ABSTRACT
Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae) originating from South America is one of the important insect pests that damages storage products and is found on every continent. There is a new interest in using enthomopathogens for microbial control of P. interpunctella as well as other stored product pests. Coccidia as a group of protistan entomopathogens are host specific and their pathogenic effects on the hosts are more pronounced. Although this pathogenic effect results in increased host mortality or higher susceptibility to insecticides, the suppressive potential of coccidia in natural populations has not been adequately studied. In this study, characterization, distribution and occurrence of a coccidian entomopathogen was aim to show its natural suppressing potential in P. interpunctella populations.
During the three years (from 2019 to 2021), a total of 3.432 P. interpunctella samples (2.047 dead and 413 living larvae, 932 adults and 40 pupae) were collected from fourteen populations. After macroscopic examination, suspected samples were dissected in Ringer’s solution and then prepared wet smears including host fat body were examined for presence of coccidian pathogens under a light microscope at a magnification of 400-1000X. The oocysts of the coccidian were measured and photographed using a microscope with a digital camera and soft imaging system.
The pathogen was observed in the fat bodies of the larvae, pupae and adults. Oocysts measured as 29.52±3.32 (25.27- 35.08) μm in diameter and they include 8 sporocysts. Sporocysts measured as 9.11±0.61 (8.90-9.85) μm. Forty-five of 3.432 P. interpunctella larvae, pupae and adults were found to be infected. Coccidian infections have also reached to the levels that can be considered high in some populations, as significant as 29.2%. The infection was observed in the three (21.4%) of the examined fourteen populations.
The coccidian entomopathogen presented in this study is the first Adeleid coccidian record from P. interpunctella populations in Türkiye. The detection of Adelina mesnili Perez (Coccidia: Adeleidae) in at least three populations and the infection rate reaching 29.2 percent, confirms that this pathogen has a considerable effect P. interpunctella populations that cannot be underestimated. Our results confirm that the coccidian pathogen is very effective in the larval stage.
Keywords: Plodia interpunctella, Adelina mesnili, stored product pest, distribution, biological control
INTRODUCTION
Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae) originating from South America is one of the important insect pests that damages storage products and is found on every continent. It is a harmful species in a wide range of products such as dried figs, dried apricots, hazelnuts, raisins, oil seeds, cereals, flour and products, cocoa and spices. P. interpunctella larvae feed both inside and on the surface of the food. The infestation of P. interpunctella causes direct crop loss and economic cost.
There is a new interest in using enthomopathogens for microbial control of P. interpunctella as well as other stored product pests (1,2). Among the entomopathogens, protistan entomopathogens are often prevalent and persistent in natural populations of pest insects. Coccidia as a group of protistan entomopathogens are all intracellular parasites and multiply extensively in the host insect. Most species are host specific. Coccidia carry out extensive and more multiplication cycles in the insect host, so the pathogenic effects on the host are more pronounced (3). Although this pathogenic effect results in increased host mortality or higher susceptibility to insecticides, the suppressive potential of coccidia in natural populations has not been adequately studied. In this study, characterization, distribution and occurrence of a coccidian entomopathogen were studied to show its natural suppressing potential in P. interpunctella populations.
METHODS
Insect Samples
During the three years (from 2019 to 2021), a total of 3.312 P. interpunctella samples (2.032 dead and 413 living larvae, 830 adults and 37 pupae) were collected from warehouses, shops and houses in the fourteen provinces (Ankara, Aydın, Bolu, Denizli, Gaziantep, Isparta, İstanbul, İzmir, Kastamonu, Malatya, Ordu, Samsun, Siirt and Trabzon), widely dispersed geographically in Türkiye (Table 1).
Macroscopic-microscopic Examinations
Insect specimens suspected of having disease were subjected to macroscopic examination. The most common symptoms in larvae were discolored death, slow movement, loss of appetite. After macroscopic examination, suspected samples were dissected in Ringer’s solution and then prepared wet smears including host fat body were examined for presence of coccidian pathogens under a light microscope at a magnification of 400-1000X. When an infection was found, the slides were air-dried and fixed with methanol, then stained with freshly prepared 5% solution of Giemsa stain. They were then washed in running tap water, air-dried and examined under a microscope (4). The oocysts of the coccidian pathogen detected by the light microscopy were measured and photographed using a microscope with a digital camera and soft imaging system.
Statistical Analysis
A chi-square test was used to compare observed results. A p-value less than 0.05 was considered significant.
RESULTS
During the microscopic observations, a coccidian entomopathogen were found in the populations of P. interpunctella in Türkiye. The pathogen was observed in the fat bodies of the larvae, pupae and adults. Polysporocystic oocysts of the pathogen were the evidence of the infection (Figures 1-3). Oocysts measured as 29.52±3.32 (25.27-35.08) µm in diameter and they include 8 sporocysts. Sporocysts measured as 9.11±0.61 (8.90-9.85) µm.
