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Key Takeaways

  • In the desert Southwest, pearl scales can become problematic and hard to control on bermudagrass because their life cycle is underground and they exist largely in a protected coating as encysted-encased ‘pearls’
  • Proper application timing and using the most effective insecticides is crucial to control pearl scales.
  • Local meteorological data is helpful for scheduling initial applications. If this data is not available, it is necessary to dissect soil cores to determine when the population has shifted to the non-encysted reproductive life form.
  • Under semiarid conditions, preliminary findings found that pearl scale populations were drastically reduced when select insecticides were applied once only, followed by other products also applied once only.

Pearl scales (a.k.a. ground pearls), although not a widespread pest, can cause severe perennial damage to warm-season turfgrasses and are difficult to control because (1) their entire life cycle occurs underground, (2) they persist in a hard-covered case as encysted shells (called pearls) for 95% of their life span, and (3) many previously used insecticides are no longer commercially available and newer chemistries, when tested alone, have resulted in marginal to nominal control.

Commercially available insecticides have demonstrated only partial and/or inconsistent control, and label restrictions limit specific time intervals for “same-product reapplications.” This is an important consideration for managing pearl scales based on their life cycle susceptibility to direct exposure of applied insecticides. Product and test literature alludes to applying products when the ground pearls are at their highest degree of environmental exposure – i.e., when they emerge from their encysted pearl stage as pink-colored females to produce another brood of juveniles when the soil warms in the late spring. The objective of this article is to provide the reader with recent research that was conducted to elucidate which insecticides, when applied at short application intervals, would reduce pearl scale infestation levels. All applications in these trails were made when the pink female stage accounted for 75% or more of the total pest populations, based on core samplings and physical counting of both encysted pearls and exposed pink females.

Objectives:  

The research objectives were as follows: 

  • Identify successful product application sequences as “treatment series” programs, where effective insecticides are applied once, followed by additional applications of other insecticides, used only once as well.  

  • Test the effect of repeating yearly applications of the best-performing treatment sequences versus the results of a single-year application program. 

This “treatment series” approach allows products to be used at the maximum single-application label rate and remain in full compliance with all remaining label requirements. As part of this research, we also investigated – by soil sampling the root system – when ground pearls had largely changed in population composition from encysted scales to the predominately naked/non-encysted female reproductive life span phase, when they would likely be at their most vulnerable. This field research was conducted with replicated plots confirmed to have surface physical damage and verified ground pearl infestation. Completing these objectives would be highly beneficial for turfgrass managers who have long-term problems with this perennial underground pest.

Materials and Methods: 

The test site was a ‘Tifgreen 328’ bermudagrass chipping green in Yuma, Arizona, with long-term pearl scale infestation. Symptoms included turf discoloration and decreased density that was often accompanied by visible dead shoot material and bare soil. Core samples were collected across the affected surfaces to confirm the presence of pearl scales and establish individual field plots for insecticide treatments.

In 2017, 4.25-inch diameter by 5-inch depth soil cores were collected from highly infested plots on four dates from May to early June in order to determine when the population shifted from typical encysted pearls to the emerged, pink-colored, non-encysted, egg-laying females. The greatest proportion of females occurred on May 26 when over 90% of the population were emerged females. The Arizona Meteorological Network (AZMET) weather data collected daily at the Yuma location showed an accumulation of 1,750 accumulated heat units from the 55/85 Fahrenheit sine growth heat unit model on this date. Note that heat unit values increase rapidly in late spring, so it is important to check these values on a daily basis. In these trials, heat units were used as the indicator to initiate insecticide treatments. This heat unit value was used to determine the date of the first product application for all treatment series in both 2017 and 2018.

All plots were aerified with 3/16-inch hollow tines on a 2-inch by 2-inch spacing 24 hours before insecticide applications. Revolution soil surfactant was applied to the entire test area prior to the first application event in both 2017 and 2018. All plots were irrigated with a hand-held hose nozzle following product application. The green was mowed six days per week at 0.140-inch height of cut with clippings removed, fertilized with 0.38 pounds of nitrogen per 1,000 square feet monthly and irrigated to avoid visible moisture stress. Treatment sequence series applications are included below for years 2017 and 2018:

  1. In 2017, six individual products were included in the trial (Table 1). These products constituted six different treatment series programs for testing. Each of the six treatment series included six different products applied once each on six closely spaced dates from May 1 to June 15, 2017. 

  2. The three best-performing treatment series from 2017 were applied again in 2018, from May 21 to May 29, over the same treatment series plots from 2017 in order to test a two-year protocol program. 

  3. Also in 2018, the same three best-performing treatment series from 2017 were applied on additional replicated plots which were not treated at all in 2017. This was done to compare the effects of a single-year initial application only, but now observed over multiple years for confirmation purposes.

  4. We express the treatment series program results in terms of the number of ground pearls per plot through evaluation of the 4.25-inch diameter by 5-inch-deep soil samples. The mean number of ground pearls was calculated from two plugs from each of the three plot replications – six soil samples in total. Pearl scale samples were taken and completed by hand counts on October 5, 2017, and October 6, 2018.

