1. Hand-picking the caterpillar is generally sufficient for control in mild infestations.
2. Floating row covers in the spring are helpful, but need to be removed by mid May. The row covers create a barrier that keeps insects out but allows for moisture, light and air circulation. In keeping insects out, it also bars pollinators.
3. Diverse gardens that include a symbiotic mingling of flowers and vegetables are a poor environment for insect pests, but also attract many natural predators. Encourage natural predators such as predatory wasps by mingling varying plants and flowers blooming at different intervals throughout the season.
4. Insecticidal soaps will also help keep their populations under control, but must be applied on a regular basis in heavier infestations.
5. Neem Oil and Neem based products are also effective. Neem acts as a growth regulator interrupting the insect's growth cycle resulting in its death. Its active ingredient, Azadirachtin, halts the life cycle progression of pests and has very low toxicity ratings, and no known insect resistance.
6. Bt bacillus thuringiensis has proven effective against almost all species of Moth and worm, it is a bacterium / organic pesticide that is readily available to home gardeners. It's harmless to people and pets. Apply it at either bloom or petal fall, or both. Bt is a stomach poison and must be ingested.
It is more effective when applied during warm, dry weather while the larvae are actively feeding. Bt breaks down quickly in nature so multiple treatments per season are necessary.
Bt is not generally harmful to beneficial insects. It is most effective when applied during warm, dry weather when the worms are feeding.
7. Other insecticides, such as Pyrethin or malathion should only be used as a last resort, generally used commercially and not really advisable for smaller plantings or home gardeners.
Sprays/Pesticides must be applied in the pre-bloom stage to prevent injury. Pesticides such as pyrethroids are more effective in cool than warm weather will work more efficiently against Celery Worms earlier in the season, and is the recommended early season chemical treatment for most worms.
Proper sanitary practices are vital to the health of your garden. Proper sanitation can help to ensure disease-free pest-free and productive gardens. Try to keep the garden free of any diseased dead or damaged plant materials. Remove cuttings from pruning and trimming and either destroy them - if diseased, or send them to the compost pile. Leaving rotting fruits and vegetables in the garden is like a written invitation to unwanted pests and diseases.
If a diseased or dead plant part has to be cut, the microorganism that caused the problem is probably on the tool you just used. Like a surgeon, sterilize all tools by washing in soap and water - rubbing alcohol wouldn't hurt either. If you pinch off diseased plant parts, wash hands before handling any other plants. Keep Weeds under control. Till the soil in the spring before planting to expose and kill larvae that wintered over in the soil.