Ideas for Student Projects and Class Demonstrations

We hope that student participation in the Great Labor Day Mosquito Count might interest them in mosquitoes or ecology, and that they might want to develop those interests with individual or group projects.  The following are some basic questions that can be asked that require very little equipment or fancy methods.  Each can be carried out with different levels of sophistication, so students at a wide variety of levels can take these on.  Some of these have been done by undergraduates at Clark University, and others have been done successfully by local children for fifth grade science projects.  

We invite students or classes who complete projects as a result of their involvement in the mosquito count to send them to us (although perhaps some teacher screening would be a good idea).  We can post good examples of these on this site, as long as space permits.  Please send electronic files of your project to:  mosquito@clarku.edu .  If there is material that must be submitted on paper, such as photos or hand-drawn graphics, please mail them to:

Todd Livdahl
Department of Biology
Clark University
Worcester MA 01610

We might not be able to return these materials, so make sure you have copies to save.

Mosquitoes are great project subjects, for many reasons.  We need to know about them, and people are interested in mosquitoes.  Most of us don't like them, but they're easy to work with.  And a student might find that their project will be a lot more unusual than the standard crystal growth project.  So, here are some ideas.  We know that some of them actually work.

A.  Adult host-seeking behavior

Female mosquitoes bite to obtain blood as a protein source for their eggs.  Many people have ideas about what makes a person attractive to mosquitoes, and what substances help to keep mosquitoes away.  Biting behavior has been studied extensively.  For students doing a project for the first time, projects that provide a simple demonstration of factors that attract or repel mosquitoes are likely to be interesting and successful, even if these factors are already known.  These can be done in the field or in the lab.

It is important to develop standard method to quantify host-seeking.  Mosquito researchers may actually allow the mosquitoes to bite an exposed area, such as an arm, for a fixed amount of time, to obtain a bite rate.  We suggest that students not be encouraged to do this, but instead to obtain a swat rate, which might be the number of mosquitoes they can swat on themselves or others within a fixed period.  Students can minimize bites by using repellent on exposed skin, wearing long pants and long sleeves, and wearing heavy clothing such as sweatshirts.  Different periods of time will be needed for different areas and times of day, because mosquitoes can vary widely in their abundance.  

Some ideas:  

1)  Does the color of clothing affect mosquito biting behavior?  

Send students to a forested area in groups, wearing clothes of different colors.  Develop a standard method of counting mosquitoes swatted per unit time, and compare the swat rates for the different color groups.  Repeat, on successive days, with different colors for the same students, in random locations in a forest.

Follow-up studies might come from observations that particular students are more attractive than others, or that particular locations have more mosquitoes than others.  Students might attempt to determine why this might be so.

2)  Does human activity affect mosquito biting behavior?

Develop groups of students into two experimental groups-- one active and one inactive.  After a standard period of activity, compare swat rates.

3)  Does mosquito biting activity change through the course of the day?

4)  How does biting activity correspond to weather conditions?  Expect significant reductions on windy days, and increases in warm, humid conditions.

5)  How late into the fall does mosquito biting continue?  Can the end of biting be traced to any weather events, such as a frost?

6)  Lab experiments with adult mosquitoes.  Rear some larvae to adulthood, and keep them in a screen cage.  Fill balloons with warm water, and they will try to bite the balloons.  Try different balloon colors (my daughter found that they prefer blue over white), and count the number landing on each during a 5-minute trial.  Try spraying some repellent on a balloon for comparison.

7)  Field trials of repellents and various mosquito deterring products on the market.  You might find surprising results-- some devices that attract and trap mosquitoes may actually draw more mosquitoes to an area than would normally be there.  There is no evidence I know of that the ultrasound-emitting devices have any effect on mosquitoes, although a lot of money has been made from them.

