Western flower thrips is becoming an increasing problem in orchards
It takes vigilance to prevent damage by some stone fruit pests because they can return to the orchard after leaving for other hosts during the summer.
Tim Smith, Washington State University Extension educator for north central Washington, said one such pest is the green peach aphid, which has a wide range of other hosts besides peaches and nectarines.
Smith explained the pest’s unusual life cycle during the Lake Chelan Hort Day in January. Although green peach aphid has a huge host range in the summer, it overwinters only on fruit trees, in the egg stage. Adults lay eggs in clusters in the fall. The eggs are small, black, and oval shaped. As the tree wakes up in the spring, so do the aphids. Eggs hatch into wingless stem mothers, which produce live young without mating and reproduce rapidly. The aphids can quickly build up to large numbers. Like other aphids, they feed on green tissue and while doing so inject a toxin that distorts the growth of the shoots. An infestation is easy to spot at this stage, Smith said, but a lot of damage will already have been done to the fruitlets.
"You have to be in there early," he told growers.
In the summer, winged aphids develop, which fly off looking for summer hosts, such as mustards, potatoes, tomatoes, and many other vegetables. Green peach aphid is a vector of leaf curl virus of potato and plum pox virus in tree fruits, Smith said.
In the fall, winged aphids leave the leafy green plants and fly back to orchards after being gone all summer long.
"Controlling them in the spring does not control them in the fall," Smith emphasized.
The winged adults give birth to nymphs, which develop into egg-laying females. They mate with returning males and lay eggs on the tree that overwinter and complete the life cycle.
Western flower thrips have become an increasing problem in orchards lately, Smith reported. Thrips are so small they can only be seen clearly with a hand lens. They are bigger than a mite, but smaller than an aphid, Smith explained. Thrips feed on flowers in the spring and can be monitored by shaking flowers over a dark piece of paper. Thrips have many hosts and can infest any flowering plant in the neighborhood of the orchard, including dandelions and bitterbrush. When those hosts stop flowering and begin to dry up, the thrips can move back into the orchard.
During the summer, when there are not many flowers around, they will feed on foliage, including growing shoot tips, Smith said, and when the leaves grow, they are rumpled. Some thrips will attack fruit.
"If you’ve cleaned up the first generation, that doesn’t mean the second generation won’t come back into your orchard and damage the fruit," he said. "You can’t control them in your orchard and assume they’re under control. You need to monitor and realize if you have a problem every year, you will have a problem next year, too."
Because the entire life cycle of a thrips takes only six to ten days, they can build up rapidly. The pupae drop to the ground, and within two to five days they can emerge as adults and start mating and laying eggs.