Jun 10, 2021

Summer options for controlling San Jose scale in Michigan tree fruit crops

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

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 fruit is a significant concern in apples, this type of indirect damage to the woody tissues is the primary concern in Michigan stone fruit orchards.

Scale insects have a unique life cycle that makes them difficult to control. Immature female and male scale overwinter underneath a waxy, turtle shell-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, males 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 typically two generations per year in Michigan.

Adult flight was first recorded on May 24, 2021, at the Trevor Nichols Research Center, and scale crawlers for this generation are expected 400-450 growing degree days (GDD) base 51 F after this biofix. First generation crawlers are typically emerging mid- to late June and second generation crawlers in mid-August. Targeting the first generation crawlers will prevent mating and reproduction thereby minimizing the population of the second generation.

Warrior, Assail, Belay and Closer are nerve poisons that will have the best efficacy when targeting crawlers as they emerge. The insect growth regulators (IGRs) Esteem and Centaur also target crawlers, but are somewhat slower acting because they disrupt insect development. 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 mobility characteristics within the tree tissue. Sivanto displays translaminar movement and is xylem mobile, meaning the spray material will move in the expanding foliage. On the other hand, Movento is phloem and xylem mobile and activity on the target pest follows plant penetration. 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.

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

– John Wise, Michigan State University Extension, Department of Entomology, and Nikki Rothwell, Michigan State University Extension