Abstract: Background and Objective: The purple scale, Lepidosaphes beckii (New.) (Hemiptera: Diaspididae) is considered one of the most important pests of citrus around the world. Biological control represents a sustainable alternative for save control of L. beckii. The present work was conducted to study the factors affecting the biological control of L. beckii by Aphytis lepidosaphes compere (Hymenoptera: Aphelinidae) in Egypt Valencia orange. Materials and Methods: The parasitism rates by A. lepidosaphes were determined throughout two years on the scales of L. beckii which attacks leaves (upper and lower surfaces) and fruits of Valencia orange. The effectiveness of 5 insecticides namely, dinotefuran, imidacloprid, pyriproxyfen, azadirachtin, abamectin also were tested against L. beckii as well as A. lepidosaphes. Results: It was found that all nymphal instars and adult of L. beckii were parasitized by A. lepidosaphes. However, percentage parasitization was higher in the 4th instar males and gravid and non-gravid females compared with the 2nd and 3rd instar of males and 2nd instar of females. Parasitism percentages of L. beckii on fruits were significantly lower than its on the leaves. The differences in the parasitization percentages with regards to the seasons were determined to be statistically significant. The tested insecticides showed high effectiveness against L. beckii and A. lepidosaphes. But a better effect, reflected in high reduction percentages in L. beckii compared with low reduction percentages in the parasite was found for the soil application with dinotefuran, imidacloprid. Conclusion: The host stage preference showed that the parasitoid had a significant preference for the older host stages. The soil application of insecticides was less harmful to the parasite.
INTRODUCTION
Scale insects (Hemiptera: Coccoidea) are sap sucking hemipterous insect pests widely distributed in different worldwide ecosystems. The family Diaspididae is one of the most commonly encountered of scale insects families with the most number of genera (420 genera) of armored scale insects1-3. The adult females of family Diaspididae are wingless and have reduced antennae and legs and are often protected by waxy protective secretions while the adult males haven’t mouthparts and have one pair of delicate wings, well-developed legs and antennae. The purple scale Lepidosaphes beckii (New.) is a cosmopolitan citrus insect pest; it infests citrus in all Mediterranean countries. The purple scale which also known citrus mussel scale attach citrus leaves, fruits and small branches. In addition to the direct injury of L. beckii that causing by sucking the trees parts sap, the presence of this insect on the citrus fruits reduces their commercial value4 and consequently decrease the export demands on citrus fruits. The control of scale insects is still relying essentially on the applications of synthetic insecticides. However, excessive use of insecticides for effective pest control has generated a lot of concerns about the insect pest's resistance for insecticide and also includes effects on the human health5. The use of resident natural enemies either alone or together with other control methods (integrated pest management) is a more sustainable approach to scale insects management. Both parasites and predators have potential to control the densities and alter the individual traits of hosts or prey6. Throughout the world, the Aphytis Genus (Hymenoptera: Aphelinidae) is an important parasite of several species of armored scale insects. The female of Aphytis species deposits its eggs beneath the cover of the scales insects and upon hatching, the larva feeds on the scale (Ectoparasite)7-12. Knowledge of the ecology and behavior of natural enemies of the scale insects is essential for the performance of biological control strategies. Keeping in view the above information, the present experiments were conducted to study the factors affecting the biological control of the purple scale, L. beckii by A. lepidosaphes in Valencia orange trees. These factors included effect of host stage preference; affect the host feeding sources and impact of prevailing climatic factors throughout the different seasons of the year. Also, 5 insecticides were tested for their effectiveness on the parasitism rates of L. beckii by A. lepidosaphes.
