The Integrated Use of Some Common Agrochemicals and Different Entomopathogenic Nematode Species in The Control of Planococcus citri (Risso, 1813) (Hemiptera: Pseudococcidae)

doi:10.21203/rs.3.rs-3396003/v1

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The Integrated Use of Some Common Agrochemicals and Different Entomopathogenic Nematode Species in The Control of Planococcus citri (Risso, 1813) (Hemiptera: Pseudococcidae)

https://doi.org/10.21203/rs.3.rs-3396003/v1

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EPNs are an important component of integrated pest management(IPM) strategies and understanding their antagonistic and synergistic interactions with other biocontrol options is of crucial importance in promoting their use. In the current study, the antagonistic and synergistic effects of azadirachtin, sulfoxaflor, mineral oil, and thyme oil on the survival and pathogenicity of S. bicornutum, S. carpocapsae, H. indica, and H. bacteriaphora were investigated under controlled conditions(25 ± 1°C). The toxicity of agrochemicals to IJs was evaluated in 12-well plates and the IJs of each EPN species (5000IJs/50µL ddh₂O) were exposed to the recommended field concentrations of agrochemicals. In the pathogenicity tests, the agrochemical and IJs solutions were applied to potato tubers at the concentration of 3000IJs/ml. The results revealed that all tested EPN species were quite compatible with azadirachtin, sulfoxaflor, mineral oil, and thyme oil and showed survival over 95% after 72 hours of exposure to tested agrochemicals. EPNs and mineral oil combination induced the highest efficacy on the Planococcus citri (Risso, 1813) (Hemiptera: Pseudococcidae) adults and all EPN species caused mortalities over 90% 72 hours after treatment except for S. bicornotum and mineral oil combination. The synergistic effect was observed in all combinations of EPNs with agrochemicals except for thyme oil. The results indicate that EPNs applied in combination with azadirachtin, sulfoxaflor, and mineral oil could provide more chances for successful control of P. citri. However, further studies are required to determine the antagonistic and synergistic effects of tested chemicals on EPNs in field conditions.

Agrochemicals

beneficial nematodes

IPM

compatibility

sustainability

Citrus cultivation holds particular importance for the growers in the Mediterranean region of Türkiye due to the suitability of climatic conditions and the economic value of citrus fruits. The profitability of the cultivation greatly depends on proper management strategies of citrus pests and diseases. Of various insect pests afflicting citrus orchards, the citrus mealybug, Planococcus citri (Risso, 1813) (Hemiptera: Pseudococcidae), is considered one of the most problematic pests for citrus producers (Franco et al., 2004; Karacaoğlu and Satar, 2017). Newly emerged nymphs and adult females of P. citri are able to feed on all above-ground plant parts particularly on leaves and immature fruits which subsequently lead to deformation of leaves, general wilting, and early drop of the fruits (Urbaneja et al., 2020). Besides direct feeding damages, they also trigger the development of sooty mold which blocks sunlight and limits the photosynthesis capacity of the plants. Heavy infestations generally result in significant yield losses and a decrease in the commercial value of the fruits (Franco et al., 2004; Sirisena et al., 2013; Mahmood al., 2014; Mansour et al., 2017).

Recently there has been a notable increase in the population of P. citri in the Mediterranean region of Türkiye and considerable yield losses occurred in these outbreaks (Karacaoğlu and Satar, 2017; Özgökçe at al., 2021). The suppression of P. citri populations with insecticides is quite challenging due to their small size and cryptic living habits, protective wax covering, and resistance to many insecticide groups (Rao et al., 2006; Venkatesan et al., 2016; Mruthunjayaswamy et al., 2019). Therefore, growers are forced to make multiple applications during the growing season and exceed the recommended application concentrations. However, overuse of insecticides leads to pesticide residues on citrus fruits which generally give rise to the violation of legally established MRL (maximum residue level) limits (Sağlam and Masatcioğlu, 2020). In addition, indiscriminate use of insecticides poses a serious threat to farmers, consumers, and has an adverse effect on the natural enemies of P. citri which may be another reason behind the frequent outbreaks of P. citri (Franco et al., 2004; van Lexmond et al., 2015). Hence, it is essential to minimize the use of insecticides in the control of P. citri by combining chemicals with other management methods such as biological control.

