The crapemyrtle bark scale, Acanthococcus lagerstroemiae (Kuwana) (Hemiptera: Eriococcidae), is an invasive pest of crapemyrtles, Lagerstroemia spp. L. (Lythraceae) in the southeastern USA. Information about its temperature-dependent development and host range is limited. The objectives of this study were to evaluate the effects of temperature on the immature development of A. lagerstroemiae, and to determine plant species suitable for immature development and reproduction. Developmental time and survival of eggs were evaluated at 7 constant temperatures from 17.5 to 32 °C, and of nymphs at 20, 25, and 30 °C. Results suggested that 27.5 °C was the optimum temperature for egg hatching with the shortest time (10 d) and the highest hatching rate (95%). The developmental time for A. lagerstroemiae from nymph to prepupa and gravid female was 56 d and 68 d at 30 °C, respectively. Five plant species besides crapemyrtle (Lagerstroemia indica × fauriei L.) were able to support the immature development and reproduction of A. lagerstroemiae under no-choice conditions, including Lawsonia inermis L., Heimia salicifolia Link, Punica granatum L., Lythrum alatum Pursh (all Lythraceae), and Callicarpa americana L. (Lamiaceae). At wk 12 from inoculation, the density of gravid females was 482 ± 92 (mean ± standard error) on L. indica × fauriei, 200 ± 70 on C. americana, and < 150 on other species. Using data from developmental time and host range, integrated pest management (IPM) practitioners can implement preventive strategies for A. lagerstroemiae.
The crapemyrtle bark scale, Acanthococcus lagerstroemiae (Kuwana) (Hemiptera: Eriococcidae), is an invasive pest of crapemyrtles, Lagerstroemia spp. L. (Lythraceae) (Wang et al. 2016). Native to Asia, this pest was first reported in Richardson, Texas, USA, in 2004 (Merchant et al. 2014), and it is currently present in 11 other states including Louisiana (EDDMapS 2017). Acanthococcus lagerstroemiae is a sexual dimorphic with the adult female being sessile on the bark for most of her lifetime (Wang et al. 2016). Honeydew secreted by this scale facilitates sooty mold accumulation on the crapemyrtles, thus reducing aesthetic values as well as producing limited photosynthesis (Gu et al. 2014). Crapemyrtles are ornamentals with the highest economic value in the southeastern US (USDA NASS 2014). With more than 130 cultivars, Lagerstroemia spp. have a wide range of plant size, flower, foliage, and bark color (Chappell et al. 2012). Before the arrival of A. lagerstroemiae, crapemyrtles were valued as an ornamental with low pest problems (Knox 2003; Chappell et al. 2012). Current management of A. lagerstroemiae relies on insecticides such as imidacloprid, cypermethrin, and dinotefuran both in China (He et al. 2008; Zhang 2011) and the US (Gu et al. 2014; Robbins 2014), though most of these chemicals have been prohibited on bee-attractive plants including crapemyrtle (Riddle & Mizell 2016).
Temperature is one of the most important abiotic factors influencing the survival and development of insects and consequently population growth (Ratte 1984; Amarasekare & Savage 2011; Régnière et al. 2012). For A. lagerstroemiae, most of its phenology comes from field observations both in the native range and the US (Gu et al. 2014). In China, the number of generations of A. lagerstroemiae increased latitudinally from 2 to 4 generations from 32 to 26 °N (Jiang & Xu 1998; Luo et al. 2000; He et al. 2008; Ma 2011). Despite the importance for the development of phenological models, there is no information on the immature survival and developmental times of A. lagerstroemiae at constant temperatures. By understanding the developmental time of a pest, effective management plans can be developed, such as better timing of insecticide applications, delivering preventive strategies, or releasing biological control agents (Waage et al. 1985; May et al. 1988; Tang et al. 2010).
