Last week Mark Viette called me up to his house for a weed/insect photo shoot. He had been walking through his gardens and noticed a lot of insect pest activity on some of the weeds that were scattered throughout the beds around his house. This provided a strong argument for keeping the garden free of weeds because weeds often harbor lots of insect pests. Better get busy Mark!
The worst infestation and the one that could potentially cause major problems in both the flower garden and vegetable garden was the whitefly on the velvetleaf that had grown up in the garden. When these plants were bumped, clouds of tiny white flies billowed up from the foliage. Yikes!
Once you have whitefly in the garden or worse yet, on your indoor plants or in the greenhouse, they are extremely hard to control. The life cycle of whiteflies is usually completed in 30-40 days and unfortunately, all stages of this life cycle can coexist at any one point in time which makes control of this pest difficult at best. The problem is that most of these life stages are resistant to insecticides.
Whitefly eggs (loads of them) are laid on the underside of leaves. These eggs hatch out after about 5-7 days and go through four nymphal stages (instars). The first instar, the crawler stage, is mobile and crawls around on the leaf eventually settling down to feed on the leaf juices. This stage lasts only a few hours but is more vulnerable to insecticides. The later nymphal stages are sedentary and flattened out rather like scale insects. At these later stages, the nymphs are harder to control.
The adult whitefly emerges from the fourth nymphal stage and the cycle begins again and again … with many overlapping generations throughout the season.
All stages of the whitefly feed by sucking the juices from the leaves of the plant. This causes the leaves to become wilted or even deformed. They eventually turn yellow and may even fall off. As they feed, the nymphs excrete honeydew which attracts ants and also encourages the growth of black sooty mold which makes the plant unattractive in the garden. Severe infestations of whitefly can cause plant health to decline. The adult whiteflies are known to transmit viruses to other plants.
In colder zones, whiteflies are troublesome in the garden only during the summer and early fall because they cannot survive the winter. They can however persist year around in the greenhouse and indoors on houseplants where they can cause major damage to plants if the populations buildup.
Whitefly control is tricky since only certain stages are killed when the plants are treated; other stages are able to survive. Because of this, a strict spray schedule must be followed in order to obtain good control. Horticultural oils such as Bonide All Seasons Oil and PureSpray Green, as well as neem oil and insecticidal soap sprays can help control whitefly when used according to the label directions. When you spray, it is imperative to coat the lower surfaces of the leaves because this is where all the whitefly stages are found.
Some systemic insecticides such as Bayer Advanced All-In-One Rose & Flower Care and Bonide Systemic Insect Spray have been shown to be effective in controlling whitefly when used according to the label directions. These products may not be available in all areas. Always read and follow the label directions.
In the greenhouse, yellow sticky cards (whiteflies are particularly attracted to the color yellow) have been used to monitor populations of adults and in some cases can offer some amount of control. Indoor plants and outdoor flower containers can be treated with Bayer Advanced 2-in-1 Insect Control Plus Fertilizer Plant Spikes according to the label directions (this product may not be available in all states).
If the infestation is not severe, often you can simply prune off the leaves that have heavy concentrations of eggs and nymphs (usually the lower leaves), seal them in a bag, and discard them in the trash. In fact, at this time of the year, most perennials can be cut back to the ground.
Good fall garden maintenance will go a long way towards ensuring a healthy, weed-free, pest-free, disease-free garden next season. So on these upcoming beautiful fall days, get out in the garden and do some cleaning.
Until next time – Happy Gardening!