Of many attempts that have been made for sustainable control of P. citri, entomopathogenic nematodes (EPNs) have shown great success in a number of studies (Demirci et al., 2011; van Niekerk and Malan, 2012; Negrisoli et al., 2013; van Niekerk and Malan, 2014; Najm et al., 2022). Infective juveniles (IJs) of EPNs are quite effective biological control agents of many economically important insect pests under favorable conditions and some EPN species have active host-seeking abilities to locate and kill the target host (Shapiro-Ilan et al., 2017; Zhang et al., 2021). They are also compatible with many synthetic chemicals and biopesticides and can be utilized concurrently to achieve a cost-effective and labor-saving control (Vashisth et al., 2013 Ulu et al., 2016). Most of the studies that have examined the effects of the insecticides and biopesticides on the survival and pathogenicity of IJs have demonstrated that a combination of IJs and insecticides can be used successfully (Negrisoli et al., 2010; Laznik and Trdan, 2014; Yüksel et al., 2019). However, the effectiveness of insecticides and biopesticides on the survival and pathogenicity of IJs may vary greatly according to EPN species/strains, chemicals, doses, and exposure time (Sabino et al., 2014; Khan et al., 2018). Therefore, it is crucial to determine the efficacy of different chemical compounds on the survival and virulence of IJs under laboratory conditions before initiating the field studies. In this sense, this study aimed to evaluate the integrated use of different EPN species and some common agrochemicals (insecticide, biopesticide, and mineral oils) in the control of P. citri.

The Source of Entomopathogenic nematodes and Planococcus citri

Four EPN species (S. carpocapsae E-76, S. bicornotum MGZ-4S, H. bacteriophora FLH-4H, and H. indica 216H) obtained in earlier studies were included in the experiments (Canhilal et al., 2016, 2017) (Table 1). Entomopathogenic nematodes were cultured on the last larval instar of Galleria mellonella (L.) (Lepidoptera: Pyralidae) in a growth chamber at 25 ± 2°C, 60% RH. Aqueous suspension of IJs (1000 IJs/ml) was pipetted into Petri dishes containing two filter papers at the bottom and sealed with parafilm. The Petri dishes were kept at 25 ± 2°C, 60% RH in dark conditions, and dead larvae were relocated to modified White traps after 72 h after treatment (McMullen and Stock, 2014). Emerged IJs were harvested, washed with sterile water and transferred to culture flasks (250 ml). The flasks were kept horizontally at 14°C until used for experimental treatments.

The laboratory culture of P. citri was initiated with the population obtained from the Department of Plant Protection, Faculty of Agriculture, Erciyes University and fed on potatoes sprouts (Solanum tuberosum L.) in plastic cages (60x60x60 cm) (Demirci et al., 2011). The cages were kept at 25 ± 2^oC, 60 ± 10% RH, and 16-h light period conditions.

Table 1

List of EPN species of entomopathogenic nematodes used in the experiments.
Entomopathogenic nematodes species	Strain	Habitat	Locality	GenBank Accession Number
S. bicornutum	MGZ-4S	Strawberry	Melikgazi-Kayseri	KX462912
S. carpocapsae	E-76	Grassland	Melikgazi-Kayseri	KX462907
Heterorhabditis indica	216-H	Olive	Karatas-Kahramanmaraş	KP970842
H. bacteriaphora	FLH-4H	Orchard	Felahiye-Kayseri	KX462939

Galleria mellonella

Larvae were originally obtained from the stock culture of Erciyes University (Department of Entomology) and kept in autoclaved glass jars (20 cm high and 10 cm in diameter) containing an artificial diet composed of Wheat and corn flour, milk powder, yeast powder, honey, and glycerin. The jars were covered with plastic wire meshes to allow ventilation and maintained in a growth chamber at 30 ± 2°C, 60% RH (Metwally et al., 2008). After 4 weeks of the incubation period, the pupae were gently collected by hand and transferred to new jars containing filter papers. The hatched adults were supported with a honey-water (5%) solution daily as food. The eggs laid by the females on filter papers were collected daily and put into new jars.

