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Control of ground pearls using application sequences of different insecticides

By |  March 18, 2021 0 Comments
Damage caused by ground pearls on a bermudagrass putting green. Symptoms often include initial thinning and texture changes, followed by loss of turf in the forms of straw or bare soil. (Photo: David M. Kopec)

Figure 1 Damage caused by ground pearls on a bermudagrass putting green. Symptoms often include initial thinning and texture changes, followed by loss of turf in the forms of straw or bare soil. (Photo: David M. Kopec)

Ground pearls, also known as pearl scales, can damage warm-season grasses but are especially troublesome on bermudagrass greens. Although not widespread, damage to infested turf is problematic as populations increase from year to year when greens are irrigated and fertilized. Transmission mainly occurs from the movement or transfer of infested soils in sod, plugs, soil cores or excavated root-zone media. Once introduced, the ground pearls feed on the turfgrass roots, and populations almost always increase in irrigated and fertilized turf.

Symptoms and life cycle

Initial turf symptoms include a slight loss of turfgrass color in small oval or semicircular patches, increasing in size from year to year, often yielding larger circles or crescent shapes (Figure 1). New offspring that hatch on the edge of the healthy turf increase the size of infected turf areas. Symptomatic turf can be evident throughout most circular regions, with the most significant number of scales usually appearing near or at the advancing edge of infestation.

Ground pearls have an interesting life cycle, which is why insecticidal control of the pest is complicated. Ground pearls attach themselves as young scales in the early summer to begin root feeding. Shortly after successful root attachment, the scales produce a hardened shell or case, at which time, they are in their “encysted” or “pearl” stage. They continue at this commonly observed stage all year until the next late spring/early summer when they emerge from within the pearl and appear as exposed pink females (Figure 2).

Table 1

Table 1

The females then lay eggs without the need for fertilization. The eggs quickly hatch, and the small newborn scales (now in the crawling stage) travel short distances to attach to viable roots and start feeding. They then surround themselves with the round, hard shell, only to repeat the full life cycle in the next spring/summer as emerged females. The hard shell protects them from external stress and insecticides because they live as encysted ground pearls for practically 95 percent of their life span.

Control approaches

For the reasons noted above, chemical control is inconsistent, primarily for contact insecticides. Systemic products used alone in repeat applications produced nominal or inconsistent results. With the loss of many older insecticides, many of which had environmental concerns, there are virtually no insecticidal products recommended for ground pearl control.

The life cycle of ground pearls. (Photo: Danny Reiland)

Figure 2 The life cycle of ground pearls. (Photo: Danny Reiland)

Many publications instead emphasize minimizing symptoms through cultural management practices (see the Arizona Golf Industry Best Management Practices Resources Guide), including modified irrigation and fertility practices, some of which can be counterproductive in maintaining ultradwarf bermudagrass cultivars.

Investigating new insecticides

We investigated the application of newer systemic insecticides that closely coincide with when ground pearls are most susceptible to body exposure.

In 2017 at Desert Hills Municipal Golf Course in Yuma, Ariz., we used a chipping green with highly visible symptoms of ground pearl damage for these trials. Each plot received applications of six different insecticides differing in active ingredient groups (Table 1).

Each plot received every one of the selected products in a different rotation sequence. After the first product was applied, plots then received an additional single application of each of the other five products within a short time. The order of product applications, which is the same as the “Treatment Sequences,” are listed in Table 2.

We applied the maximum label rate of the first product in each treatment sequence (Table 1). After that, the next labeled product was used at its maximum rate within a short time window when the population of ground pearls was 90 percent or more pink females. Thus, in 2017, six treatment sequences were started on May 24, 2017.

We counted exposed females and pearls in soil cores sampled on several dates in mid-May. Treatment sequences were started when the population of ground pearls was greater than 90 percent exposed pink females. (Photo: David M. Kopec)

Figure 3 We counted exposed females and pearls in soil cores sampled on several dates in mid-May. Treatment sequences were started when the population of ground pearls was greater than 90 percent exposed pink females. (Photo: David M. Kopec)

Turf management

The entire green was core aerified with 5-inch-long (12.7-centimeter) 0.1875-inch (0.47-centimeter) solid tines on a 2- by 2-inch (5- by 5-centimeter) spacing. Revolution soil surfactant was applied at the rate of 6.0 ounces product per 1,000 square feet (1.9 milliliters/square meter) and watered in 24 hours before the first application series on May 24, 2017.

The green was mowed six times weekly at 0.1375 inch (3.5 millimeters) with a triplex mower with clippings removed. Irrigation occurred six days per week to avoid drought stress. Fertilizer was applied during the first week of the month in May to September from a 21-7-14 fertilizer at the rate of 0.375 pound of nitrogen per 1,000 square feet (1.8 grams/square meter).

We sprayed the plots in the early morning with the CO2 sprayer, delivering 56 gallons per acre (523.8 liters/hectare), while Nimitz was applied with a hand shaker. Each treatment series appeared three times in a completely randomized field design. Plots were hand irrigated with 0.375 inch (9.4 millimeters) of water within 30 minutes after the final product application.

Table 2

Table 2

Plots were harvested for ground pearl counts on Sept. 27, 2017, by taking two standard 4.0-inch-diameter (10.2-centimeter) cup cutter samples to a 5-inch (12.7-centimeter) depth (Figure 3). We stored all samples immediately in a refrigerator during the counting procedure, which took place within five days after harvest.

Ground pearl counts

All plots in the trial that received chemical products had visual symptoms, confirmed by observing ground pearls found in the 2.0-inch-diameter (5.1-cm) soil core samples were taken at a depth of 2.5 inches (6.4 centimeters). The plot size was 20 inches by 60 inches (50.8 by 152.4 centimeters). For data analysis of the treatment sequences (TSs), two standard 4.0-inch greens cup cutter plugs (10.2 cm) were taken to a soil depth of 5 inches (12.7 cm) and analyzed for effectiveness by counting the number of ground pearls per plug.

We used the average of the two samples per plot as the individual plot data response. Also, we included severely infested areas as untreated controls and plots that showed no symptoms (asymptomatic controls). We conducted a statistical analysis on the number of ground pearls counted in each of the treatment sequences harvested at the end of September each year.

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