|
|
|
 |
Printer Friendly Version P. A. Stansly Mandarin: Citrus reticulata x C. sinensis (L.) Murcott California red scale (CRS), Aonidiella aurantii had been previously reported in Florida from citrus and some ornamental crops, but not even old timers remember it ever being a problem on Florida citrus. However, during the 1998-growing season, I received reports of outbreaks of armored scale on some mandarin varieties as well as orange and grapefruit in southwest Florida. An unusual aspect of these reports was the prevalence of CRS. In response to grower reports and my observations of high CRS populations in some groves during the 1998 season, I conducted a survey of scale incidence and pesticide use in Florida citrus. A total of 73 responses were received from growers in the Southwest, Indian River, and Ridge regions (unpublished data). Nine growers reported recent problems with CRS and all had used Nexter 75WP (pyridaben). All but 2 of an additional 11 who had used Nexter saw higher than normal armored scale populations, albeit of different species. In contrast, only 9 of 55 growers who had not used Nexter reported unusually heavy armored scale infestations. Nexter also reportedly caused increases in CRS density of up to 8 times the untreated control in California (Grafton-Cardwell & Reagan, 1999). The following experiments were conducted with the objective of evaluating the effect of Nexter on CRS populations in southwest Florida. Materials and Methods An 18 acre block of mature sweet orange Murcott with 80 tree rows orientated north to south was divided into 4 row plots with 3 replications. Four treatments were assigned to each 4 row plot in an RCB design. 1999Trial Time and number of applications of Nexter 75 WP @ 10.7 oz/acre (0.5 lb ai/acre pyridaben) was varied. Control trees were sprayed with AgriMek 0.15EC at 8 oz/acre (0.0094 lb ai/acre abamectin) plus 6% v/v FC 435-66 horticultural mineral oil (HMO) on 9 June to control citrus rust mite. All applications on 9 June included zinc and manganese at 1 lb/ acre and copper at 4 lbs per acre as a tank mix. Treatments were applied with an airblast sprayer at 125 gpa. Evaluations were made using pheromone traps and scale counts on fruit. The first 3 evaluations were conducted in the field using a 5x-magnifying headset to count all armored scales on each of 2 randomly chosen fruit from both of the 2 middle rows of each bed (plot) at each of 5 locations along the bed. The last 2 evaluations were conducted in the laboratory by removing the same number of fruit and counting all scale under a stereoscopic microscope. Ten additional fruit from each plot, chosen for high scale counts, were held for 21 days in 1-pint cardboard ice cream containers. Emerged parasitoids were counted after 20 days incubation at ambient temperature. An additional sample of the following year's fruit taken on 30 June 2000 and evaluated in the laboratory also served as a pretreatment sample for the following year's treatments. A pheremone lure (Trécé, Salinas CA) was attached to the center of a folded 6"x12" yellow sticky trap (Olson Products, Medina Ohio) placed centrally in the middle row of each bed. The traps were removed after 3-6 days and the CRS males were counted by placing a template with six 2.26 cm2 circles on each side of the trap, measuring a total area of 27.2 cm2. A t-test was used to test for significant treatments effects for the first 3 evaluations where treatments consisted only of treatment with Nexter versus treatment with the control. Data gathered subsequent to the second Nexter treatment was subjected to analysis of variance with mean separation by Fisher's LSD. 2000 Trial Plots were re-randomized and there were again 3 treatments plus a control: (1) Nexter75 WP @ 13.2, oz/acre (0.62 lb ai/acre), (2) Nexter @ 13.2 oz/acre + 17 oz Esteem 0.86 EC @ 17 oz/acre (0.114 lb ai/acre pyriproxyfen) + 5% Fl-435-66 HMO, and (3) as 2 without the HMO (2 replicates only). Control trees were sprayed with Micromite 25 WP @ 20 oz/acre (0.31 lb ai/acre diflubenzuron) for CRM control. All treatments included 5.6 lb/acre 98% basic copper sulfate and 15 lb/acre potassium nitrate. The application was made on 31 May using the same equipment and volume as the previous year. All trees were sprayed in August with 10 oz AgriMek 0.15EC + 2 gal. HMO and nutrients, and in September with 5 pints of Ethion 4EC, both for