Fewer Bites, Healthy Pond: Judging Three Mosquito Plans

Use microscope counts from a tub test to evaluate solutions that protect a pond's biodiversity and services.

The tub test

Read the passage and Table 1, then answer the questions.

Mosquitoes at Willow Run Pond

1Willow Run Nature Center has a 1-hectare pond beside its playground and picnic tables. Last summer, many visitors complained about mosquito bites, and the town asked the center to choose a way to cut the mosquitoes without harming the pond. Center educator Mr. Faisal Rahimi asked seventh grader Paloma Bermúdez and the Pond Study Club to help collect evidence.

2The pond does a lot for the town. Frogs, dragonflies and swallows eat flying insects, and birdwatchers come to see herons. Water from the pond flows on into Willow Run creek. Tiny animals called zooplankton, such as water fleas and copepods, eat the algae that would otherwise cloud the water, and they are food for tadpoles, small fish and dragonfly nymphs. Mosquito larvae, often called wigglers, live in calm water near the pond's edges.

3The town is considering three plans. Plan 1: spray a general insecticide along the pond edges every two weeks; it kills many kinds of small animals with jointed legs. Plan 2: add Bti every week; Bti is made from a natural soil bacterium, and its poison works inside the guts of mosquito larvae and a few close relatives, such as black fly larvae. Plan 3: stock the pond with mosquitofish, small fish that eat mosquito larvae and are not native to this part of the country.

4To test the plans, the center set up 12 outdoor tubs of 500 L each, filled with pond water and mud. Each tub got 20 tadpoles. Three tubs got no treatment, and three tubs got each plan; adults added the insecticide and the Bti following the label directions, and the fish tubs got 5 mosquitofish each. Every week, club members strained water from each tub through a fine net and counted the animals under microscopes, using three slides per tub and averaging them. They also measured water clarity: how deep they could see a black-and-white disk in a clear tube.

5The town set three criteria. A: at least 80% fewer mosquito larvae than in the untreated tubs. B: the kinds of zooplankton and the tadpoles alive must each be at least 90% of the numbers in the untreated tubs. C: water clarity of 50 cm or more.

Table 1. Tub Test After 4 Weeks (averages of 3 tubs)
Plan	Mosquito larvae per L	Water fleas per L	Kinds of zooplankton	Tadpoles alive (of 20)	Water clarity (cm)
No treatment	30	40	9	18	60
1: Insecticide	2	3	3	15	25
2: Bti	3	38	9	18	60
3: Mosquitofish	4	8	5	6	35

The Bti tubs had what percent fewer mosquito larvae than the untreated tubs? %

The untreated tubs had how many times as many water fleas per liter as the mosquitofish tubs? times as many

The insecticide tubs had the cloudiest water. Which explanation fits paragraph 2 and Table 1 best?

A. It killed most water fleas, so fewer algae were eaten and algae grew.
B. The insecticide fed the algae, which used it to grow faster than in other tubs.
C. The insecticide killed the tadpoles, and their bodies made the water cloudy.
D. The insecticide is a cloudy liquid, so it colored the water in those tubs.

Circle the two results that show Plan 3 (mosquitofish) harmed the pond's biodiversity.

The fish tubs had 4 mosquito larvae per liter.The fish tubs had 5 kinds of zooplankton, but untreated tubs had 9.The fish tubs had water clarity of 35 cm.Only 6 of 20 tadpoles were alive in the fish tubs, but 18 in untreated tubs.

Why did the club count three slides from each tub and use three tubs for each plan?

A. To make the test finish three times faster
B. To use up all the pond water they had collected
C. To have enough water to feed the tadpoles
D. So that one unusual count matters less

Weighing the plans

Use the criteria in paragraph 5 and Table 1 to answer the questions.

Sort each plan by whether it meets all three criteria in the tub test.

Plan 1: insecticidePlan 2: BtiPlan 3: mosquitofish

Meets criteria A, B and C	Fails at least one criterion

A parent says, “Plan 1 is the best plan because it cut the mosquito larvae the most.” Which response uses the evidence best?

A. Plan 1 cut larvae the least of the three plans, so the parent has the numbers wrong.
B. Plan 1 left 15 tadpoles alive, which was more than the untreated tubs had left.
C. Plan 1 cut larvae most, but it cut water fleas from 40 to 3 and clarity to 25 cm.
D. Plan 1 failed criterion A, because it left more larvae than Plan 3 did.

Use paragraph 3. Which is a trade-off of choosing Plan 2 instead of Plan 3?

A. Bti leaves more tadpoles dead than the mosquitofish do.
B. Bti must be added every week, which takes steady work.
C. Bti makes the water cloudier than the mosquitofish do.
D. Bti cuts the mosquito larvae less than the fish do.

Suppose the town chose Plan 3, and a flood later carried some mosquitofish from the pond into Willow Run creek. Which result does the tub test support best?

A. The creek's zooplankton and tadpoles could drop, as they did in the fish tubs.
B. The creek's water would get clearer, as it did in the fish tubs.
C. The fish would eat the algae in the creek, so more tadpoles would hatch.
D. The creek's water fleas would rise, since the fish would eat their predators.

10.

The tubs ran for 4 weeks and held no dragonflies, frogs or birds. Which next step would best test the leading plan before the town uses it on the whole pond?

A. Count the mosquito larvae in the Bti tubs for one more week, and skip the other counts
B. Use Bti in part of the real pond all summer, counting larvae, zooplankton and frogs
C. Put twice as many mosquitofish in the fish tubs and run the test for 4 more weeks
D. Repeat the tub test with a stronger insecticide to push the larvae count even lower

11.

Write a recommendation to the town. Make a claim about which plan it should use at Willow Run Pond, show with numbers from Table 1 whether each plan meets criteria A, B and C, and explain how your choice protects both the pond's biodiversity and the services it gives people. Name one trade-off.