MATERIALS AND METHODS
This study was conducted from January, 2018 until December, 2019 at Nubaria district, Beheira Governorate, Egypt in a Valencia orange orchard infested by L. beckii. The trees were 15 years old and spaced 5 by 5 m apart. During the period of this study no chemical treatments were applied and for higher yield, all the horticultural practices were followed as per the package of practices. The orchard was sampled twice a month and the samples which consisted of 50 leaves and 50 fruits (when available) were randomly collected from ten trees, packed in polyethylene bags and transferred directly to the laboratory. Scales on the leaves (in upper and lower surface) and fruits were counted, examined using stereoscopic microscope binocular after turned over the cover and the stage specific parasitism recorded for 2nd instar males, 2nd instar females, prepupal and pupal males, non-gravid and gravid females. Parasitism was indicated by the presence of eggs, larvae, prepupae or pupae of A. lepidosaphes. Also, unparasitised scales for each stage were counted and the data were recorded separately for each of leaves (lower and upper surface) and fruits.
Effect of different insecticides on the purple scale insect and its parasitoides: Five insecticides were tested against the purple scale insect, L. beckii and its parasite, A. lepidosaphes. The used insecticides were dinotefuran, pyriproxyfen, imidacloprid, abamectin and azadirachtin. Details of these insecticides are listed in Table 1. Two methods were used to apply each of dinotefuran and imidacloprid, the first was throughout the soil and the second was the foliar spray as the other three treatments. In foliar spray, to ensure complete coverage of all parts of the tree, the trees were sprayed with a rate of 5-6 L/tree with conventional hand knapsack sprayer.
| Table 1: | Evaluated insecticides with their formulations, common names and rates of application |
The soil insecticides were applied with a rate of 4 L/tree. The specific dose was diluted with four liters of water in a container and drenched around the Valencia orange tree within 20-50 cm at 4 points of the tree trunk (cardinal directions) and then irrigated. Insecticides were distributed in a randomized complete block design (RCBD) in five treated replications and untreated control. Water was used as control treatment. The applied of the tested insecticides was in two dates, the 1st date was set to target 1st nymphal instar (crawlers) of the purple scale and the 2nd to target gravid and non-gravid females. Samples consisted of forty Valencia orange leaves infested with L. beckii were randomly collected from each treatment immediately before spraying and after 1, 2, 3, 4 and 5 weeks of application. After collection, the samples were packed in paper bags, taken to the laboratory and divided into two parts. In the 1st part, the live and dead scales of L. beckii were recorded. Dead scales were visually distinct (shriveled, discolored and can separate easily from the leaves). In the 2nd part, except L. beckii the other scales were removed from the leaves by a needle and the leaves were placed inside a plastic jars (15×20 cm). The jars covered with muslin cloth and tight with rubber bands for securing the emerging parasitoids. The weekly emerging parasitoids from each of the seven treatments and/or control were recorded. The reduction percentages of L. beckii and its parasite, A. lepidosaphes were calculated according to Henderson and Tilton equation13 as following:
where, Cb is the mean of population density of L. beckii or A. lepidosaphes in control plots before treatment, Ta is the mean of population density of L. beckii or A. lepidosaphes in treated plots after treatment, Tb is the mean of population density of L. beckii or A. lepidosaphes in treated plots before treatment and Ca is the mean of population density of L. beckii or A. lepidosaphes in control plots after treatment.
Statistical analysis: Thus, all the gathered data were subjected to the one way analysis of variance (ANOVA) and the mean values compared with LSD test14.
RESULTS
Mean population density of various developmental stages of L. beckii: Densities of all developmental stages of L. beckii on Valencia orange leaves (upper and lower surface) throughout two successive seasons (2018 and 2019) at Nubaria district, Beheira Governorate, Egypt are illustrated in Fig. 1a-b. Statistical analysis of data revealed that during the 1st season the gravid female was the highly dominant stage (26.99 and 28.17% of the total population). The 2nd instar female comes in second place (21.18 and 23.53%), followed by non-gravid female (17.93 and 32.21%), 2nd instar males (16.38 and 13.4%), 4th instar males (pupa) (12.81 and 10.17%) and finally 3rd instar males (pre-pupa) (4.72 and 4.08%) in upper and lower leaf surfaces, respectively. Similar results were obtained in the 2nd season, whereas the general means of gravid female reach 27.29% and 26.11 of the total population in the upper and lower surface of leaves, respectively. The 2nd instar females recoded 23.53 and 16.74% followed by non-gravid females 18.51 and 31.62%, 2nd instar males (17.01 and 13.02%), 4th instar males (pupa) (10.29 and 9.23%) and finally 3rd instar males (pre-pupa) (3.37 and 4.3%).
