San Jose scale attacks all the delicious fruit trees: apple, peach, pear, and plum. Some of you may already know that this insect is particularly difficult to manage, because most life stages are very small, even tiny, so they are not obvious until there are lots of them! In fact, you’re going to need a legit hand lens or magnifying glass to see some of the life stages clearly. In this article, I’ll try to cover the big picture things you need to be aware of to detect and protect your trees from San Jose scale.
Figure 1. An apple branch infested with San Jose scales, which are denoted with white arrows (A) and a close-up image of the dark-colored, protective covering produced by scales, along with yellow, mature female scales that have their protective scale covering removed. Photo credit: John Obermeyer.
Signs and symptoms. It may be difficult to catch San Jose scale infestations before they reach high levels because they are so small! But looking for signs and symptoms can help you detect this pest before the population gets out of control. Here’s what you can look for, but take care to look closely and carefully:
A key sign of San Jose scale infestation is the presence of small, hardened, dark-colored bumps or “scabs” on tree twigs and branches (Figure 1A & B). The mature scales may be found on the tree from late July through early September, but note that multiple life stages may be present at the same time (more info below in the life cycle section). A key symptom of San Jose scale infestation that is particularly eye-catching is the presence of red, circular spots on fruit (Figure 2). These red spots are caused by toxins that the scale injects into plant tissue while feeding. Once you see symptoms of injury on fruit, the population is likely at a level that management is necessary to protect trees from decline
caused by this insect pest.
Figure 2. An apple with diagnostic symptoms of San Jose scale feeding: red, circular spots caused by toxins injected by scales during feeding. Photo credit: G. Krawczyk
The life cycle of San Jose scale. This insect pest transitions through four stages during its development: 1) immature crawlers, which look like tiny, flattened-yellow disks, but note they are active and mobile; 2) immature scales, which are fixed in place on the tree and are also the overwintering life stage; 3) adult males, which are free-living and look like tiny, delicate, gnat-like flies; and finally, 4) the mature scale, which is immobile and fixed to one spot on the tree; male scales are a bit smaller and more oblong than the female scale. You won’t see eggs of this pest because female scales give live birth to offspring! There are typically two generations each year in our region and note that there may be overlapping generations during the growing season, which means you may detect free-flying adult males, immature crawlers, and the immature or mature scales, all at the same time in the orchard.
Monitoring strategies and Degree-day models for San Jose scale. The adult males (tiny gnat-like flies) can be monitored after they emerge using pheromone-baited sticky traps. These traps should be placed prior to bloom (anytime between ‘tight cluster’ and ‘pink’), because adult males typically emerge to mate with immobile female scales around ‘petal fall.’ Once males are captured on pheromone-baited sticky cards, be on the lookout for immature crawlers 4-6 weeks later. Be sure to check the trap daily however, to make sure you know when the first male is captured. Monitoring traps are hung from a tree branch with a string or wire and males are captured on the sticky trap when they fly in to investigate the attractive pheromone odor. Traps can be purchased from Great Lakes IPM and there are ready-to-use kits available!
If pheromone-baited sticky cards are used in conjunction with the San Jose scale degree day (DD) model (calculated based on San Jose scale developmental temperature of 51 °F), you can use first capture of adult males on the traps to begin tracking degree days. Research indicates that ~300 DD51 after the first males are captured, you can begin monitoring for the tiny, yellow crawlers by placing a piece of black electrical tape (sticky side out) around branches that are infested with San Jose scale. Check the tape twice a week for the crawlers, and remember to have your hand lens or magnifying glass handy when you inspect the tape! The crawlers begin emerging between 380-400 DD51 after adult males were first captured, and peak crawler activity is estimated to occur between 600-700 DD51 after adult males were first captured. This is the best time to apply insecticides to combat crawlers in late spring, because they have not yet settled and developed the protective, waxy scale.
Best management strategies. Add biological control agents! If you know there are trees with San Jose scale infestations, a good cultural strategy is to prune out infested branches, which may also improve penetration of sprays into the canopy.
Next, dormant oil is your friend! Because San Jose scale overwinters as an immobile, immature scale on the tree, they can’t move or evade early spring applications of dormant oil, which works by suffocating the immature scales. If you know there are certain areas or trees in your orchard that have a history of infestation by San Jose scale, make note or flag the trees, so you can be sure to apply dormant oil to these trees in the early spring. Research suggests that an application of dormant oil at ‘green tip’ is very effective against immature scales, BUT (and I know you’ve heard this a million times) thorough coverage is critical (i.e., use high volumes of water) to see the best control. If you miss the window to apply dormant oil, you can apply insecticides to combat San Jose scale in the late spring (mentioned above), but know that mature scales are protected from insecticides by their waxy scale covering, so you must target immature crawlers to reduce the population and infestation of fruit later in the season.
Last but not least, some may be wondering if there are natural enemies in the orchard that attack San Jose scale, and the answer is yes! There are tiny, parasitoid wasps that attack the immobile scales, laying their eggs inside, which hatch and eat the scale as the new parasitoid develops, eventually killing it. Thank goodness we’re not insects, right?! If infestations are not severe, these parasitoids can provide effective biological control, but note they are sensitive to insecticides too, so try to avoid using broad-spectrum insecticides when you can to help keep these “good bugs” around.
This article was written in response to a reader request 😊! I hope it was helpful, and if you have suggestions for other fruit insect topics, you are welcome to email me: long132@purdue.edu