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Common Rose Pests - Part 1: Aphids and Thrips

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Common Rose Pests - Part 1: Aphids and Thrips

By Mark Whitelaw

Spring is near for those of us in the Northern Hemisphere. And along with spring comes a plethora of critters which try to take the fun out of gardening. I'm speaking of pest insects and mites, of course!

For the next few weeks, I'll be offering a short primer on some of the more common pests which frequent our rose garden.

Aphids

Description: The Rose Aphid (Macrosiphum rosae) is only one of the 4000-plus species of aphids which vary in size, color, and even mating habits. Aphids are usually restricted in their preference for food - attacking a specific host or a closely related species. Rose Aphids, for example, attack not only the rose, but may attack many of its cousins like the photinia, pyracantha, and many of our fruit trees.

Aphids are divided into two major groups - the chief difference being those that only lay eggs (like the Pine Aphid) and those that bear live young. The Rose Aphid falls into this second group.

Biology: In temperate climates, the female lays a fall batch of eggs in nooks and crannies throughout the landscape, near host plants. In the spring, the eggs hatch and become "stem mothers" - that is, they are all female. They also already have their young within their bodies. These stem mothers produce more females, which produce even more females - all without mating. Within a few days, thousands of aphids can be born.

In late summer and early fall, the females also produce a few males. Females born at the same time mate, and eggs are laid for next season's offspring.

Damage: As a group, aphids attack a plant by sucking its fluids from tender new growth. They are attracted to the concentrated nitrogen in these new growth areas. The results are deformed leaves and new bloom stems.

In addition to deforming new growth, aphids cause another problem. Their exudate is a substance called "honeydew" - a sweet, syrup-like material which appears on leaves and stems. It is a food source for many insects, both pest and beneficial. But honeydew causes two more problems: "Sooty mold" and ants.

Sooty mold is a fungus which grows in the honeydew. It makes the leaves look dirty and black. Many unknowing gardeners spray a fungicide on the sooty mold, attacking the symptom rather than the cause - the aphid.

Honeydew is also a food source for ants. Ants will fiercely defend their honeydew "factories" against all natural predators, like lacewing and lady beetle larvae. They have even been known to move aphids from one plant to another so additional honeydew sources can be created, much like a dairyman moving his cows from one pasture to another.

Controls: As numerous as the aphids are, so are the methods of control. These include:

1) Reducing the amount of nitrogen applied to the rose;

2) Removing them by hand;

3) High-pressure watering wands especially designed for insect control;

4) Natural predators and parasites to achieve a micro-environmental balance; and

5) A variety of organic and synthetic pesticides.

Note: Research has shown that aphids can quickly mutate to pesticide-resistant strains. If using organophosphates or carbamates, rotate between two or three different chemical types.

Thrips

Description: What most gardeners call thrips are the larvae of a flying insect entomologists group in the suborder Terebrantia. I mention this because some thrips in another suborder, Tubulifera, are actually beneficial and prey on aphids, mites and other thrips. Terebrantia, the pest thrips, lay their eggs inside the plant's epidermis, the results of which can drive you mad trying to get rid of them.

Unlike aphids, thrips are not particularly host plant-specific although some species, such as the Banded Greenhouse Thrips (Hercinothrips femoralis ), prefer a limited range of food sources.

In North America, our roses are most likely attacked by one of three or four species, depending on where we live and how we cultivate our roses. These are

1) The Flower Thrips (Frankliniella tritici) and Florida Flower Thrips (in the southeastern U.S.);

2) The Western Flower Thrips (Frankliniella occidentalis), until 1980 thought to be confined to the U.S. west of the Mississippi River, but now prevalent throughout North America and possibly Asia and Europe; and/or

3) The Greenhouse Thrips (Heliothrips haemorrhoidalis).

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Biology: The thrips that damage your roses are in there because mama thrips made a small slit in the bud or new growth tissue, then laid her eggs. After a few days, the eggs hatch and go about sucking plant fluids until they get fat enough to pupate, commonly in the soil. One species, the Western Flower Thrips, sometimes pupates in plant litter or protected areas on the plant as well.

Eventually they emerge as a flying adult, mate, and the process renews. This whole life cycle, from egg to adult, takes only about two weeks during warm weather, and about a month in cooler weather.

Damage: Damage to the rose bud is most noticeable in light colored roses, although thrips may attack all roses. If the buds open at all, the petal edges may look brown or discolored. Sometimes the buds will only partially open. (Although this is not a single indicator, as high humidity will also keep roses from fully opening - called "balling".) Sometimes, the buds will simply wither and die.

My technique for checking for thrips is to take a blossom and, with your fingers, pull back the petals. If you see small slivers of creme, yellow, brown or black scurrying about headed for cover, these are thrips… well, actually the larvae. A hand lens will assist you in identifying these critters as they are only about 1 - 3 mm long.

Controls: As with the aphid controls, we rosarians have an arsenal of weapons with which to combat these pesky critters. These range from those high-pressure watering wands designed for insect control to natural predators like lacewings and predatory mites to botanical and chemical pesticides.

Next time, we'll look at more plant sucking rose pests including spider mites, scales, mealybugs and whiteflies. In future articles, we'll talk about plant chewing critters like beetles, caterpillars, borers, and the Leaf Cutter Bee.

Note: Entomology information contained within this series is distilled from numerous on-line and text sources including those of Cornell, Florida State, Kentucky, North Carolina State, Oregon State, and Texas A&M universities; the Bio-Integral Resource Center, Berkeley, CA; U.S. Dept. of Agriculture; and Agriculture Canada. Additional reference materials from Insects of North America (McGavin; Longmeadow Press: Stamford, CT; 1993); Common Sense Pest Control (Olkowski, Daar, and Olkowski; Taunton Press: Newtown, CT; 1991); The IPM Practitioner (William Quarles, ed.; BIRC: Berkeley, CA); and Simon & Schulster's Guide to Insects (Arnett and Jacques; Simon & Schulster, Inc.: New York; 1981)

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