Q: This is my first year with a garden. Most of it has done well. I planted one watermelon, one cantaloupe and one Magda squash. My problem is they became covered with what I think were aphids (at least that was what the local nursery told me). I used insecticidal soap and neem oil, but have not gotten rid of them. The plants are a mess. What should I have done or used to remedy this problem?
Marbi Wise, Cameron Park
A: According to UC master gardener Rachel Tooker, aphids are a common challenge to the vegetable gardener. Almost every plant has one or more aphid species that occasionally feeds on it.
Aphids are small, soft-bodied insects with long, slender mouth parts that they use to pierce stems, leaves, and other tender plant parts and suck out plant fluids. Although low to moderate numbers of leaf-feeding aphids are not usually damaging in gardens, large populations can cause curling, yellowing and distortion of leaves and stunting of shoots. Aphids also excrete a sticky honeydew that attracts ants and other insects (beneficial, as well as pests).
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Although aphids can be controlled chemically, these methods are less desirable around food crops and also can destroy beneficial insects. Fortunately, there are many other cultural and biological methods that should help you to keep the population in bounds.
Start by checking your plants regularly for signs of aphids – at least twice weekly when plants are growing rapidly. Many aphids cause the greatest damage when temperatures are warm but not hot (65 to 80 degrees F.). By catching infestations when populations are low, it will be easier to implement management measures. Once leaves curl with the aphids inside, it is harder for either beneficial insects or insecticides to reach them.
When monitoring, pay most attention to the upwind edge of your garden and under leaves. Look for evidence of natural enemies, such as lady beetles, lacewings, syrphid fly larvae and the mummified skins of parasitized aphids. If there are high populations of these beneficial insects, and conditions are kept right (which means no spraying with insecticides) to keep the natural enemies healthy, the aphid population may well be reduced within a week or two.
Where aphid populations are localized, the best control may be to cut out and dispose of the leaves or new shoots on which they are clustered. Also, high levels of nitrogen fertilizer favor aphid reproduction by promoting growth of lots of tender leaf shoots. Never use more nitrogen than necessary. Use less soluble forms of nitrogen and apply it in small portions throughout the season rather than all at once.
Because many vegetables are susceptible to serious aphid damage during the seedling stage, losses can be reduced by growing seedlings under protective covers in the garden. Another easy way to reduce aphid populations on sturdy plants is to knock them off with a strong spray of water. Most dislodged aphids will not be able to return to the plant, and their honeydew will be washed off as well. Spray early in the day so that the plants dry off rapidly and don’t become a host to fungal diseases.
As a last resort, insecticidal soap can be applied. However, this material must be applied thoroughly to all sides of the leaves and stems. The spray will only kill aphids present on the days the plants are sprayed, so applications may need to be repeated. Do not apply spray when temperatures reach over 90 degrees, as it can damage foliage.
For more information, consult UC IPM Pest Management Guidelines for Aphids, Publication 7404, which can be downloaded at ipm.ucdavis.edu/PMG/PESTNOTES/pn7404.html.
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