Understanding the host range of exotic pests is critical to determine potential risks and economic losses (Venette et al. 2010; Zalucki et al. 2012). In Asia and Hungary, A. lagerstroemiae was reported to attack 13 species of ecological and economic importance (Hoy 1963; Hua 2000; Kozar et al. 2013). Some of the reported hosts of this scale are also important crops in the US, including pomegranate, Punica granatum L. (Lythraceae), persimmon, Diospyros kaki Thunb. (Ebenaceae), and edible fig, Ficus carica L. (Moraceae) (USDA NASS 2012). In addition, polyphagous pests including A. lagerstroemiae may expand or shift the host range in the adventive area (Strong 1979). These changes in the host range have been reported for invasive scales in different regions (Hemiptera: Coccoidea) (Cham et al. 2011; Culik et al. 2013; Silva et al. 2017). However, there are no studies in the US of the host range of the population of A. lagerstroemiae.
The purpose of this study was to understand the temperature-dependent development and host range of A. lagerstroemiae. The specific objectives were (1) to assess the effects of temperature on the development and survival of immature stages; and (2) to determine the host range of the scale under no-choice conditions. Temperaturedependent development was evaluated at constant temperatures in the laboratory. Thirteen plant species from 7 families were tested under no-choice conditions. Preventive strategies and improvement of IPM plans for this scale are discussed.
Materials and Methods
INSECT AND PLANT COLONIES
Branches of crapemyrtles infested with different stages of A. lagerstroemiae were collected in Shreveport (32.5500°N, 93.7800°W), Louisiana, USA, from Apr 2016 to Jul 2016. Upon arrival at the laboratory, infested branches were immediately placed in a growth chamber at 25 ± 1 °C with a photoperiod of 12:12 h (L:D). Experiments were conducted 1 or 2 d after the field collection to ensure that the insects were alive.
Crapemyrtles, Lagerstroemiae indica × fauriei ‘Natchez White’ (Lythraceae) in 1 L pots were purchased from local nurseries in Baton Rouge, Louisiana. Other plant species were purchased from local nurseries or were obtained from the Louisiana State University Agricultural Center (LSUAC) Hammond Research Station, Hammond, Louisiana, with container sizes ranging from 1 to 3.8 L. All plants were placed under full sun, fertilized every 3 mo with 14 g of a controlled release fertilizer (OsmocotePlus®, 15N-9P-12K; The Scotts Miracle-Gro Company, Marysville, Ohio, USA), and watered daily.
The immature development and survival of A. lagerstroemiae were examined at 7 constant temperatures (17.5, 20, 22.5, 25, 27.5, 30, and 32.5 ± 1 °C) in environmental growth chambers (Series 101, Percival Scientific®, Perry, Iowa, USA) set at 12:12 h (L:D) photoperiod. Short branches (< 5 cm) containing gravid females were placed inside Petri dishes (9 cm diam) and monitored for the presence of eggs. Recently deposited eggs (< 1 d old) were gently removed using a pin, and transferred to new Petri dishes containing dry filter paper. One Petri dish was assigned to each temperature, and 40 eggs laid by at least 3 females were pooled at each temperature. A single egg was considered a replicate. All Petri dishes were examined daily under a microscope, and the numbers of crawlers were counted and recorded until all eggs had hatched or died.
