Cerambycid beetle larvae
http://ipmwww.ncsu.edu?AG271/corn_sorghum/japanese_beetle.html
Dr.
Stephen Malcolm
Introduction:
As described in the last laboratory, spatial and
temporal patterns of distribution of individuals within populations or
metapopulations is an important consequence of their responses to the
distributions of resources and conditions as well as to different ecological
processes, within the evolutionary constraints of their life histories.
It is common for populations of species to be
distributed among a series of partially isolated subpopulations and for
individuals to move occasionally from one group to another. The size, number and degree of
isolation of the units of such a METAPOPULATION and the dispersal abilities of individuals influence
the regional persistence of a species.
This makes the dynamics of metapopulations an important determinant of
species persistence and it is a major concern for conservation biologists. This is especially true because rarer
species are more likely to show fragmented populations distributions because they
are more likely to be habitat specialists than common, habitat
generalists. Generalized resource
use life histories are likely to result in random or regular distributions of
individuals in time and space. In
contrast, specialized life history patterns of resource use are more likely to
result in strongly aggregated distributions of individuals that communicate by
dispersal among the components of a metapopulation.
In this laboratory exercise we would like you to test
this idea by comparing the distributions in space of the herbivorous larvae of
two beetle species. The two
beetles are, Tetraopes tetrophthalmus, the red milkweed beetle, and Popillia japonica, the Japanese beetle. T. tetrophthalmus is a longhorn beetle in the family Cerambycidae and is a specialist
herbivore of milkweeds in the genus Asclepias, especially the common milkweed, A. syriaca. P.
japonica is an introduced pest
species in the large family Scarabaeidae, that is a broadly generalist
herbivore of over 275 different plant species and the larvae are serious pests
of lawns, grasses and nursery stock.
Hypotheses:
In this exercise, please formulate null and alternative hypotheses about
the influence that such diet breadth differences are likely to have on whether
or not these two beetle species show a metapopulation distribution of
individuals in space.
The beetle life
histories:
The Japanese beetle, P. japonica, was first reported in North America in 1916 and now
occurs in more than 20 states from southern Maine south to Georgia and west to
Kentucky, Illinois, Michigan and Missouri.
Adult beetles emerge from the soil from mid May to
late July and feed on many plant species and mate. Females lay batches of 40-60 eggs in small batches 5-8 cm
deep in the soil and larvae hatch and feed on roots, especially of
grasses. Three larval instars take
approximately 140 days to complete and this period is interrupted by
winter. Larvae overwinter in cells
approximately 13 cm from the soil surface and in spring they move up to ground
level and complete feeding and pupate.
The milkweed beetle, T. tetrophthalmus, is a very unusual cerambycid beetle because it is a
leaf- and root-feeding herbivore of milkweeds rather than a wood borer as is
typical for most members of this large and diverse family of beetles. Adult T. tetrophthalmus emerge from pupae in the soil in June and July and
crawl or fly to the tops of milkweed ramets where they feed on young leaves and
mate. Females lay long, thin eggs
into the pith of adjacent grass stems and the larvae emerge from these eggs and
drop to the soil around the underground, root-like stems of the common
milkweed, A. syriaca. This milkweed species is highly modular
and a genet (the genetic individual) is commonly made of from 10s to 100s of
ramets. Thus the probability that
larvae will find root resources to exploit is high. Larvae feed through late summer and early autumn and then
diapause in the soil near milkweed roots.
Like the Japanese beetle larvae complete their feeding in spring, pupate
and adult emerge in mid summer to complete the life cycle.
Both
beetle species have a single generation per year.
Methods:
Organize yourselves into working groups of 3 or 4 and measure the
distribution of Japanese beetle and milkweed beetle larvae both inside and
outside milkweed genets. Randomly
select samples and dig with a garden fork to the depth of the fork tines and
search carefully for larvae that look like those illustrated above. Repeat your sample digs until you have
a sample size of both beetle species that will allow you to make statistical
comparisons of their distributions both inside and outside milkweed
genets. Then replicate your sample
by repeating sampling at a further 2 milkweed ramets. This will give you N = 3 and repeated samples at each
location. Use the numbers of
larvae of each species inside and outside the milkweed genet to compare
distributions.