Jun 24, 2020
San Jose Scale crawler emergence beginning in Michigan tree fruit

Concerns of San Jose scale damage on many Michigan tree fruits have grown as pest management programs have changed in recent years. This pest has traditionally been a pest of apples as it causes direct, unsightly damage to the apple fruit.

San Jose scale on apple spur at the Northwest Michigan Horticulture Research Center on April 5, 2020. Photo by Emily Pochubay/MSU Extension.

Even as San Jose scale has been a known pest in apples, populations and resulting damage have increased in the past three years. Additionally, over the last 10-plus years, we have observed increasing populations of San Jose scale on sweet cherries in northwest Michigan and on peaches in west central Michigan. Reports of San Jose scale in sweet cherries, primarily in northwest Michigan, indicate that this pest reaches high populations in hot spot areas of the orchard block that injure trees causing branches and sometimes entire trees to die when left unmanaged.

Even if growers have been actively managing San Jose scale in sweet cherries, this pest has been difficult to eradicate from a previously infested block. While direct damage to apples is a significant concern, this type of indirect damage to the woody tissues of the tree is the primary concern in Michigan sweet cherry orchards as limb/tree death results in reduced fruit capacity and overall health of the tree. At this time, we have not observed direct damage to cherry fruit. Based on growers and researcher reports, San Jose scale populations have been on the rise in apples, cherries and peaches across the state

Life cycle of San Jose scale

Scales insects have a unique life cycle that makes them difficult to control. Immature female and male scale overwinter underneath a waxy, turtle-like covering. When sap begins to run in the spring, the overwintering scales grow and reach maturity in mid- to late May. At this time of year, male scales come out from under the scale to mate with females. Females give birth to live young rather than laying eggs – these nymphs are the crawler stage of the life cycle.

Each female is capable of bearing 150-500 offspring, also known as crawlers. These crawlers start to suck sap with their needle-like mouthparts and within three weeks, the crawlers molt and lose their old skins, legs and antennae to become a flattened sac with waxy caps. They remain attached to the trees with their mouthparts and protective covering. Weather permitting, immature scales will continue to feed, develop and mature, and depending on location, can have two to five generations. In Michigan, there are typically two generations of San Jose scale.

San Jose scale feeds on sap of sweet cherry trees, and on healthy trees, large populations are needed to cause economic injury. Depending on the size of the population, San Jose scale can kill young trees in two to three years. Older trees can also be killed by scale, but they do withstand more feeding damage than young trees; in older trees, we more often observe limb dieback rather than whole tree decline. In many cases, we have observed damage in older sweet cherries, and there is considerable dieback in the tops of the trees. In these situations, trees are not killed but the cropping potential is considerably reduced.

In addition to feeding on sap from woody tissues, San Jose scale can also feed on the fruit and leaves. Feeding on apple fruit causes bright red spots, which are unsightly and can reduce packouts. As mentioned previously, we have not identified San Jose scale feeding injury on sweet cherry fruit in Michigan.

Control strategies

Because these insects typically have two generations per year in our area, we have three optimal timings for control. An oil application during prebloom is highly effective for targeting adults by suffocating the overwintering scale. Crawlers are typically active in mid- to late June and mid-August, and insecticides applied at these timings target crawlers before they produce their protective waxy covering. Targeting the first generation crawlers will prevent mating and reproduction thereby minimizing the population of the second generation.

Be mindful that insecticides labeled for San Jose scale control have different mechanisms for their efficacy against San Jose scale. For example, products such as Lorsban (Note: Applications to the canopy are only labeled for prebloom dormant or delayed dormant; post-bloom trunk sprays are labeled; Lorsban is phytotoxic to sweet cherry foliage) and those that were not tested but are recommended in the Michigan Fruit Management Guide (ex. Warrior, Assail) are contact poisons that will have the best efficacy against crawlers if the spray material comes in contact with the pest.

Beleaf is also labeled for scale control when used at first generation crawler timing in apples. Sil-Matrix works to thicken the cuticle of plant tissues and enhances systemic acquired resistance, thus providing suppression of San Jose scale in pome fruits.

The newer unique chemistries such as Sivanto and Movento are taken up by plant tissue and have different movement characteristics within the tree tissue. Sivanto displays translaminar movement and is xylem mobile, meaning that the spray material will move in the foliage. On the other hand, Movento is phloem and xylem mobile, meaning that this chemistry can move from foliage all the way to the tree’s roots. Because the tree takes up these materials, they are most effective against scale when the material is present in the tree prior to substantial feeding. Therefore, these materials should be applied prior to crawler emergence when there is substantial leaf tissue present to take up the material. Sivanto is not labeled for stone fruits and Movento is labeled for both pome and stone fruit.

Lastly, Table 1 shows the speed of activity of the chemistries on the crawler stage and the potential for the insecticide to flare mites.

Table 1. Insecticidal activity on crawler stage of scale insects.
Compound Labeled crops Speed of activity Mite flaring potential
Esteem All fruits Slow Low
Movento Pome and stone fruits Slow Low
Warrior/Asana Pome fruit (not on stone fruit label) Fast High
Assail (suppression only) Pome and stone fruits

(not on blueberry label)

Moderate Moderate
Beleaf Pome fruit (not on stone fruit label) Moderate Moderate
Sivanto Pome fruits

(not on blueberry label)

Moderate Low
Closer (suppression only) Pome and stone fruits Moderate Low
Centaur Pome and stone fruits Slow Low
SilMatrix Pome fruits Slow Low

, and , and , MSU Department of Entomology 





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