Results

2017 

For the six treatment sequences (TS) applied in 2017, the mean number of ground pearls per plot ranged from 1.3 to 82.6 (Table 2). The replicated, untreated turf plots averaged 94.6 encysted ground pearls per plot. Additional replicated plot areas that showed no visible symptoms had, on average, only one or two ground pearls per plot (data not shown). Three of the treatment series from 2017 resulted in greatly reduced numbers of pearl scales compared to untreated controls This occurred for treatment series that included the insecticides Ference, Avid, or Zylam when these products were applied in one of the first three – out of six total – application dates in 2017 (see TS 4, TS 7, and TS 8). While the treatment series themselves were devised from simple numerical ordering of the selected products, the products themselves – and how they appear in the relative application orders – do shed light on the probable importance of each product’s contribution, as well as the importance of product placement in the rotational timing scheme. 

For the overall treatment sequences alone, Ference was effective when applied once on any of the first three application dates (see TS4, TS7 and TS8). Divanem was most effective when applied on any of the first two application dates (see TS4 and TS7), and when Zylam was initially applied followed by the Divanem application (see TS4) (Table 2).

These results lead to a reasonable conclusion that the observed effects from treatment sequences TS4, TS7 and TS8 – which had the best initial results – were most likely provided by Ference, then secondly followed by Zylam, with Divanem producing a somewhat-less, but more-than-nominal, effect. The additional observation that TS8, which averaged a low 1.3 pearls per plot, was comprised of Ference applied at the first application date, which was then followed by Nimitz and then Arena, also alludes to the observation that Ference has a major effect against pearl scales. Both Nimitz and Arena produced larger pearl scale counts when they were used as the first application in their respective treatment sequences (see TS11 and TS1 vs. TS8) (Table 2).

The importance of application timing is also strongly suggested by the fact that Ference had reduced activity when applied during any of the last three application dates in 2017 (June 9, 12 or 15), as evidenced in TS1, TS2 and TS11 – which had 22.0, 59.6 and 82.6 pearls per plot for the 2017 harvest, on average. Meanwhile, TS4, TS7 and TS8 had an average of 1.0, 8.6 and 1.3 ground pearls, respectively, at the end of 2017 (Table 2).

2018

In 2018, the treatment series that produced the better first-year results (see TS4, TS7 and TS8) were also applied on infested turf which did not receive any products in 2017. This was done to corroborate whether the best-performing treatment series would indeed produce low pearl scale counts repeatedly. In this case again, applying Divanem as the first product led to slightly greater pearl counts, when compared to the other two treatment sequences that started with either Ference or Zylam.

Most importantly, when applied over the same turf plots for two years (2017 and 2018) the treatment sequences TS4+4, TS7+7 and TS8+8 resulted in only one or two pearls per plot in the fall of 2018 (Table 3). Again, these treatment sequences keyed on rotations of Zylam, Divanem, and Ference (Tables 2, 3). Replicated untreated control plots averaged 95 and 167 pearls per plot in 2017 and 2018, respectively, showing that pest pressure was robust and that the chemical effects were real. It remains to be determined what the result would be from other treatment series such as Ference only, Ference followed by Zylam, Zylam only, or Zylam followed by Ference only.

Note that in 2017, greens core plugs were sampled and hand screened on four dates from May through June to ascertain when the ground pearls had changed from the cream-colored, encysted pearl form to the pink female form. On May 26, 2017, female scales constituted 90% of the sampled population – 10% were still encysted pearls. The application date was initiated corresponding to these findings, to which AZMET weather data showed a heat unit cumulative total of 1750 heat units using the 55/85 Fahrenheit model. These same units were used to schedule the first application made in 2018.

Summary of Preliminary Findings: 

1. Under semiarid conditions, pearl scale populations were drastically reduced when select insecticides were applied once only, followed by other products also applied once only (defined as a unique treatment series). In 2017, six products were applied over a 21-day period. In 2018, three products were rotated over a 10-day application window. 

2. Ference (also sold as Mainstay), Divanem (same active ingredient as former Avid product) and Zylam, when applied as single products in closely spaced concurrent applications, essentially eliminated ground pearl cysts from ‘328 Tifgreen’ bermudagrass turf when these products were applied to the same turf surfaces for two years in a row via defined treatment sequence application programs.

3. Initial applications of the first product in the treatment sequence should be made when the majority of the pest population exists as non-encysted, pink-colored females. Timing applications according to this standard was achieved by correlating specific AZMET accumulated heat unit values when the pest population consisted of over 90% non-encased, naked pink females. This occurred at 1750 accumulative AZMET heat units for the 85/55 Fahrenheit plant growth model. If treating for pearl scale in locations not applicable to AZMET data, it is critical to determine the best time to begin an insecticide program based on multiple soil core sampling across multiple dates, starting in the late spring when temperatures begin to increase and soil temperatures rise.

Click here to access the AZMET data. Select Data Archive and choose the nearest weather station to your location. Under the Special Report column, select Heat Units and observe the accumulated heat units in the 86/55 F column. 

4. There is limited peer-reviewed research on pearl scales, and this preliminary research provides practitioners with helpful information on how to manage these pests. However, more research is needed to fully understand the complexities of pearl scales.