Intraspecific Competition between Insects and a Study of
Predation on Haole Koa Seeds
In this lab we will ask how Anthribid (Araecerus levinennis) beetles select the laying
sites for their eggs.
Haole Koa, Ekoa, or Leucaena leucocephala (the scientific name) is an abundant,
fecund, fast growing tree typical of dry areas in the Hawaiian Islands. This plant is not native,
but was introduced to the islands. It produces elongate seedpods containing numerous large
seeds. The seeds occur in separate chambers within the pod and you can see the seeds as bumps
on the pod surface. Haole Koa seeds suffer a high mortality caused by the larvae of an Anthribid
beetle. The beetles chew a small hole in the pod, right above or below a seed, and deposit one
egg in the pod, next to the seed. The egg hatches and the seed serves as a food source for the
Anthribid larva which then pupate and metamorphose into an adult. The adult emerging adult
chews a larger hole in the pod wall through which it escapes. Thus pods can be read like a book,
the numbers and sizes of Anthribid holes telling the fate of the seeds inside.
Emergence holes
(in brown pods)
Seeds
Egg laying
scars
If seed pods of appropriate age are available, we will attempt to determine if anthribid
mothers lay eggs at random on the seeds or whether they space them out, e.g., only one egg to a
seed, (what would be the selective advantage of spacing the eggs 1 per seed?) or cluster them.
If events such as "attacks" on seeds are rare, or relatively rare, and occur at random, the
frequency distribution of these events approximates a Poisson distribution, named after a
mathematician, Mr. Poisson.
Frequency
of events
100
50
0
1
2
3
4
5
Events per unit time (or space)
6
7
For instance if AM's (anthribid mothers) deposit eggs at random and if eggs are rare
relative to the # of seeds, most seeds will have no eggs (scars), fewer will have 1, fewer still 2,
etc.
10
8
Frequency of
eggs per seed
6
4
2
0
0
1
2
3
4
Number of eggs per seed
5
6
If eggs are clustered (“clumped”) we would expect to see a distribution more like this:
8
Frequency of
eggs per seed
6
4
2
0
0
1
2
3
4
Number of eggs per seed
5
6
7
8
The principle point of this exercise is to show you that you can learn something about the
behavior of these beetles by studying how they have dispersed their eggs.
Once we know the average number of eggs/seed we can use the Poisson distribution to calculate
the number of seeds we expect to have 0, 1, 2, 3, etc. eggs if the MB's are laying eggs at random.
The formula for calculating the expected frequencies for each of the classes, 0 eggs/seed, 1
egg/seed, etc., is:
X r (# of seeds) = Expected frequency in the rth class
(X̄r/r! ex̄ ) * (total number of seeds)
where: x̄ = average eggs/seed
r = respectively 0 eggs/seed;1 eggs/seed; 2 eggs/seed etc.
e = base of natural logs
r!= r factorial e.g. 3! = 3*2*1,
4! = 4*3*2*1
The observed frequencies of # of eggs/seed can be compared to the expected frequencies using a
test you've learned about.
Reading
Krebs pgs. 41-54, 88-94, Chap 9
Andrewartha, M. G. 1961, Introduction to the study of animal populations. Univ. Chicago Press.
Southwood, T.R.E. 1966. Ecological Methods. Methuen.
Whittaker, R.H. 1975. Communities & Ecosystems. MacMillan.
http://en.wikipedia.org/wiki/Poisson_distribution
http://www.itl.nist.gov/div898/handbook/eda/section3/eda366j.htm