Agrochemicals

The most commonly used agrochemicals in the control of P. citri were included in the experiments (Table 2). To test the efficacy of agrochemicals on the survival and virulence of EPNs, recommended field doses were used.

Table 2

Agrochemicals used in the experiment.
Active ingredient	Trade name	Recomended Dose	Formulation	Company
Mineral oil 850 g/l	Protective Summer Oil	1250 mL/100 lt	SL	Koruma Klor Alkali Inc.
Sulfoxaflor 240 g/l	BREAKER™ 240 SC	20 mL/100 lt	SC	Dow Agroscıences LTD.
Azadirachtin 10 g/l	NeemAzal T/S	1000 mL/100 lt	EC	Trifolio-M Gmbh Ltd.
Thyme oil (Pure)	Thyme oil	3 mL/100 lt	EC	Doğan Baharatçılık Inc.

Experimental Design

Survival tests

The toxicity of agrochemicals to IJs was tested in 12-well plates. Aqueous suspension of IJs of each EPN species (5000 IJs/50 µL ddh₂O) was exposed to the recommended field concentrations of agrochemicals in 2 mL of double distilled water (ddh₂O) (Table 2). Controls treatments were performed only with ddh₂O. The well plates were covered and put in an orbital shaker-incubator (25 ± 2°C, 120 rpm) to prevent the settling down of agrochemicals at the bottom. The survival of IJs was evaluated after 24, 48, and 72 hours of exposure to agrochemicals. After each exposure time, 0.1 mL suspension retreated and 100 IJs were examined under the stereo microscope (Zeiss, Germany). Motionless IJs that did not respond to gentle prodding with a single hair brush were considered dead. The experiment consisted of 4 replicates for each of the agrochemicals and the bioassays were conducted twice.

Pathogenicity Bioassays

The infectivity of H. bacteriophora, H. indica, S. carpocapsae, and S. feltiae in combination with agrochemicals was tested against the adult female of P. citri under controlled conditions. The bioassays were performed in plastic containers (10x8x8cm) containing one filter paper at the bottom. Twenty adults were transferred gently to the upper side of sprouted potatoes using a soft camel hair brush. Then, potatoes were put into the containers, and mealybugs were allowed to feed on the tubers for 4 hours prior to treatments. The agrochemicals and nematode solutions were applied to potato tubers by using a mini spray gun (Nozzle size 1 mm) at the concentration of 3000 IJs/ml agrochemicals-nematode solutions. Each container was treated with a 2 ml nematode-agrochemical solution. Then, the containers were covered with a perforated lid to allow airflow and maintained at 25 ± 1°C, 60 ± 10% RH, and 12 hours of light/dark photoperiods. The mortality of adults was checked daily for three days. In control treatments, ddh2O water was used instead of agrochemicals, and the rest of the procedure was repeated. Each treatment consisted of 4 plastic containers (4 repetitions) and the bioassays were conducted twice.

Statistical analysis

No mortality occurred in the control treatments of both bioassays (Survival and pathogenicity) during the experiment. Data from both experiments were subjected to normality tests and arcsine transformed. All statistical analyses were performed with SPSS software (22.0). Repeated measure ANOVA was performed to detect differences between treatments. Post hoc multiple comparisons were conducted using Tukey’s test (P ≤ 0.05).

Survival of IJs

All treatments and their interactions had a significant effect on the survival of IJs (Table 3). The survival of IJs of tested EPN species was above 92% at all treatments during the experiment. The mortality of IJs did not exceed 5% after 24 and 48 hours of exposure to agrochemicals. However, the survival of the IJs of S. carpocapsae exposed to azadirachtin and mineral oil dropped to 93.6 and 92.8% 72 hours after treatment, respectively. Similarly, thyme oil induced 6% mortality on the IJs of S. bicornotum 72 hours after treatment (Table 4).