B.  Adult egg-laying behavior and ecology

Many mosquitoes survive for weeks in nature, and females may produce several batches of eggs.  Each batch requires that a mosquito obtain blood from a vertebrate, usually a mammal or a bird.  After obtaining a meal, the female will rest for a few days while she makes eggs from the protein that the blood provides.  When the eggs are ready, she will look for a place to lay her eggs.  She must make decisions about where to lay them.

The Mosquito Count is focused on container-breeding mosquitoes.  These mosquitoes prefer small bodies of water, and their preferences for different kinds of habitats for their offspring can be easily studied by comparing the number of eggs laid in traps that have been modified in various ways with control traps.  

1)  Do females care if there are already eggs present in the trap?  Obtain egg batches from traps, count the eggs, and place them into new traps.  Count the number of eggs added to these and control traps with no eggs after one week.  Randomize trap positions.

2)  Do female mosquitoes avoid containers with mosquito larvae in them?  They might actually find that larvae can attract egg-laying mosquitoes, because they're a good sign that the container won't dry up.  Hatch some eggs by submerging them in a yeast or broth solution, and raise them in pans for one week.  Provide them with ground up dry dog food to eat, then add the larvae to the cups that are used for egg traps.  Count the number of eggs laid in cups with different densities of larvae.

3)  Are female mosquitoes attracted to different colors of water?  Add food coloring to some traps, or color the plastic.

4)  Do mosquitoes prefer traps that have a food source for their families?  Add decaying organic material, such as leaves, to some traps.

5)  Do mosquitoes prefer containers at different heights in a forest?  Throw a rock with a cord attached to it over a high limb of a tree, and  suspend egg traps from the cord at different heights in the canopy.

6)  Does egg-laying change in response to weather conditions?  I would expect this to relate to humidity, temperature and wind.

7)  Do mosquitoes avoid predators in containers?  Find some predators, such as dragonfly nymphs in pond bottoms, or small fish, and add them to the traps.  Compare the average number of eggs laid in a week with control traps.  Better yet, add predators that container-breeding mosquitoes have some experience with-- the predaceous treehole mosquito, Toxorhynchites.  These are fantastic animals:  mosquitoes that eat other mosquitoes as larvae, but which don't bite as adults.  They occur as far north as the southern Midwest, so many of you can find them.  [add link to the Toxorhynchites video].  The problem is that they're not easy to gather in large numbers, but if you look carefully you will see their eggs on the water surface in the egg traps.  The eggs are brilliant white, shaped like footballs, and barely touch the water.  If you blow gently on the water, the eggs scatter.  Collect these eggs by pouring egg trap contents into a jar, and keep an eye on the water in the jar for the next 2 or 3 days.  The Toxorhynchites eggs should hatch into nasty little predator larvae.  Another problem in working with Toxorhynchites larvae is that they eat each other, so you should separate newly-hatched larvae into smaller containers, such as vials or ice cube trays.  Then they need to be fed, which is another problem.  Freshly-hatched Aedes or Ochlerotatus larvae are their favorite food, so hatch some of those from the egg traps.  As they get older, they will, of course, need larger prey, so you'll need to keep rearing a food source for them.  It is worth the effort, though-- if you get them through the pupa stage, you'll see some of the most beautiful insects you've ever laid eyes on.


C.  Egg hatch experiments

The primary species you'll be collecting-- Eastern Treehole Mosquito, Asian Tiger Mosquito, and Yellow Fever Mosquito, all lay their eggs above the water line where they wait for the water to rise above them.  They will not hatch without being submerged.  The stimulus for this is a decline in oxygen in the water, which occurs when bacteria become active after a rainfall.  This is easy to demonstrate:

1)  What is the influence of oxygen on egg hatch rate?  Bubble air into some jars using an aquarium pump and aerating stone, and not into others.  Place egg batches into each, and count the number hatched after a fixed period of time.  You should find that the container-breeding mosquitoes of the sort we're likely to catch in our traps will be inhibited from hatching in high oxygen conditions.  Anything that promotes lower oxygen tends to stimulate egg hatch.