Parasitoid’s host stage preference: The parasite, A. lepidosaphes oviposited on the various nymphal instars and adults of L. beckii. The mean rates of parasitism by A. lepidosaphes on the adults and various nymphal instars of L. beckii are shown in Fig. 2a-b. There were significant differences between the numbers of parasitized scales on the different stages of L. bekcii (parasitism rates) in both the upper and lower leaf surfaces throughout 2018 and 2019 seasons. However, the highest parasitism percentages were observed on the gravid females of L. bekcii in both upper and lower surfaces. While the lowest rates of parasitism were recorded on the 2nd male and female instars. On the other hand, there was no significant difference in the parasitism percentages between the third and fourth instars of the male nymphs and non-gravid females of L. beckii.
Effect of feeding source of the host, L. beckii on the parasitism rates by A. lepidosaphes: In spite of the significantly increasing presence of scales of L. beckii on the Valencia orange fruits (as a measurement unit) comparing with its on the leaves. The parasitism rates on the scales which attack fruits were significantly lower (p<0.05) than that obtained in case of the scales on leaves. When the L. beckii developmental stages feed on leaves, the parasite, A. lepidosaphes attack significantly more scales than when they were feed on fruits (df = 28, p<0.05, t = 4.465, f = 8.979 for 1st season and t = 7.55, f = 7.759 in the 2nd season). Data of the rates of parasitism on the immature and mature stages of L. beckii by A. lepidosaphes in both of leaves and fruits showed the same trend of the two years of study (Fig. 3a-b).
| Fig. 1(a-b): | General means of different developmental stages of Lepidosaphes beckii per a Valencia orange leaf throughout, (a) 2018 season on upper (F = 38.37, LSD = 0.4521) and lower surface (F = 70.974, LSD = 0.10535) and (b) 2019 season on upper and lower surface (F = 107.794, LSD = 0.0974) |
Means followed by the same letter are not significantly different (p<0.05), small letters for upper surface and capital letters lower surface |
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Effect of the prevailing climatic factors throughout the different seasons of the year: Regarding the rates of parasitism by A. lepidosaphes during the different seasons, results illustrated by Fig. 4 clarify noticeable variations in the parasitism rates from season to another. These differences may be due to the prevailing climatic factors throughout the considered seasons. The rates of parasitism reach maximum in summer season (39.83 and 42.29% in 2018 and 2019, respectively) at 31.74, 27.93 and 24.13 as an average of maximum, mean and minimum temperature of summer of 2018 and 31.37, 27.83 and 24.29 for the summer of 2019. In winter and fall seasons, the general means of parasitism rates significantly decreased to 10.31 and 30.51% in 2018 year and 27.08 and 33.75% in 2019 year, respectively. In general, results of the two consecutive years on the means of parasitism rates reveal that environmental conditions prevailing throughout summer are the most favorable for the development of parasites and its population increase.
| Fig. 2(a-b): | Host stage preference of different life stages of Lepidosaphes beckii parasitized by Aphytis lepidosaphes, (a) 2018 season in upper (F = 22.62, LSD = 9.79) and lower surface (F = 22.78, LSD = 8.85) and (b) 2019 season in upper (F = 55.005, LSD = 7.8828) and lower (F = 54.125, LSD = 7.0251) Valencia orange leaves |
Means followed by the same letter are not significantly different (p<0.05) |
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Effect of different insecticides on L. beckii and its parasite, A. lepidosaphes: The effect of the first spray which achieved in mid-March to target 1st nymphal instar (crawlers) is illustrated in Fig. 5a. Among the foliar insecticide sprays, the overall mean of reduction percentages of L. beckii at five weeks post treatment can be arranged in descending order as follows; imidacloprid (85.36%), pyriproxyfen (80.56%), dinotefuran (69.98%), abamectin (59.33%) and azadirachtin (49.73%). In all treatments, the parasite, A. lepidosaphes was more affected by the tested insecticides than its host (L. beckii) whereas the reduction percentages of A. lepidosaphes were 86.77, 83.9 and 76.52% for imidacloprid, pyriproxyfen and dinotefuran, respectively. The differences in influence were statistically significant in case of abamectin (75.93%) and azadirachtin (72.77%). In contrast, in case of soil applications of imidacloprid and dinotefuran, the purple scale, L. beckii was more affected than its parasite whereas the reduction percentages recorded 66.39 and 56.2%, respectively for L. beckii and 58.42 and 49.82%, respectively for A. lepidosaphes.
