is competition? e. If only 50% of the nuts are infested,
what is the regulating factor? f. If 5
parasitic species are introduced, and the 3rd introduction results in a codling
moth density drop to less
than 1.5% infested nuts, what is the key factor affecting the caterpillar
density on walnuts? g. What controlled the caterpillar problem? Problem 11: The navel
orangeworm, Amyelois transitella, enters walnuts that
have been penetrated by the codling moth in July, and in walnuts whose husks
have cracked in late August. Assuming that there are no effective parasitoids
of orangeworm nor codling moth, a. What is the regulating factor in July?
In August? b. What limits the naval orangeworm
population in July? In August? c. Where is competition? d. If the introduction of a parsitoid against the
codling moth lowers this pest's density to less than 1.5% infested nuts, what
is the key factor affecting the navel orangeworm density on walnuts? e. What level of control would you expect
of orangeworm in July? In August? Problem 12: The Dutch elm
disease kills American elm trees after the vector Scolytus multistriatus
inoculates the inciting fungus. Consequently, in eastern North America the
native elms have been reduced to less than 5% of their former population
density. a. What regulates the American elm density
now? b. What are some environmental limiting
factors? c. Is there a key factor? d. Where is competition? e. Where is control? f. What function has the pathogen in
determining elm density? g. What might cause an increase in the
American elm population density? |