Thousands of insects can be found in your garden, and many of them are beneficial to your plants. Aphids, on the other hand, are tiny soft-bodied bugs that stress plants by sucking sap from leaves, stems and buds. When they show up on herbs, and other edible plants, it is a good idea to carefully consider how to deal with them.
Insecticidal soap and horticultural oil are considered mechanical insecticides. When sprayed on plant surfaces, they kill aphids and other soft-bodied insects by smothering them. A benefit of this approach is that you are not applying a poison to something that could end up on your dinner plate.
Soaps and oils are ecologically sound solutions. They target aphids but do not harm beneficial insects like honeybees, lacewings and ladybugs. Butterfly lovers appreciate mechanical controls like these, because they don't endanger butterfly larvae which feed on dill, parsley, fennel and rue.
Help From The Good Guys
The use of beneficial insects is considered a biological control. Ladybugs are particularly helpful when you are faced with an aphid infestation. According to Ric Bessin, Extension Entomologist for the University of Kentucky College of Agriculture, one ladybug can eat up to 5,000 aphids in its lifetime. Adult ladybugs lay eggs on plants prone to aphid infestation, and the larvae that hatch have even more voracious appetites than their parents. Gardeners need only have the patience to let them do their job.
Anyone who has ever stared at a shelf full of insecticides may have been overwhelmed. It is important to understand that there are systemic and contact varieties. Each has its own place in the garden. Plants that you will eat, like herbs, should only be treated with contact insecticides like pyrethrin.
Pyrethrin is produced from a kind of chrysanthemum and, along with its synthetic version permethrin, will kill aphids on the spot. The difference between pyrethrin and permethrin is how long they remain active. Pyrethrin can be sprayed in the evening when good bugs like bees are not active, and it will dissipate by the next morning. According to the National Pesticide Information Center's technical fact sheet, permethrin will take days to break down. This puts a far greater number of beneficial insects at risk.
Keep A Watchful Eye
The best way to prevent severe aphid infestations in your herb garden is to spend time with your plants. Get down on their level and give them thorough inspections a few times a week. This can be a pleasurable task with herbs, because as you touch their foliage their fragrances fill the air. Turn over leaves, examine stems, and take note of anything unusual. Aphids are often found clustered together, but it might take a magnifying glass to spot them individually.
The Southern Living Garden Problem Solver states that there are over 1,300 types of aphids in North America. It can be tough to separate all of those from insects that might not require your attention at all. When in doubt, consult experienced professionals. Grab your clippers and take a small section of any plant containing a mystery bug. Place your sample in a sealed plastic bag or container and bring it to a county extension agent or reputable garden center for identification. They see insects all the time and can help you feel confident in whichever solution you choose.