CRM control. A pretreatment sample was taken on 30 May, and post-treatment samples taken on 29June, 27 July, 11 September 2000 and 11 January 2001, using the same sampling scheme as above. Fruit were taken to the laboratory and inspected under a stereoscopic microscope for CRS and classified by instar. Scale covers were removed from 2nd and 3rd instars and were further classified as alive, parasitized or dead. Citrus rust mite population density was evaluated on 10 Aug and 14 Sep. using a Bausch and Lomb 10x Hastings triplex hand lens by observing 2 lens fields from either side of fruit sampled as for CRS. Percentage peel damage from CRM was estimated visually on fruit sampled using the same scheme on 26 Jan. 2001. Male CRS flight activity was monitored with a single pheromone trap placed in 2 centralized plots on 29 April and increased to 3 plots (Treatment 4) on 21 June. One quarter of the double-sided 6"x12" trap was exposed at a time (6" x 6"), with a new surface exposed at approximately 10 d intervals. Pheromone was changed after 4 exposures, or at approximately 40-d intervals. Results 1999 Trial Density of male scales was consistent among treatments on 2 and 9 September. On 4 November there were again no significant differences among treatments although 6 times more scales were seen on traps in plots treated with Nexter in June and Sep compared to those treated in June only (Table 1). Highly significant differences were observed in numbers of CRS on fruit from trees treated with Nexter compared to the control (Table 2). Observations in the laboratory following the second (September) treatment showed significantly more CRS on fruit randomly sampled from trees sprayed both in June and September compared to all other treatments, with no differences among the other treatments (Table 3). However, no significant treatment effect on the next year's fruit crop was observed on 30 May 2000. CRS observed on fruit incubated for parasitoid emergence were more numerous from trees sprayed with Nexter on both dates. However, there were significantly more scale from the June only treatment compared to the control on both dates. Percent parasitoid emergence from fruit collected on 15 November was greatest from trees sprayed with Nexter in June and least from trees sprayed with Nexter in September. 2000 Trial Captures in pheromone traps peaked 8 May, with secondary peaks observed 18 July and 4 October, possibly indicating 3 generations per year (Fig. 1). Captures were considerably sparser than the previous year. Proportional representation by the 3 instars sampled on fruit did not change appreciably over the season (Fig. 2). There were no significant differences between CRS densities in plots treated with Esteem and Nexter, with or without HMO (F = 0.55, df = 3,121, P < 0.65), so results from these treatments were combined for analysis. There were no significant differences in total number of CRS observed from the pre-treatment fruit sample or the first post-treatment sample on 29 June (Table 4). All 3 subsequent samples showed significant treatment effects. Two months after treatment on 27 July over 4 times more scales were seen on fruit from trees sprayed with MicroMite and HMO compared to trees sprayed with Esteem and Nexter or Nexter and HMO alone. Scale counts were still highest on MicroMite-treated fruit for the 11 September sample, although they had also risen significantly on trees treated with Nexter and HMO relative to those also receiving Esteem. By 11 January 2001 there were no differences between the MicroMite and the Nexter plus HMO treatments which exceeded the number of scales observed on fruit receiving Esteem by an order of magnitude. There were no significant post-treatment differences in proportional representation of instars except on 11 Jan 2001 (Table 6). However, on this date only 3 of 100 fruit from trees treated with Nexter plus Esteem had scales. Likewise, there were few differences in the proportion of dead and/or parasitized 2nd and 3rd instar CRS except for significantly more parasitized scales observed on 27 July from fruit treated with MicroMite and HMO (Table7). CRM density tended to be greater on fruit treated with Esteem plus Nexter and HMO than other treatments (Table 8). The same trend was observed with CRM damage on mature fruit, although differences were not great. Discussion A considerable, though short-lived