The Room 10 class, at Balclutha Primary School, asks :-

How can we remove greenfly from our class-room plants?

Cody Fraser, who looks after the vegetation in the butterfly house at the Otago Museum, responded.

Greenfly, or aphids, are small soft-bodied insects that eat sap. They can be found on the soft green parts of many types of plants. Sap is the fluid inside plants that circulates to move water and nutrients around the entire plant, and aphids use their mouthparts to drill into the plant to reach it. If the population of aphids is large, plants can lose too many nutrients and be weakened which can prevent proper growth. Aphids can also spread diseases between plants.

There are many different species of aphid in New Zealand. They can reproduce without males, some giving birth to live young, which allows populations to multiply rapidly in warm conditions.

There are a few simple methods you could use to remove a population of aphids from plants. If you only have a few aphids, squashing them or cutting off the affected sections of the plant can solve the problem. Sometimes rinsing the plant with water to knock the aphids off will do the trick.

For larger aphid populations soapy water made with a small amount of dishwashing detergent can be used to kill them. Little is known about how soaps and detergents work as insecticides, but it is believed that they damage cell membranes or break down the waxy protective layer on the outside of the insect leading to excessive loss of water from the body. The soapy solution needs to contact the insect to work, which means spraying the plant with the solution. Unfortunately soap could also damage the plant or kill other insects, s ome of which might be beneficial to your plants.

If you want a more plant-friendly solution, you might like to try the 40% white vinegar-60% water mixture that Otago Museum sometimes sprays on affected plants in the Tropical Forest to remove aphid populations without hurting the butterflies or plants. Or you could enlist the help of some biological control agents by introducing another insect to the plants. Ladybird larvae eat aphids, so ladybirds can help reduce their numbers. Other insects that like to eat aphids include praying mantises, hoverflies and lacewings.