Table 3

Repeated measures analysis of variance (RM-ANOVA) parameters for the main factors and their associated interactions for the mortality of infective juveniles after 24, 48, and 72 hours of exposure to different agrochemicals (P ≤ 0.05).
Sources	Df*	F-value	P-value
Nematode species (N)	3	16.954	< 0.001
Agrochemicals (A)	3	8.361	< 0.001
N*A	9	12.223	< 0.001
Error-1	48
Exposure time (t)	2	245.193	< 0.001
t*N	6	23.933	< 0.001
t*A	6	2.621	0.021
tNC	18	9.205	< 0.001
Error-2	96
*Degree of freedom

Table 4

Survival rates of entomopathogenic nematodes after 24, 48, and 72 hours of exposure to different agrochemicals.
EPN Species	Exposure time	Survival rates of Infective Juveniles (%±SE)
EPN Species	Exposure time	Azadirachtin	Sulfoxaflor	Mineral oil	Thyme oil
Steinernema carpocapsae	24 hours	99.6 ± 0.5A^aa^b	99.1 ± 0.4Aa	99.2 ± 0.5Aa	99.0 ± 0.6Aa
Steinernema bicornotum		98.3 ± 0.5Aab	98.7 ± 0.4Aa	98.2 ± 0.4Aa	96.8 ± 0.3Bb
Heterorhabditis indica		97.6 ± 0.2Ab	100.0 ± 0.0Ba	99.2 ± 0.9Ba	98.4 ± 0.7ABa
Heterorhabditis bacteriophora		99.1 ± 0.4Aa	98.6 ± 0.4Aa	98.8 ± 0.3Aa	99.6 ± 0.2Aa
Steinernema carpocapsae	48 hours	99.2 ± 0.2Aa	98.5 ± 0.8Aa	98.6 ± 0.4Aa	98.8 ± 0.5Aa
Steinernema bicornotum		95.7 ± 0.8Ab	98.1 ± 0.3Ba	97.7 ± 0.1Ba	96.4 ± 0.3Ab
Heterorhabditis indica		97.5 ± 0.2Aab	98.6 ± 1.1Aa	98.0 ± 0.3Aa	97.5 ± 0.8Aa
Heterorhabditis bacteriophora		98.8 ± 0.2Aa	98.2 ± 0.4Aa	97.4 ± 0.5Ba	99.2 ± 0.3Aa
Steinernema carpocapsae	72 hours	93.6 ± 0.4Aa	96.1 ± 1.5Ba	92.8 ± 1.9Aa	97.9 ± 0.1Ba
Steinernema bicornotum		95.4 ± 0.5Ab	97.6 ± 0.3Ba	97.4 ± 0.3Bb	94.0 ± 1.1Ab
Heterorhabditis indica		96.9 ± 0.2Ab	97.9 ± 1.4ABa	98.0 ± 0.3Bb	96.8 ± 0.4Aa
Heterorhabditis bacteriophora		97.4 ± 1.3Ab	97.1 ± 1.8Aa	96.0 ± 0.2Ab	96.6 ± 0.8Aa

^aCapital letters indicate statistical differences among agrochemicals for each entomopathogenic nematode species. ^bLowercase letters indicate statistical differences among entomopathogenic nematode species for each agrochemical (Tukey, P ≤ 0.05).

Effect of Agrochemicals on Pathogenicity

The mortality of P. citri Risso (Hemiptera: Pseudococcidae) was affected by all main factors and their associated interactions (Table 5). A time-dependent increase in mortality rates was observed at all combinations of IJs and agrochemicals. In general, the highest efficacies were obtained from mineral oil combinations of IJs for all EPN species, except for S. carpocapsae, which performed the best 24 hours after treatment when combined with Sulfoxaflor. Thyme oil had an antagonistic effect on the infectivity of H. bacteriophora and S. bicornotum and thyme oil combinations of IJs of these EPN species showed poor efficacy on the P. citri at all exposure times.

