Answer: There are a number of methods for releasing ladybugs, all intended to encourage them to stay in the garden longer. Moisture and humidity seem to be important, as does the number of aphids present when they are released. If the ladybugs consume all of the aphids immediately, they will leave to find more food!
Ladybug larvae are even more effective aphid eaters than the adults. It is possible that your ladybugs reproduced before disappearing and have left you with an ongoing supply. (If you plan to try ladybugs again, you might wish to consider some such as those available from Gardens Alive which are ready to reproduce upon arrival. They state their stock "will produce over 10,000 aphid-eating larvae within 30 days." Gardens Alive can be reached by calling 812-537-8650.)
In addition to using the strong blast of water from the garden hose to knock the aphids off, or using insecticidal soap, another sprayable control is neem, which supresses the aphids' desire to eat and interrupts their life cycle. The active ingredient is an extract from Neem tree seed. If you use this, be sure to follow the label instructions carefully with regard to timing and repetition of the spray. As with any spray you might wish to test it on a few leaves first before you treat all your plants and be sure to use the recommended dilution rate.
Here are a few non-chemical "home remedies" you might consider: handpick the aphids and squish them or drop them into soapy water; try the following mixture as a repellent spray: 2 tablespoons hot, red pepper, 2 cloves garlic, 1 tablespoon liquid soap mixed into one quart of water; use water or sticky traps. Water trap: small bright yellow pan filled with soapy water (I think a margarine tub would probably work); sticky trap: place 6 x 8 inch bright yellow cards or plywood or plastic vertically in the garden. Coat with a sticky substance such as Tanglefoot.
Finally, healthy vigorous plants will withstand insect attacks best and sometimes a little damage is acceptable. If your aphid problem is not too severe, the next good rainstorm should do a lot to help knock them down. In the meantime, good luck with your plants!
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