Pieter has had a severe aphid infestation on his zucchini plants, and applying a dishsoap/water spray not only killed the aphids, it also killed the plants. He wants some tips on a better way to deal with aphids.
Aphids are fairly easy pests to get rid of without resorting to chemical pesticides. Aphids have a lot of natural predators, but the sprays will also kill these, resulting in more intense pest infestations in future.
Moderate infestations of aphids can be blasted off plants with a jet of water from a hose or a spray bottle set to ‘stream’. For plants with hairy leaves (such as zucchinis) apply water early in the day so that leaves can dry before night. If you blast them with water before serious damage occurs, predators will be able to control any escapees.
Severe infestations can be removed with a soap spray, but not with detergent or bathroom type soap, which has a caustic soda base. The soap used in soap sprays to kill pests has a potassium base, and is available from nurseries, or can be purchased by mail order from Green Harvest. ‘Natrasoap’ does not leave a residue on plants, nor will it burn leaves if used in temperatures below 30° C.
In order to prevent future infestations of aphids, avoid overdoing nitrogen fertilisers because aphids are pests that are attracted to a flush of soft, sappy growth that excess nitrogen, or erratic watering, produce. Controlling weeds in the growing area can also help prevent aphid attack. Many weeds are host plants for aphids, allowing a large colony of pests to establish before you notice them. As aphids can carry viruses for which there is no cure, prompt removal of these tiny pests is advised.