Table 5

Repeated measures analysis of variance (RM-ANOVA) parameters for the main factors and their associated interactions for the mortality of *Planococcus citri* Risso (Hemiptera: Pseudococcidae) 24, 48, and 72 hours of exposure to different entomopathogenic nematodes in combination with agrochemicals.
Sources	Df*	F-value	P-value
Nematode species (N)	3	9.070	< 0.001
Agrochemicals (A)	3	8.361	< 0.001
N*A	9	3.812	< 0.001
Error-1	48
Exposure time (t)	2	1348.625	< 0.001
t*N	6	8.797	< 0.001
t*A	6	9.434	< 0.001
tNA	18	2.794	< 0.001
Error-2	96
*Degree of freedom

The highest mortality (61%) was achieved by H. bacteriophora and mineral oil combination 24 hours after treatment (Table 6). Mineral oil combinations of Heterorhabditis species induced remarkable mortality on the P. citri adults 48 hours after treatment compared to Steinernema species and mortalities over 97% were recorded. A similar trend was observed in Azadirachtin combinations of H. bacteriophora and H. indica 72 hours after treatment. The highest synergistic interaction of Sulfoxaflor was recorded for S. carpacapsae 72 hours after treatment and exhibited 80% mortality in combination with S. carpocapsae. Thyme oil showed a synergistic effect only in conjunction with H. indica after 72 hours of treatment and induced 72% mortality (Table 6).

Table 6

The mortality rates (% ± SE) of *Planococcus citri* Risso (Hemiptera: Pseudococcidae) 24, 48, and 72 hours of exposure to different entomopathogenic nematodes in combination with agrochemicals.
Treatments*	Mortality rates (%±SE)
	Exposure time
	24 h	48 h	72 h
Sc	8.75 ± 4.8A^aa^b	41.25 ± 8.5Ba	60 ± 10.0Ca
Sc + Aza	26.25 ± 4.8Ab	57.5 ± 6.5Bab	71.25 ± 9.4Cab
Sc + Sulf	52.5 ± 16.6Ac	65 ± 14.7Aab	80 ± 10.8Bab
Sc + Mo	46.25 ± 8.5Ac	77.5 ± 8.7Bb	92.5 ± 11.9Cb
Sc + To	10 ± 4.1Aa	38.75 ± 8.5Ba	60 ± 7.1Ca
Sb	8.75 ± 10.3Aa	28.75 ± 8.5Ba	45 ± 10.8Ca
Sb + Aza	35 ± 7.1Ab	66.25 ± 8.5Bb	76.25 ± 8.5Bb
Sb + Sulf	13.75 ± 4.8Aa	41.25 ± 4.8Bab	65 ± 7.1Cab
Sb + Mo	40 ± 14.7Ab	71.25 ± 8.5Bb	80 ± 5.8Bb
Sb + To	1.25 ± 2.5Aa	21.25 ± 12.5Ba	43.75 ± 12.5Ca
Hi	12.5 ± 11.9Aa	36.25 ± 11.8Ba	57.5 ± 16.6Ca
Hi + Aza	21.25 ± 6.3Ab	66.25 ± 4.8Bb	82.5 ± 8.7Cab
Hi + Sulf	22.5 ± 2.9Ab	65 ± 10.8Bb	75 ± 10.0Ca
Hi + Mo	45 ± 7.1Ac	97.5 ± 5.0Bc	98.75 ± 2.5Bb
Hi + To	6.25 ± 4.8Aa	38.75 ± 8.5Ba	72.5 ± 8.7Ca
Hb	20 ± 8.2Ab	42.5 ± 11.9Bb	61.25 ± 14.4Ba
Hb + Aza	43.75 ± 14.9Abc	78.75 ± 6.3Bc	87.5 ± 5.0Bb
Hb + Sulf	28.75 ± 8.5Ab	51.25 ± 11.8Bb	71.25 ± 8.5Cab
Hb + Mo	61.25 ± 10.3Ac	98.75 ± 2.5Bd	98.75 ± 2.5Bb
Hb + To	2.5 ± 2.9Aa	16.25 ± 11.1Aa	46.25 ± 9.5Ba

