No Need to Panic Over a Few Summer Pests

Written by Ron Vanderhoff

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Summer is upon us. Not only are temperatures rising, but so is the likelihood of a few garden pests. The three that seem to be getting the most attention right now, turning up in gardens from Laguna Beach to Huntington Beach, are Rose Slugs, Giant Whiteflies and Citrus Leafminer.

If you have one or all three of these pests, don’t panic. Learn a bit about them and then decide on the best strategy – which might be to do nothing at all. Where possible I always suggest organic or natural solutions, which are not only more environmentally appropriate, but build healthier gardens over time.

Rose Slugs get busy every year once the weather warms up, usually about May or so. By July the damage is very noticeable on rose leaves. The leaves become riddled with hundreds of holes and irregular scratches. Heavy infestations can skeletonize the entire leaf, leaving just the veins.

The culprit is the larva of an insect called a Sawfly, more commonly referred to as a Rose Slug, even though it’s not a slug at all. If you turn over a few chewed leaves and look very carefully at the backside you will see the little green larvae. If you have only a few roses, or a lot of patience, you can choose to pick off the larva by hand. You can also knock off the little guys with a strong spray of water from the hose. Horticultural oils and natural plant oils can be used when Rose Slugs are first attacking. Oils are an environmentally safe approach, but should be used with caution in very warm weather. Spinosad, a naturally occurring biological insecticide, seems to be the treatment of choice for most rose enthusiasts. Always spray during the evening hours, in order to minimize the negative effects Spinosad will have on honeybees and native pollinators.

Giant Whitefly still gets lots of attention. Crossing the Mexico border in 1992, this is a noticeable summertime pest on several subtropical plants, especially hibiscus, giant bird of paradise, citrus and canna lilies.

An exciting development in whitefly control has come to light over the past several years. Plants that are well mulched with worm castings repel a variety of pests, especially whiteflies. Apparently the worm castings raise the level of the chitinase  enzyme in the plants, which whiteflies find distasteful and avoid.

I recommend spreading a one-inch layer of worm castings around infected plants and periodically applying more, especially through the summer months. It takes a little time for worm castings to move up into the foliage and for the enzyme to form. Small plants are usually free of whiteflies in three or four weeks, but larger plants may take a while longer. In the meantime, a forceful stream of water directed at the whitefly colonies is as effective as any insecticide. Syringing the plant weekly will also reduce the whitefly infestation dramatically and will not interfere with the natural biological balance of predators and parasites, which are important allies in controlling these pests.

Citrus Leafminer is the newest of the three pests discussed here, also entering from Mexico as recently as 2000. Citrus Leafminer larvae feed by creating shallow tunnels, referred to as mines, in young citrus leaves. As the larvae mine the leaves, whitish trails appear and cause leaves to curl and distort.

Since only young, new leaves are preyed upon by Citrus Leaf Miner, mature citrus trees are not adversely affected, although the pest will be seen. These established trees will continue to grow, blossom and fruit fine, with little or no overall impact to the plant. However, very young or newly planted trees may need treatment. In this case, the same earthworm casting treatment mentioned for the Giant Whitefly is also quite effective. Spinosad can also be effective, but because it is toxicity to citrus pollinators I suggest it only as a secondary choice.

If any of these pests are being seen in your garden, there’s no need to run for the hard chemicals. A little knowledge about the pest and the right strategies will ensure that the plants in your garden continue to thrive all summer long.

Ron Vanderhoff is the Nursery Manager at Roger’s Gardens, Corona del Mar and his profile can be seen at www.theMulch.com/my-profile/userprofile/Ron Vanderhoff.

Questions from Readers July 17, 2010

My Marguerite Daisies are looking pretty tired. Are they getting too much sun? Need more fertilizer? They were beautiful, but not anymore. What can I do to get them going again?

Julie, Huntington Beach

Answer:

Marguerite Daisies, along with related daisies like Euryops, Arctotis, Dimorphotheca and other so called “African Daisies” are cool-weather plants. (See my column dated June 26.) These plants flower and grow happily during the cool half of the year. When the days get long and the temperatures rise, they withdraw. Putting them in shade won’t help, neither will feeding them more or anything else. Seasonal rhythms and preferences are deeply embedded in a plant. A gardener’s futile struggle overcome these rhythms is a cause of much frustration and failure. Better to just learn you plants seasonal habits and adjust your plantings, design and maintenance accordingly.