As I travel around the country, one of the common scenarios I use to demonstrate how to let the garden guardians control garden pests is with the rose aphid. I always start by asking what people do to treat rose aphids and I am delighted to say that most people just talk about squirting with them off with the hose, soapy water, homemade chilli or garlic sprays, or squashing them. Occasionally someone will also say they use milk sprays to control aphids. While I am delighted that these remedies have changed drastically over the past 10-20 years and we are no longer reaching for the nasty garden chemicals such as Rogor, Malathion, or Rose Gun. However I don’t even like the nontoxic methods such as soapy water, chilli or garlic sprays or squashing, so I will tell you why.
Firstly, while rose aphids are a common pest problem, just because you grow roses doesn’t mean you will automatically get aphids. These sap-sucking insects are pale green or brown in colour and are particularly fond of developing shoots and flower buds. They have a very short generation time and can build up in numbers very quickly as the weather warms up. A few aphids can be tolerated, however severe infestations can result in new buds being deformed, a loss of flowers and even foliage. They will not however kill your rose!
Firstly it is important to understand that roses which are growing in the right position and nourished properly so that they are happy and healthy, are less likely to get aphids in the first place. Roses that get aphids may be too shaded, overwatered, overfed, soft and sappy, or growing in the wrong spot with issues such as tree root competition. The first step to getting rid of aphids is to make your roses happy and healthy.
Traditionally, many gardeners have reached for insecticidal sprays however the problem with such sprays, even those such as low toxic, pyrethrum-based, garlic, chilli, oil or soap sprays is that they do not distinguish between beneficial insects such as ladybirds, and the rose aphid. Something as seemingly harmless as soapy water desiccating all insects it touches, good or bad.
The best method of control if rose aphids are a problem in your garden is to do nothing, except encourage natural biological control. There are a number of natural predators of the rose aphid that are present in most gardens around Adelaide, provided that excessive amounts of insecticides are not used.
The first is the ladybird. Most gardeners know that ladybirds love to eat aphids, however most gardeners are not aware of the juvenile form or larvae of the ladybird, which looks nothing like the adult ladybird and actually looks rather menacing. This juvenile form is elongated in shape rather than the expected round shape of the adult ladybird and is with a number of tiny orange spots on its back in a square pattern. These baby ladybirds even eat more aphids than the adult ladybirds do. People often kill these juveniles thinking that they are harmful. If you see a ladybird amongst the aphids on your roses, you will probably find a patch of ladybird eggs. These are yellow-orange in colour and usually laid in a tight group amongst the aphids, so that the babies have something to eat when they hatch. One of the reasons that I don’t recommend people squash aphids is that they are most likely squashing ladybird eggs without realising it.
Another natural predator of the rose aphid is the rose aphid parasitic wasp. This wasp lays its eggs inside the rose aphids and then the larvae parasitises the aphids as it develops inside the aphid’s shell. A shiny brown, paper-like skin, known as a ‘mummy’, is all that remains of an aphid after it is parasitised. The larvae develops into its adult form in this round skin before chewing its way out through a hole in the rear of the aphids abdomen. As the wasps themselves are tiny, these mummies are usually the only way of checking for the wasp’s presence. Look for brown, bloated aphids that don’t move and appear paralysed. These mummies are frequently seen in between live green aphids on buds and the undersides of leaves, and can range between one and three millimetres in size. The other reason I don’t recommend squashing aphids is that people invariably squash these rose aphid parasitic wasp mummies with baby wasps inside them.
Other beneficial insects which also demolish your aphids include hoverflies and lacewings. The adults of these insects are nectar feeders so come for flowers especially those with a daisy or umbel form, however they lay their babies in and around the aphids. These babies, which again look nothing like the adult insect, feast on the aphids. Small insectivorous birds and micro-bats will also eat aphids so keep an eye on your domestic cats so they don’t hurt these precious garden guardians.
Now the challenge with biological control is that it is not immediate and patience is required. Generally it will take the predators anywhere from 10 to 14 days to build up in numbers to be effective against a surge of rose aphids. There won’t be lots of beneficial if there is nothing for them to eat! Unfortunately it is in this period of time that some gardeners cannot stand it and jump in to deal with the aphids themselves.
Firstly, let me assure you that just because you have a few aphids on your roses – don’t panic – it doesn’t mean that you are a bad gardener. However, if you can’t bear to do nothing and let the natural biological control take effect, the best thing to do is simply squirt the rose aphids off with your hose on high pressure. You will need to do this every few days as a new crop of aphids will quickly be back to replace the old. Does this work? Not really … but it keeps you occupied while we let Nature come in and clean up the problem!