*Sc: Steinernema carpocapsae; Sb: Steinernema bicornotum; Hi: Heterorhabditis indica; Hb: Heterorhabditis bacteriophora. Aza: Azadirachtin; Sulf: Sulfoxaflor; Mo: Mineral oil; To: Thyme oil. ^aCapital letters indicate statistical differences among exposure times for each treatment. ^bLowercase letters indicate statistical differences among different treatments for each exposure time (Tukey, P ≤ 0.05).

The sole activity of agrochemicals on the P. citri was also investigated in this study. Results demonstrated that mineral oil applications had superior activity on the P. citri compared to other tested agrochemicals. Mineral oil applications provided 56% mortality 72 hours after treatment and followed by Sulfoxaflor with 35% mortality. Thyme oil showed the lowest efficacy at all exposure times and the mortality did not exceed 15% 72 hours after treatment (Table 7).

Table 7

The mortality rates (% ± SE) of *Planococcus citri* Risso (Hemiptera: Pseudococcidae) 24, 48, and 72 hours of exposure to different agrochemicals
Treatments	Mortality rates (%±SE)
	Exposure time
	24 h	48 h	72 h
Azadirachtin	5.0 ± 5.7ab^a	15.0 ± 9.1b	30.0 ± 11.5ab
Sulfoxaflor	10.0 ± 4.0ab	26.2 ± 8.5c	35.0 ± 10.8ab
Mineral oil	21.2 ± 2.5b	38.7 ± 7.5c	56.2 ± 10.3b
Thyme oil	1.2 ± 2.5a	1.2 ± 2.5a	15.0 ± 10.8a

^a Lowercase letters indicate statistical differences among different treatments for each exposure time (Tukey, P ≤ 0.05).

The current study investigated the compatibility of different EPN species with the main agrochemicals used in the control of P. citri. The results revealed that the IJs exposed to tested agrochemicals showed survival of over 90% 72 hours after treatment. These results are in agreement with the findings of Krishnayyaand and Grewal (2002) who reported 84% IJ survival of S. feltiae after 120 hours of exposure to Neem plant extract. Similarly, Mahmoud et al. (2016) reported a 99% survival rate for the IJs of S. carpocapsae and H. bacteriophora that were exposed to neem products. Sulfoxaflor is a potent insecticide that causes neurotoxic damage to targeted insects (Sparks et al., 2013). In a previous study, Sulfoxaflor exhibited the lowest toxicity to soil-dwelling roundworm, Caenorhabditis elegans among tested insecticides and did not induce significant lethality which confirms our findings (Zhou et al., 2022). However, Laznik and Trdan (2014) found a high degree of variation in the susceptibility of different strains of EPN species to tested chemicals including azadirachtin. Barua et al. (2020) reported that thyme oil at 1% concentration was highly toxic to S. feltiae and H. bacteriophora juveniles and caused 100% mortality. Consistencies in the experimental design are also an important factor that affects the survival of EPNs. For instance, Barua et al. (2020) evaluated the survival of EPNs at 1% thyme oil concentration which is quite high compared to the concentration used in the present study. The tolerance of nematodes to agrochemicals may vary according to species/strains, chemicals, exposure time, environmental conditions, and application concentrations (Wang et al., 2010; Raheel et al. 2017; Özdemir et al., 2020). Therefore, determining the compatibility of biological control agents with agrochemicals is of crucial importance before initiating field studies. In the present study, agrochemicals did not cause significant mortality on the IJs of tested EPN species. The lethality of EPN species in combination with agrochemicals was also evaluated on the P. citri in the current study. The results revealed that the mineral oil combination of EPNs generally provided the highest efficacy on P. citri. This is probably due to the protective effect of mineral oils against desiccation. EPNs are susceptible organisms to desiccation and the viability and persistence of EPNs are adversely affected by desiccation, especially on exposed surfaces (Shapiro-Ilan et al., 2014; Ulu and Susurluk, 2014). Mineral oils act as a barrier to help prevent moisture loss, thus increasing the survival chance of EPNs post-treatment. Additionally, mineral oil may have provided a film layer for the IJs enhancing their host-seeking ability and tolerance to environmental extremes. Similar to our findings, Aquino-Bolaños et al. (2019) evaluated the effectiveness of S. carpocapsae, S. websteri, and H. bacteriophora in oil emulsions on adult ticks and reported mortalities ranging between 80 and 100% 48 hours after treatment. However, EPN species/isolates exhibit a significant adaptive genetic variation in their susceptibility to environmental extremes as suggested by Ulu and Susurluk (2014) and Somvanshi et al. (2008). The differences in adaptation capability and morphological characteristics may also have had an impact on the pathogenicity of EPNs. Timper and Kaya reported that Heterorhabditis species retained their J2 cuticles for a more extended period of time than Steinernema species which might have provided advantages to Heterorhabditis species in our study to tolerate environmental extremes. Another variable that may have led to the difference in the mortality rates may be the endosymbionts of EPNs. The bacterial associates of EPN species play a pivotal role in the pathogenicity of IJs by producing various secondary metabolites with insecticidal and immunosuppressant activities (Joyce et al., 2006; Hinchliffe et al., 2010). However, earlier studies revealed that biochemical compounds released by endosymbionts of EPNs vary greatly among bacteria species/ strains and have varying degrees of toxicity against insect pests (Darsouei and Karimi, 2018; Hasan et al., 2019; Vicente-Díez et al., 2021).