| Fig. 3(a-b): | Host stage preference of different life stages of Lepidosaphes beckii parasitized by Aphytis lepidosaphes on Valencia orange fruits throughout two successive seasons, 2018 and 2019, (a) General means of population density of different stages in 2017 (F = 52.247, LSD = 2.0306) and 2018 (F = 23.218, LSD = 2.0421) and (b) Rates of parasitism on different stages in 2017 (F = 49.410, LSD = 1.27495) and 2018 (F = 16.642, LSD = 2.2068) |
Means followed by the same letter are not significantly different (p<0.05) |
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| Fig. 4: | General means of parasitism percentages by Aphytis lepidosaphes on Lepidosaphes beckii during the different seasons of years of 2018 (F = 20.782, LSD = 8.0706) and 2019 (F = 4.908, LSD = 8.8155) |
Means followed by the same letter are not significantly different (p<0.05), small letters for 2018 season and capital letters 2019 |
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| Fig. 5(a-b): | Reduction (%) of the purple scale, (a) Lepidosaphes beckii (F = 26.097, LSD = 7.071) and its parasite, Aphytis lepidosaphes (F = 20.210, LSD = 8.3072) after 1st spray which set to target 1st nymphal instar (crawlers) and (b) Lepidosaphes beckii (F = 20.327, LSD = 7.06975) and its parasite, Aphytis lepidosaphes (F = 10.209, LSD = 10.5475) after 2nd spray which set to target gravid and non-gravid females |
Small letters for L. beckii and capital letters for A. lepidosaphes |
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Considering the extracted general mean of reduction percentages after the 2nd application (achieved in mid-July to target the gravid and non-gravid females) of the different performed insecticides treatment (Fig. 5b). It is noticed that the treatment of imidacloprid was the most effective one in reducing the number of L. beckii and increased their reduction percentages up to 76.01% (foliar spray) and 83.56% (soil application). The treatment with dinotefuran gave a reduction percentage of 73.73% (foliar spray) and 72.94% (soil application) after 35 days post 2nd spray. The growth hormone, pyriproxyfen ranked the third and gave a reduction percentage of 65.96%. On the other hand, application of abamectin and azadirachtin gave the least reduction percentages of L. beckii (55.51 and 51.89%, respectively) (Fig. 5b). The detected effect of applied treatments in the 2nd spray against the parasite, A. lepidosaphes confirmed or revealed again that the foliar spray of imidacloprid and dinotefuran were proved to be the superior treatments achieving reduction (82.32%) and (76.9%) all over the inspection periods post-spraying followed by 72.09, 62.17 and 59.74% reductions for pyriproxyfen, abamectin and azadirachtin, successively. Meanwhile, the soil application treatments with imidacloprid and dinotefuran gave low reduction values of 57.34, 47.93%. The tested insecticides significantly reduced the population of L. beckii for up to 5 weeks after treatment.