For nymphal development, 50 newly hatched crawlers (< 1 d old) were inoculated on a potted crapemyrtle plant, and 4 plants (replicates) were used per temperature (20, 25, and 30 ± 1 °C; photoperiod 12:12 h [L:D]). Each infested plant was kept inside a 49 L plastic wastebasket (20 × 30 × 45 cm; MainstaysTM, Kenmore, Virginia, USA) that was modified by removing the plastic material from each of the 4 sides and the bottom, then covering with fine mesh. The fine mesh served to maintain air ventilation and humidity inside the container and prevented the crawlers from escaping. The top of the basket was covered with transparent plastic wrap. Because of the minute size and similar morphology among different A. lagerstroemiae instars (Wang et al. 2016), it was difficult to differentiate each molting during the nymphal stages. However, the presence of white waxy coverings of male prepupa and gravid female was considered in this study to be the end of the nymphal stage for male and female, respectively. Because females produce a white covering when they are ready to lay eggs, the developmental time for female nymphs measured in this study could be overestimated. All plants were examined daily, and individuals with the presence of white coverings were recorded and marked with a permanent marker on the bark. Because most nymphs cannot finish their development at 20 °C, we harvested all plants at 7 mo. For the nymphs that were left on the plant, we confirmed the mortality under a microscope by leg movement. Developmental time and survival per observed life stage were compared among temperatures using 1-way analysis of the variance (ANOVA) in PROC MIXED (SAS Version 9.3; SAS Institute 2011), and the LSMEANS were compared using Tukey's Honestly Significant Difference (HSD) test at α = 0.05.
HOST RANGE TEST
The immature development and reproduction of A. lagerstroemiae reared on different plant species were examined under no-choice conditions. A total of 13 plant species were selected based on 3 criteria: (1) plants previously were reported as hosts (reviewed in Wang et al. 2016), (2) plants are closely related as determined by the centrifugal phylogenetic method (Wapshere 1989), and (3) Callicarpa americana L. (Lamiaceae) that was observed infested with A. lagerstroemiae in the field (Wang et al. 2016) (Table 1). Plant species reported as hosts in Asia and found in the United States were Buxus microphylla Siebold & Zucc. (Buxaceae), Celtis laevigata Willdenow (Combretaceae), Diospyros kaki Thunb. (Ebenaceae), Ficus carica L. (Moraceae), Punica granatum L. (Lythraceae), and Rubus fruticosus L. ‘Kiowa’ (Rosaceae). According to the phylogenetic analysis of Lythraceae (Myrtales) (Graham et al. 2005), another 4 plant species were selected including Cuphea ignea A. DC., Heimia salicifolia Link, Lawsonia inermis L., and Lythrum alatum Pursh. Four plants (replicates) of each species were used in this study, and Lagerstroemiae indica × fauriei ‘Natchez White’ was considered the control. Plants were inoculated by tying infested branches (8-10 cm in length) to the main stem of test plants for 1 wk. Then each plant was placed inside a cage (61 × 61 × 91 cm) (BioQuip? Compton, California, USA) and allowed to grow under greenhouse conditions. Gravid females, recognized by the white ovisacs found on each plant, were counted and recorded every wk for a total of 14 wk. The experiment was conducted from Apr to Oct 2016. Plant species that supported complete life cycle development from egg to adult and the reproduction of adults were defined as host plants of A. lagerstroemiae (Heard 1997). When no gravid females were found after 4 wk of inoculation, plants were re-infested using the same protocol to confirm the non-host status. The total number of gravid females by wk 12 were compared among host plants using 1-way ANOVA in PROC MIXED (SAS Version 9.3; SAS Institute 2011), and the LSMEANS were separated using Tukey's HSD test at α = 0.05.
Plant species as host candidates of Acanthococcus lagerstroemiae used in no-choice tests.
Developmental time differed among temperatures for eggs (F = 1076.0; df = 5,159; P < 0.001; Fig. 1), male nymphs (F = 84.9; df = 2,48; P < 0.001), and female nymphs (F = 350.2; df = 1,65; P < 0.001; Table 2). Mean developmental time for eggs decreased from 36 d at 17.5 °C to 10 d at 27.5 °C, and then increased to 11 d at 30 °C (Fig. 1). Development time from nymph to male prepupa increased from 56 d at 30 °C to 154 d at 20 °C, and the time from nymph to gravid female was 68 and 137 d at 30 and 25 °C, respectively.
