Parsley (Petroselinum crispum) is a common culinary herb frequently used in companion planting, leading to the belief that its strong aroma repels insects. This investigation explores the scientific evidence behind this claim, distinguishing between its repellent compounds and its role in garden ecology.
Understanding Parsley’s Repellent Capabilities
Parsley contains compounds with insecticidal properties, but these effects are highly specific and require concentration. The plant’s essential oil is rich in phenylpropanoids, primarily apiol and myristicin. Laboratory studies show that concentrated parsley oil, particularly fractions high in apiol, exhibits larvicidal activity against the Aedes aegypti mosquito and toxicity against cattle ticks (Rhipicephalus microplus). However, the whole, uncrushed plant is generally not a strong repellent against common household pests like flies or mosquitoes. Its primary contribution to garden pest management is attracting beneficial insects. When allowed to flower, parsley’s blooms attract hoverflies and parasitic wasps, whose larvae prey on pests like aphids, thrips, and caterpillars.
Applying Parsley for Pest Management
Gardeners commonly employ parsley in two ways to manage pests: companion planting and topical extracts. Companion planting involves placing parsley near vulnerable crops for a protective effect. For example, planting it near tomatoes may deter the destructive tomato hornworm, and its strong scent can confuse pests like the carrot fly. It is also planted alongside asparagus to help deter the asparagus beetle. The second method involves creating extracts or using the juice of the leaves as a topical application against biting insects. However, the repellent effect from a simple homemade spray is short-lived and weak compared to concentrated essential oils, as the active compounds are not present in high enough concentrations.
Stronger Natural Alternatives
For robust, scientifically recognized natural protection against biting insects, several other herbs offer stronger alternatives. Oil of Lemon Eucalyptus (OLE), derived from the lemon-scented gum, is highly effective. Its active compound, p-menthane-3,8-diol (PMD), is recommended by health authorities and provides protection comparable to lower concentrations of DEET for up to six hours. Another powerful option is catnip, whose essential oil contains nepetalactone, a potent mosquito repellent. Citronella oil is popular but evaporates rapidly, limiting its duration of effectiveness. Neem oil, containing azadirachtin, also offers protection against mosquitoes and various garden pests. These alternatives contain higher concentrations of broadly effective repellent compounds than unrefined parsley preparations.