increase in CRS populations in response to Nexter was observed in 1999. One explanation consistent with the data is that the June spray suppressed parasitic wasps, allowing scales to multiply rapidly. More scales would have attracted more wasps, reducing CRS on trees sprayed in June only. However, trees sprayed again in September would not have recovered as quickly due to further suppression of parasitism by Nexter. Indeed, we did observe on 15 Nov. that parasitism as judged by wasp emergence was highest on trees sprayed with Nexter in June and lowest on trees sprayed in September. Nevertheless, scale populations apparently did not have time rebound on trees sprayed only in September. Broad-spectrum insecticides were not used in the grove, so parasitic wasps were probably able to reestablish by the following spring, negating any further effect of Nexter through 30 September 2000. How then do we explain the results observed in 2000? One clear conclusion is that Esteem provided slow acting but persistent and effective control of CRS. Scale density did eventually become greater on trees treated with Nexter but not before they had risen significantly on trees treated with MicroMite. Since there could unfortunately be no untreated control, we are left with two possibilities, both equally consistent with the data: (1) both Micromite and Nexter caused or allowed scale populations to increase, or (2) neither had any effect on scale populations. This time the data on parasitism doesn't give much help. It is necessary to compare trends over years to better understand the responses of CRS populations. Scale densities were considerably reduced in 2000 compared to 1999. Although there are no populations estimates for 1998, this grower like many others that year was concerned about CRS and brought the problem to my attention. I myself observed heavy CRS infestation that year in his and other groves. In contrast, the grower was quite satisfied with the condition of the fruit in 1999 and 2000, regardless of treatment, and there have been virtually no further reports of CRS infestations. Therefore, it would appear that conditions were especially favorable for CRS development in 1998, a so-called El Niño year, characterized by an unusually wet winter and dry summer (Fig. 3). This reversed the normal pattern, approaching the typical Mediterranean rainfall pattern favorable to CRS. Perhaps a wet cool weather is the key to CRS survival in winter. Indeed, we observed low populations in the spring of 2000 after a dry, warm winter. Thus, weather conditions were particularly favorable for CRS in 1998, allowing the population to respond strongly to localized parasitoid suppression. Conditions were less favorable in 1999, although a large number of scales must have carried over from 1998 and we did see a response to Nexter, especially early in the year. By 2000 populations had dropped globally in response to increased pressure from parasitoids against a background of unfavorable weather patterns typical of Florida. Under these conditions even less response was observed to insecticidal perturbation. Thus, under typical conditions we should not expect to see CRS infestations exacerbated by Nexter. However, if weather conditions again become favorable to CRS in Florida, i.e. approach a Mediterranean rainfall pattern, we could see populations increase in response to insecticidal suppression of parasitoid activity. In such a situation, pyriproxyfen could probably be used successfully for CRS suppression. Cardwell, E. E. and C. A. Reagan. 1999. California red scale insecticide efficacy trial, 1997. Arthropod Management Tests. 24:91. Table 1. Density of male CRS on yellow sticky cards baited with pheromone attractant in 1999.
Table 2. Mean number of CRS observed in the field on 20 fruit, summer 1999.
Table 3. Mean number of CRS observed on 20 fruit in the laboratory from 1999 Trial.
Table 4. Mean number of parasitoids emerging per 10 fruit selected for high CRS infestation in 1999.
Table 5. Mean number of CRS from 20 fruit in 2000 and 2001
Table 6. Proportional representation of CRS instars by treatment in 2000-2001.
Table 7. Mean number of parasitized and dead plus parasitized CRS per 20 fruit in 2000-2001.
Table 8. Mean number of CRM in 2000 and mean percentage damage from CRM feeding on 20 fruit in 2001
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|
|
|
|
|
||