Survival was different for eggs (Fig. 1) and nymphs at different temperatures (F = 7.4; df = 2,9; P < 0.01; Table 2). Lower egg survival (≤⃒ 55%) was recorded for temperatures lower than 25 °C, and most eggs hatched (≥⃒ 90%) when the temperature ranged from 25 to 30 °C (Fig. 1). No eggs hatched at 32 °C. For nymphs, the highest survival rate (30%) was found at 25 °C and the lowest (16%) at 20 °C (Table 2).
Results under no-choice conditions indicated that La. inermis, H. salicifolia, P. granatum, Ly. alatum, and C. americana supported nymphal development and reproduction of A. lagerstroemiae. The number of females on all hosts was lower than 100 after the first 6 wk, then increased to different levels (Fig. 2). The number of gravid females at wk 12 also differed among species (F = 8.5; df = 5,19; P < 0.001; Fig. 2). Crapemyrtle (L. indica × fauriei) had the highest number of gravid females (482 ± 92; mean ± SE), followed by C. americana (200 ± 70), and lower numbers (< 150) were obtained on the other 4 plants (Fig. 2). Sooty mold accumulated on all these plant species, and the amount of accumulation varied with the density of A. lagerstroemiae. Branch dieback was reported for L. indica × fauriei and C. americana.
The developmental time and survival for A. lagerstroemiae eggs and nymphs varied among temperatures. The optimum temperature for egg hatching is 27.5 °C, which was determined by the shortest hatching time and highest hatching rate. Constant temperatures below 25 °C resulted in lower egg hatching whereas temperatures above 32 °C led to complete mortality. However, the ovisacs of scales could prevent heat and moisture exchanges and maintain a relatively stable microenvironment inside (Gullan & Kosztarab 1997); thus, air temperature may not represent the best predictor for temperature inside the ovisac. Nymphs of A. lagerstroemiae have a much slower growth rate than other scales. The development from crawler to gravid female was 137 d at 25 °C for A. lagerstroemiae, but 54 d for Pseudaulacaspis pentagona (Targioni-Tozzetti) (Hemiptera: Diaspididae) (Erkiliç & Uygun 1997), 65 d for Hemiberlesia rapax (Comstock) (Hemiptera: Diaspididae) (Blank et al. 2000), and 24 d for Phenacoccus solani Ferris (Hemiptera: Pseudococcidae) (Nakahira & Arakawa 2006). Nymphal survival of A. lagerstroemiae was lower (< 35%) compared to other scales (70-90%), including P. solani (Nakahira & Arakawa 2006), Paracoccus marginatus (Hemiptera: Pseudococcidae) (Amarasekare et al. 2008), and Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae) (Prasad et al. 2012). Another factor that may lead to the slower development and lower survival in this study could be less favorable conditions with artificial light and constant temperatures inside the growth chamber (Colinet et al. 2015).
Mean (± SE) developmental time (d) and nymphal survival (%) of Acanthococcus lagerstroemiae at 3 constant temperatures. Means within each row followed by different letters are significantly different (P < 0.05; Tukey's HSD).
The developmental time of A. lagerstroemiae estimated in this study can be used to understand the phenology of this pest in the field. The time for A. lagerstroemiae to complete 1 generation is about 4 mo at 25 °C and 3.5 mo at 30 °C, and could be shorter at relatively warmer temperatures. Nymphs of A. lagerstroemiae stayed quiescent and did not reach the reproductive stage at constant 20 °C. According to the National Climatic Data Center ( https://www.ncdc.noaa.gov/), the daily average temperatures in subtropical areas such as Louisiana and Texas increase above 20 °C from mid-Apr to Oct and decrease to lower than 20 °C from Oct to mid-Apr, suggesting the potential time of crawler emergence and beginning of overwintering for A. lagerstroemiae. Therefore, A. lagerstroemiae should have more than 2 generations per yr in Louisiana and Texas. The information obtained from this study could help build models combined with data collected by our collaborators to predict the population dynamics in different locations (Yurk & Powell 2010), as demonstrated for the population growth of P. solenopsis (Fand et al. 2014), and for crawler emergence of Unaspis yanonensis Kuwana (Hemiptera: Diaspididae) (Kim & Kim 2013).
