Whiteflies on Tomatoes
Description and Life History:
Well, I originally planned on writing about the birch leaf miner and I was so organized that I already wrote my Pest of the Month for September. I planned ahead before I went on a wonderful camping trip in NE Oregon. When I returned the other night and started to check out the garden, I found swarms of whiteflies on my neglected tomatoes. Ugh, everything was looking great when I left and now you can barely see the tomatoes through the white cloud of flies. Now my girlfriend is wondering how I can be a pest management specialist if I can’t even keep up on my own pests. I’m thinking the same too; so to make up for this, whiteflies are the pest of the month!
Whiteflies are very interesting critters. They behave much like aphids in the sense that they can reproduce very quickly and abundantly; a female whitefly will lay 80-100 eggs in her short lifetime. Eggs are laid on the underside of leaves. The “crawler stage,” that hatches after a few days of development, is called so because this is the only mobile stage of the whitefly’s immature life. After the crawler molts into the next immature stage, it loses its legs and antennae, becomes flattened and glues itself to the leaf surface. The immature stages look like a light green flattened blob with waxy hairs. For the next few molts, until the whitefly emerges from the pupa (not a true pupa but a last resting stage before the adult stage) to become an adult, the immature whitefly is sessile (stationary). The development of whiteflies can take anywhere from 18-38 days depending on the weather and temperature. Whiteflies have multiple generations per year and become most populous during mid to late summer.
Like aphids, whiteflies have piercing-sucking mouthparts so the damage caused is very similar to that of aphids. Direct damage to tomato plants causes deformed new growth and wilting, chlorotic leaves. Whiteflies can also transmit some plant viruses. If your plant gets infected with a virus, do your neighbor a favor and pull it out. Whiteflies, like aphids, secrete honeydew, causing the opportunity for sooty mold to grow. Feeding by whiteflies can also cause deformed fruit and discoloration of your tomatoes.
Check the undersides of leaves regularly with a hand lens to monitor for both adults and nymphs. Look for flying whiteflies regularly by knocking tomato leaves. This is an easy way to monitor adult activity and abundance because they are easily disturbed and will flutter away. Your local garden shop will also sell yellow sticky cards for measuring the number of adult whiteflies that are flying (this is what most commercial growers do). In a small garden setting, sticky cards may also manage your whitefly populations by killing the adults. Monitoring your populations, especially during hot dry weather will help you decide if or how to manage whitefly problems. Generally, tomatoes (and other plants) can tolerate some whiteflies (up to 10-25 nymphs per leaf) but population explosions (50 and up nymphs per leaf) like the one I’m experiencing can quickly rob you of your tomatoes.
An important tactic for avoiding whitefly problems is to buy clean plants. Inspect your tomato starters before you purchase them. Look for the small sessile immatures on the undersides of the leaves. I believe this is where I really screwed up. My plants that I bought probably had whiteflies on them already.
Gardeners in western Washington generally do not need to apply insecticides for whitefly control and should not use insecticides without other control tactics. With regular monitoring at this time of year (I know that was my fault for not looking before we left for vacation), you can keep them down by the most reliable insect control method: squishing them. Insecticidal soaps and oils are effective for killing whiteflies but be sure to read the label first. Also, test the product on a leaf first before applying to the entire plant; these products can burn foliage. Spraying water from the gardening hose underneath the leaf will also cause adult mortality and reduce the number of eggs laid. I am using the water hose method, hoping to avoid another large egg-laying session by drowning the adults.
Most exciting about this whitefly problem, and my real reason for sharing this experience with you, is that the majority of the whitefly immatures on my tomato plant are parasitized! Some leaves approach 90-95% parasitization. The parasitoids that are attacking the whiteflies with such voraciousness are wasps (Hymenoptera) in the family Aphelinidae. The two types that attack whiteflies in our area are from the genera Ecarsia and Eretmocerus spp. Both tiny wasps, less than one millimeter in length, are prolific and effective. You can monitor your whiteflies for parasitoids by looking for mummified whiteflies. These mummies will appear dark blue or black. Mummies are formed when the parasitoid pupates inside the whitefly.
To review, a parasitoid is an insect that uses another insect to feed their babies. They develop much like a parasite, but they kill the host (unlike a parasite that keeps its host alive). The adult wasp lays an egg inside the whitefly nymph host. The wasp larva hatches and feeds on the whitefly internal organs and tissue, eventually killing it. The larva develops, pupates and emerges as an adult to do the whole thing over again. Another added benefit of these particular whitefly parasitoids is that they host feed. This means, if they size up a whitefly that isn’t suitable for developing their babies, they stab it with their stinger and drink the whitefly’s guts. These wasps act as predators and parasitoids, killing many whiteflies for each wasp that develops.
For my specific problem, since I missed the problem to begin with, I have the unique opportunity to see the results of my favorite IPM strategy: the Do Nothing strategy. So now implementing a good, diligent adult whitefly management program and many hungry parasitoids working for me, I’m not too worried about the tomato plant. I’m reducing the potential of more whiteflies by killing the adults while relying on the parasitoids to balance the remaining whiteflies and ultimately impressing my girlfriend with my savvy pest management skills. All I had to do is nothing!
To reach Todd Murray please call (360) 676-6736 or e-mail him at: email@example.com