DISCUSSION
The results of the present study implied that, the general means of the parasitism rates throughout the 1st year of this study were 30.49 and 27.84%, while during the 2nd year these rates reach to 38.4 and 33.13% in upper and lower Valencia orange leaf surfaces, respectively. The parasitoid had a significant preference for the older host stages. Abdel-Fattah and El-Saadany15 recorded different rates of parasitism throughout the different months of the year reach to 84% in October. Also, from these results, it's obvious that the parasitism percentages in the upper surface of the Valencia orange leave were superior to the lower surface. The results also revealed that, when the L. beckii stages feed on leaves, the parasite, A. lepidosaphes attack significantly more scales (about 5-6 times more) than when they were feed on fruits. These results are in conflict with the observations of Boyero et al.16 who found higher levels of parasitism rates by Aphytis melinus and A. chrysomphali on California red scale, Aonidiella aurantii which recorded on the citrus fruits than in twigs. Environmental conditions particularly climatic factors prevailed during winter and autumn seasons were quite unfavorable for the parasite, A. lepidosaphes. On contrast of these results Hafez et al.17,18 observed the highest percentages of parasitism on L. beckii by immature stages of Aphytis spp. during the winter season followed by spring, summer and finally autumn season while the period from March to August was the appropriate period for emerging the adults of the parasite. Also, these results are slightly different than those of Aly19 who found that the rates of parasitism by A. lepidosaphes on L. beckii were low in summer and increased in autumn season but she didn't noticed the parasite during winter season.
The results also showed that, using the five insecticides (imidacloprid, dinotefuran, pyriproxyfen, abamectin and azadirachtin) in integrated pest management programs (IPM) reduced scale density on leaves but gives adverse side effect on the parasitoid of L. beckii. Meanwhile, use of imidacloprid and dinotefuran as a soil application can eliminate this defect. These results are in harmony with the results of Machigua20 in their study on the calico scale (armored scale) and its natural enemies. They found that the soil application of the systemic insecticides, dinotefuran reduce the density of this scale on the leaves and did not reduce the population of its natural enemies. In this work, when the crawlers was the target (the 1st application in did-March) the insect growth regulator pyriproxyfen ranked the 2nd after imidacloprid. Meanwhile, dinotefuran was in the same rank when the females were the target. Quesada et al.21 tested pyriproxyfen, chlorantraniliprole, spiromesifen and spirotetramat against Chionaspis pinifoliae and Aspidiotus nerii (armored scales) and two species of soft scales Eulecanium cerasorum and Toumeyella pini. They found that the four insecticides were killed armored scales when the crawler stage was the target of application. Also, these results are in inconsistent with the results of Sadof and Sclar22 who found that the soil applications of imidacloprid did not effectively control euonymus scale because armored scales do not feed on plants in the same manner as other sucking insects that are easily killed by this pesticide. Dewer et al.23 tested five insecticides azadirachtin, pyriproxyfen, acetamiprid, emamectin benzoate and summer mineral oil and their mixtures against L. beckii and its parasitoid A. lepidosaphes they found that each of acetamiprid and emamectin benzoate show less reduction percentages to the parasitoid while each of azadirachtin, pyriproxyfen and summer mineral oil have given the highest reduction (%) values for L. beckii and its parasitoid.
CONCLUSION
The main biological control agents of L. beckk are the parasitoid A. lepidosaphes. All nymphal instars and adult females of L. beckii were parasitized by the parasitoid, A. lepidosaphes. The parasitoid had a significant preference for the older host stages. The parasitism percentages of L. beckii which attacks the fruits of Valencia orang were significantly lower than its on the leaves. The tested insecticides (imidacloprid, dinotefuran, pyriproxyfen, abamectin and azadirachtin) showed high effectiveness against L. beckii and its parasite, but a better effect, reflected in high reduction percentages in L. beckii compared with low reduction percentages in the parasite was found for the soil application with dinotefuran, imidacloprid.
SIGNIFICANCE STATEMENT
This study discovers the performance of Aphytis lepidosaphes (Hymenoptera: Aphelinidae) as a biological control agent of Lepidosaphes beckii under field conditions. Also, the study provided that the soil application of systemic insecticides of L. beckii was less harmful to its parasite that can be beneficial for try to maximize the role of biological control of Lepidosaphes beckii in the integrated pest management programs.
ACKNOWLEDGMENT
The author would like to thank Mohammed Mabrouk Ragab Attia lecturer at Plant Protection Department, Faculty of Agriculture, Damanhour University for his efforts during these experiments.