Acanthococcus lagerstroemiae is polyphagous, and can develop and reproduce on at least 5 species from different genera and families. Four out of the 5 plant species are phylogenetically related to the crapemyrtle (Lythraceae), but the American beautyberry (C. americana; Lamiales) is relatively distant to Lythraceae phylogenetically (AGP II 2003). Reasons for the polyphagy of A. lagerstroemiae are unknown but one speculation is that these plant species could share somewhat similar plant chemistry (Ehrlich & Murphy 1988; Erbilgin et al. 2014), or simply that A. lagerstroemiae has the adaptations to overcome the chemical defense of plants in multiple families and orders (Dicke 2000; Harrison et al. 2016). The phylogenetic relationship of scales in Acanthococcus (= Eriococcus; Eriococcidae) is still ambiguous (Cook et al. 2002; Kozar et al. 2013), and the host ranges for these scales are poorly investigated. However, several phylogenetically related species to A. lagerstroemiae including Acanthococcus (= Eriococcus) macedoniensis Fetyko & Kaydan, Acanthococcus (= Eriococcus) melnikinensis (Kuwana), and Acanthococcus (= Eriococcus) onukii (Kuwana) were collected from several families and orders of plants (Kozar et al. 2013). Furthermore, the host species of A. lagerstroemiae found in this study are different from reports in Asia, except for pomegranate (P. granatum) (Wang et al. 2016). Considering the potential of a wider host range, additional plant species having been reported to be suitable in the native range, or phylogenetically related to confirmed hosts, should be evaluated.
Prevention should be the primary approach to manage A. lagerstroemiae in nurseries growing potential host plants. Host species of A. lagerstroemiae found in this study are economically and ecologically important. Pomegranate (P. granatum) is a fruit crop produced in 13,309 ha in the US as recorded in 2012 (USDA NASS 2014), with a value of about US $184 million reported in California alone (CDFA 2016). American beautyberry (C. americana) (Wiersema & Leon 2016) and winged loosestrife (L. alatum) (Clute 1901) are important native plants that also are grown as ornamentals in nurseries. Sinicuichi (H. salicifolia) is valued for its medicinal traits (Baxter et al. 2001). Though not commercially planted in the US, henna (L. inermis) is an economically important crop in India and several other countries for its medicinal and cosmetic uses (Kumar et al. 2005; Semwal et al. 2014). On all these host species the density of A. lagerstroemiae increased over time, with injuries appearing, including accumulation of black sooty mold and branch dieback. If not detected and controlled in time, A. lagerstroemiae could exert severe impacts on these plant species. Scouting is recommended for all plants in the host range of A. lagerstroemiae, and immediate responses, such as spraying insecticides or removing infested plants, should be carried out to prevent further spread of this invasive scale (Kim et al. 2006; Zalucki et al. 2012).
In summary, temperature-dependent development of A. lagerstroemiae can help to time the delivery of control tactics on development of population growth models. Five out of 13 plant species chosen from different genera and families were found as suitable host species of A. lagerstroemiae. Inspections in all potential host plants are recommended with appropriate treatments in order to prevent the spread of A. lagerstroemiae and potential economic losses.
We thank Joey P. Quebedeaux and Gina Dimm Hebert of the Louisiana State University Agricultural Center, Hammond Research Station, Hammond, Louisiana, for providing test plants, and Otto Castillo of the Louisiana State University Agricultural Center, Entomology Department, Baton Rouge, Louisiana, for maintaining plant conditions in the greenhouse. This work was funded in part by the National Institute of Food and Agriculture, USDA, under award number 2014-70006-22632, and by the Louisiana State University Department of Entomology.