In the present study, tested agrochemicals exhibited low to moderate insecticidal activity against P. citri. Thyme oil showed the poorest efficacy (15%) among them. Similar to our results, Szczepanik et al. (2012) reported the poor acute toxicity of thyme oil, and mortality rates of Alphitobius diaperinus Panzer (Coleoptera: Tenebrionidae) did not exceed 10% after 3-days. Raphael et al. (2011) reported a 43% mortality in P. citri after 8 days of exposure to seed kernel extract of Azadirachta indica. Phytochemicals are a complex mixture and consist of different biochemical compounds that act as an endocrine disruptor, suffocant and repellent (Odeyemi et al., 2008). To be effective, these compounds may need to be absorbed or ingested by the target pest. However, in this study, results revealed that the contact efficacy of thyme oil and azadirachtin remained limited probably due to the protective coating of P. citri. In addition, the presence of developed mechanisms of detoxication in P. citri populations was documented in earlier studies which may be the reason behind the low levels of activity of azadirachtin and thyme oil (Venkatesan et al., 2016).

The results demonstrated that agrochemicals had a negligible effect on the survival of S. bicornutum, S. carpocapsae, H. indica, and H. bacteriaphora and no significant mortality occurred on the IJs of EPNs. The combinations of EPNs with agrochemicals provided better control against P. citri adults compared to separate applications of EPNs and agrochemicals except for thyme oil. The highest synergistic effect was observed when EPNs were applied in combination with mineral oil. However, further studies are needed to confirm their antagonistic and synergistic interaction with these agrochemicals in field conditions.

EPNs

Entomopathogenic nematodes

IJs

Infective juveniles

Acknowledgements

This study was supported by Scientific Research Projects Unit of Erciyes University (FYL-2020-10167).

Ethics approval and consent to participate

Not applicable.

Consent for publication

This study does not contain any individual person’s data.

Competing interests

The authors have no competing interests.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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The Integrated Use of Some Common Agrochemicals and Different Entomopathogenic Nematode Species in The Control of Planococcus citri (Risso, 1813) (Hemiptera: Pseudococcidae)

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Abstract

Figures

Introduction

Materials and Methods

Galleria mellonella

Agrochemicals

Experimental Design

Survival tests

Pathogenicity Bioassays

Statistical analysis

Results

Survival of IJs

Effect of Agrochemicals on Pathogenicity

Discussion

Conclusion

Abbreviations

Declarations

References

Status:

Version 1