2)  What is the influence of larvae on egg hatch rate? We have done this experiment many times, and it consistently works.  You should find that larvae inhibit egg hatch if you add enough of them.  We think this is because larvae remove bacteria from the surfaces of unhatched eggs, so the eggs that have been "grazed" don't receive as strong a hatch stimulus.

Eggs are also the stage at which the Eastern Treehole Mosquito and the Asian Tiger Mosquito pass the winter.  The eggs go into a resting state (diapause) as the winter approaches, and they do this by detecting a shortening length of day (photoperiodism).  The Yellow Fever Mosquito originated in tropical Africa and can't do this, which might explain why they don't occur in North America north of the Gulf Coast states.  Only two participating groups in Florida and South Carolina, are likely to find any of these.  Potential projects on diapause include:

3) determining the timing of diapause in your area.  Collect eggs through the fall, and submerge them in a yeast or broth solution right after collection.  You should see a sharp decline at a certain time in the fall, as the daylength shortens past the so-called critical photoperiod.  It would be great if multiple groups could combine their data for different latitudes-- if that could be done in a consistent way, we should see that the northern populations go into diapause earlier than the southern ones to prepare for an earlier freeze.

4) determining the time of release from diapause.  This is the reverse of (3) above.  Eggs also use daylength to determine when it is safe to hatch again in the spring.  Leave eggs exposed to winter day lengths, and submerge a portion of them each week as the spring advances. Plot % hatched against date, and you should see a sharp rise.  In our area, it's about April 15, but most of you should see this earlier.

5) determining winter egg mortality.  This is important to know, and very little has been done.  It should be easy enough to place egg trap liners outdoors, perhaps in coffee cans, to experience winter conditions, and to determine how many of them can hatch after the days have become long enough.  It's particularly important to our understanding and prediction of the spread of the invading Asian Tiger Mosquito, because low winter temperatures are expected to place a limit on how far north they can come.  

D.  Behavior and ecology of mosquito larvae

1)  Behavioral responses.  Mosquito larvae are easy to take care of, and there is a lot that can be done with them.  They make an interesting demonstration on an overhead projector, and with dissecting microscopes students will find them interesting to watch.  You can demonstrate a dramatic avoidance of light, shadows, and disturbance of the water surface.  

2)  Competition.  Mosquito larvae are known to be limited by the amount of food.  In nature, they eat bacteria and protozoans, as well as bits of debris, using comb-like filters attached to their mouths.  A good experiment to demonstrate food limitation is to vary food levels (using something easy to measure, like ground dog food or brewer's yeast) or to vary the initial number of larvae per volume.  As competition becomes more intense, larvae take longer to develop, survive at lower rates, and become smaller adults.  Note that competition experiments can take a long time, because larvae can remain alive for several months with very little food.

3)  Predation.  How effective are predators at finding and eating mosquito larvae?  This can be studied by placing predators in jars, and exposing them for a standard period (say, 24 hours) to different numbers of mosquito larvae.  A plot of the number eaten against the number of prey shows the "functional response" of the predator.  It should rise and then level off as prey become so abundant that predators are completely stuffed.  How fast this curve rises tells us something about the "attack rate," which is a good summary of how effective the predator is at finding the mosquito larvae.  How high the curve gets tells us something about the "handling time," which summarizes how long it takes the predator to process the prey after capture.  Low handling time -> high plateau, and vice versa.  Possible predators that aren't hard to find include small fish, dragonfly nymphs, various waterbugs that swim on the surface of ponds and slow-moving streams, and of course Toxorhynchites.  Functional response curves can be compared for different kinds of predators, and also for different sizes of mosquito larvae as prey.  They can also be used to compare predator effectiveness under different experimental conditions.  Formal analysis of data can get quite statistical, but the experiments are easy to do if you can get enough predators and prey, and students can compare the curves by eye.