Aphids are small, soft bodied insects that pierce plant tissues to suck on the sweet juices of the phloem. The phloem is a system of cells that transports sugars produced by plant leaves down the stem to other leaves, fruits and roots. If aphids take too much, the plant starts to suffer. This is why aphids are sometimes "pests" of crops and flowers.
Aphids can turn into serious pests even though they look like helpless bags of plant juice. Why? The following Characteristics of Pestiness, that's why!
- reproductive capacity is tremendous (females give rise to live female offspring)
- good dispersal mechanisms (when they get crowded, they produce winged aphids)
- hide in the folds and curled tissue of the plants (their split makes plant leaves curl)
- piercing mouthparts allow them to avoid "stomach poison" type pesticides
- partnership with ants (see below)
Aphids can also transmit viruses from plant to plant, spreading disease. If that wasn't enough, some aphids have their own guardian angels in the form of ants. Ants feed on "honeydew" coming out of the back end of aphids. (Isn't that a nice way to refer to aphid poop?) The ants then protect these aphids from predators and parasites.
The cereal aphid we will work with today has a complex lifecycle involving wingless females that give live birth to other female nymphs. The adult females are said to be viviparous (vivi=live, parous=birth) and they are also parthenogenic, meaning, they do not require mating to produce these offspring (parthenos = virgin). So you see, if every single member of the population is giving birth, that population increases more rapidly than if half the population is male!
Still, aphids ARE actually helpless bags of plant juice - they can hide, but they can't run. They are susceptible to systemic insecticides, all contact poisons and dessicants (if you can hit them with it), and many biocontrol agents including predators of all sorts (wasps, ladybug adults and larvae, and lacewing adults and larvae) and tiny parasitoid wasps and fungal diseases. In this activity, we will compare the effects of some of these on colonies of cereal aphids on oats. (This can be done with other aphids on other plant material.)
- Learn about comparative control methods
- Learn about the dynamics of using biological controls (see previous background section)
- Observe aphid population (distribution, stages of aphid growth, behavior)
- Observe predator behavior
- plants with aphids on them
- mesh and/or plastic covers for plants
- biocontrols and other types of control
- ladybugs adults or larvae; lacewing adults or larvae
- Safer's soap, diatomaceous earth,
- household pesticide if certified applicator can apply it
- other (vinegar and water, pepper or garlic spray, etc.)
- hand lenses
- data sheet (available below)
Instructors will have prep time in growing the plants, acquiring aphids, inoculating plants with aphids, making "tents" to keep plants covered, keeping plants watered and acquiring control measures. Older students may be put in charge of some of this if a greenhouse or sunny window is available at the school.
Set up in classroom: 20 minutes
Observations over time: 2-3 observations times at 15 minutes each to take data
Discussion time: Variable, depending upon how lesson is used.
Before beginning, discuss the biology of aphids and the types of controls with the students. Outline how each type of control might effect an aphid. Explain the experiment to them and ask them to form a hypothesis about what they think will happen to the aphid populations with each type of control. Which do they think will work or fastest or longest?
Have students form some simple hypothesis to test:
E.g. pesticides will kill aphids faster than biocontrols or soap will kill more aphids than water.
Have them students write their hypothesis on their data sheets.
1. Work in teams of 3 or 4 students. Each team is in charge of one of the treatments.
2. Each team obtain two aphid-infested plants
- one plant will be the control and will not receive a treatment
- the other plant will receive one of the control treatments
3. Assess beginning aphid population on each plant by doing the following:
- randomly identify three fully developed leaves
- mark these three leaves with a dot with a sharpie (or twistie if big leaves)
- count the number of aphids on each leaf and record this number on the data sheet (the aphids may be very small so use hand lenses to be sure)
4. Next, calculate the variability in your samples
A simple way to do this for small sample sizes is to identify the range.
Range = highest # of aphids counted on a leaf minus the lowest # of aphids counted on a leaf in your sample. Calculate the range for both plants (treatment and control).
5. Apply the experimental treatment to one of your two plants. The other plant is the control.
Group 1: Biological control - Ladybeetle adults. Remove tent over plant and place 12 beetles inside the container, quickly replacing the tent over the plant to prevent escapees. Do nothing to the control plant.
Group 2: Biological control - Lacewing larvae. Remove tent over plant and place 1 tablespoon (larvae with the rice hull carrier) on the soil beneath the plant. Do nothing to the control plant. Replace tents over plants.
Group 3: "Biorational" pesticide - Safers soap. Remove tent over plant and spray on the plant according to label directions. Spray plain water onto the control plant. Replace tents over plants.
Group 4: "Conventional" pesticide -- Do this only if your teacher is certified to use pesticides in the school. Students observe as instructor applies common household pesticide in the same manner as Group 3.
6. Observations: observe the aphids before and after treatment is applied. Observe any interactions between the predators and pests. Record your observations on the data sheet. Repeat at each observation time (up to two weeks or longer).
Have each group present their results to the class, beginning by stating their original hypothesis. Put the results on the board for all to see. Are the results different from what was expected in the hypothesis? With everyone's data reported discuss opinions on each control option used.
What do your results tell you? Did your treatment "work" - or in other words, would you try this at home?? Why or why not? With background information on types of biological control, which type were the students practicing?