During the study, 3.432 samples of P. interpunctella samples including larvae, adults and pupa were dissected and searched for the coccidian infections in the fourteen localities of Türkiye between the years 2019-2021. Totally, 2.047 dead and 413 living larvae, 932 adults and 40 pupae were examined for the presence of the coccidian pathogen, 45 of 3.432 P. interpunctella larvae, pupae and adults were found to be infected. Total infection occurred as 1.3% for the pathogen (Table 2). On the other hand, the average of coccidian infections for all populations was found as 0.2% for dead larvae, 0.2% for living larvae and 4.2% for adults. Coccidian infections have also reached to the levels that can be considered high in some populations, as significant as 29.2% (Tables 2, 3). The infection was observed in the three (21.4%) of the examined fourteen populations (Figure 4).
DISCUSSION
Light microscopic observations of the fresh and Giemsa-stained preperations indicate that the coccidian pathogen found in P. interpunctella populations belongs to the genus Adelina. The recorded parasite has typical characters of the genus Adelina such as shape and size of oocysts (Figures 1-3), number (3-30) of sporocyst per oocyst and number of sporozoites per sporocyst (3). In the literature there is no coccidian record from P. interpunctella populations in Türkiye. The Coccidian entomopathogen presented in this study is the first Adeleid coccidian record from P. interpunctella populations in Türkiye. Number of sporocyst per oocyst and the host affinity is generally recognized as a valid taxonomic character to discriminate the pathogen at the species level. According to Yaman et al. (2), up to now fifteen species belonging to the genus Adelina have been described from host insects; their distinctive characteristics are shown in Table 4. The oocyst dimension and sporocyst number per oocyst is a good feature for comparison of the fifteen Adelina species from host insects. As seen in Table 4, our coccidian differs from thirteen Adelina species in oocyst size and similar with Adelina mesnili (8 sporocysts per oocysts). The number of sporocyst in an oocyst varies from 6 to 8 with 8 being the most common. Pérez (5) recorded the number of sporocysts in each of A. mesnili as generally 6 to 8, rarely 9 in the original description. The morphological features of the pathogen show similarities with other species of the genus Adelina (Coccidia: Adeleidae) and especially resembles A. mesnili, described in lepidopteran hosts by Pérez (5) and observed in the artificial cultures of P. interpunctella and Ephestia kühniella by Steinhaus (6). Therefore, the coccidian pathogen was identified as the Turkish strain of A. mesnili. A. mesnili found in the present study was observed first in the larvae, pupae and adults of P. interpunctella in Türkiye.
No statistical difference between both dead and living larvae was found. As seen in Table 3, the infection rates in the adults are higher than those in pupae. There is statistically significant difference in the infection levels of adult and pupa of P. interpunctella (Pearson chi-square, p=0.000 <0.05). Coccidians occur naturally in Lepidoptera. So, have been recognized as potential biocontrol agents against Lepidoptera. However, the use of pathogenic protist species as a control agent should be in the early stages of development. At the same time, extensive research is required to be used as a protective agent (7).
There have been several studies on pathogens and parasites of stored-product pests, mainly focused on isolation and characterization of pathogenic microorganisms. A few of them were carried out on the protistan entomopathogens of P. inteerpunctella. Until now, microsporidian pathogens, Nosema plodiae (8,9), Vairimorpha plodia (1,10-12), neogregarine pathogen, Mattesia dispora (13), gregarine pathogen, Leidyana sp. (14), have been studied as microbial pathogen in P. interpunctella. However, there is only one study on the distribution, occurrence and potential of microsporidian entomopathogen, V. plodia in P. interpunctella under natural conditions (1). There is no any study on distribution, occurrence and potential of microbial pathogens about A. mesnili (Coccidia: Adeleidae) in P. interpunctella under the natural conditions. In this study, characterization, distribution and occurrence of the coccidian entomopathogen of P. interpunctella from 14 localities representing all Türkiye between the years 2019-2021 is given in an extensive field study for the first time by confirming its first record from Türkiye and effectiveness on natural populations. A. mesnili was detected in three populations (21.4%).
CONCLUSION
Coccidians occurring naturally in insect pest populations are highly pathogenic for them, therefore they have been considered as potential natural suppressing factor in insect pest populations. The detection of A. mesnili in at least three populations and the infection rate reaching 29.2 percent, confirms that this pathogen has a considerable effect in P. interpuctella populations that cannot be underestimated.
Ethics
Ethics Committee Approval: Ethics committee approval is not required as it is studied on harmful insects.
Informed Consent: N/A.
Peer-review: Externally peer-reviewed.
Authorship Contributions
Concept: M.Y., Design: M.Y., Data Collection or Processing: T.S., Ö.E., Analysis or Interpretation: M.Y., T.S., Ö.E., Literature Search: M.Y., T.S., Ö.E., Writing: M.Y., T.S.
Conflict of Interest: No conflict of interest was declared by the authors.
Financial Disclosure: The study was financially supported with a research project by the Scientific and Technological Research Council of Türkiye (project no: 118O980).
