Influence of foliar fertilizers on the development and reproduction of the citrus red mite, Panonychus citri (Acari: Tetranychidae), and the growth of citrus seedlings

LIU Zhe, George Andrew Charles BEATTIE,CEN Yi-Jing, XU Chang-Bao

(1. Laboratory of Insect Ecology of South China Agricultural University/Key Laboratory of Bio-Pesticide Innovation and Application of Guangdong Province, Guangzhou 510642, China; 2. College of Horticulture and Plant Protection,Yangzhou University, Yangzhou, Jiangsu 225009, China; 3. School of Science,Western Sydney University, Penrith, NSW 2751, Australia)

Abstract: 【Aim】 The citrus red mite, Panonychus citri, is an important pest of citrus in China where foliar fertilizers are commonly used in citrus orchards. The objective of this research is to assess the influence of two fertilizers, urea and compound amino acid, on the development and reproduction of this mite, and on the growth of citrus seedlings. 【Methods】 Two foliar fertilizers, urea (0.50%) and compound amino acid (0.17%), were applied separately to potted citrus (Citrus reticulata Blanco cv. Shatangju) seedlings in the laboratory, and the seedlings sprayed with water were used as the control. The life-table parameters [the net reproductive rate (R0), mean generation time (T), intrinsic rate of increase (rm), finite rate of increase (λ) and population trend index (I)] of P. citri, the growth parameters (leaf length, width and area; stem length; plant height; number and length of new shoot leaves) of citrus seedlings and the nutrient (N, P and K) contents in leaves of citrus seedlings were investigated under controlled environment conditions. 【Results】 The duration of immature stages of P. citri was not influenced by the fertilizers, but the survival rate of deutonymphs on citrus seedlings applied with 0.50% urea (95.40%) was significantly higher than those on the control citrus seedlings (applied with water) (78.26%) and on citrus seedlings applied with 0.17% compound amino acid (75.61%). The fecundity of female mites reared on urea treated citrus seedlings (42.1/♀) was also significantly higher than that of females reared on the control citrus seedlings (33.1/♀). The longevity of females reared on compound amino acid treated citrus seedlings (19.5 d) was significantly longer than that of females reared on urea treated citrus seedlings (14.8 d) and the control citrus seedlings (14.5 d), and the longevity of males reared on compound amino acid treated citrus seedlings (17.6 d) was significantly longer than that of males reared on the control citrus seedlings (13.1 d). Overall, P. citri reared on urea treated citrus seedlings had the highest net reproduction rate (Ro)(17.88) and population trend index (I)(18.08), both significantly higher than that reared on the control citrus seedlings (10.08 and 11.17, respectively). Leaf growth (leaf length, width and area) of citrus seedlings was significantly promoted by both fertilizers. Both fertilizers also significantly increased N, P and K contents, and the ratio of N/K in leaves of citrus seedlings. 【Conclusion】 Urea and compound amino acid as foliar fertilizer both promote the growth of citrus seedlings. Compound amino acid should be recommended in preference to urea because spraying of urea increases P. citri populations in contrast to compound amino acid. However, P. citri population should be also monitored because that although compound amino acid does not facilitate population growth, it does increase the longevity of adult mites.

Key words: Panonychus citri; spider mite; foliar fertilizer; urea; compound amino acid; citrus growth

1 INTRODUCTION

Many factors, such as temperature, photoperiod and plant nutrition influence the population dynamics of insects and mites (Xiaetal., 2009; Xiaetal., 2012; Zouetal., 2016; Liuetal., 2019a). Fertilization affects the adaptability of phytophagous insects and mites to different plants or different parts of the same plant by changing the content and proportion of plant nutrients and non-nutritive secondary metabolites, and further affects the host selection, development, survival, reproductive ability and population growth of herbivorous insects and mites (Bernays and Chapman, 1994; Awmack and Leather, 2002; Luetal., 2008; Liuetal., 2019a, 2019b). The variations in phytochemistry are related to a variety of factors, such as plant species or varieties, location, environment, health status, growth phase, pests and diseases, water management, and fertilizer use (Bernays and Chapman, 1994). As soil fertilization, application of foliar fertilizers can also change the relationships between phytophagous arthropods and host plants by altering the nutritional status and growth of plants (Pang and Dong, 2013; Luoetal., 2017; Liuetal., 2020).

Foliar fertilizer sprays have been widely used on many crops and trees as an effective supplement or alternatives to soil fertilization (Reuveni and Reuveni, 1998; Yangetal., 2001; Tejadaetal., 2008; Lietal., 2017). They have become an important part of modern agricultural fertilizer use because of their effectiveness, high nutrient utilization rate, and low levels of environmental pollution. Foliar fertilizers such as urea also have reduced the severity of certain diseases (Goodingetal., 1988; Peltonenetal., 1991), which may result in a yield benefit. Additionally, application of foliar fertilizers provides opportunities to apply compatible agrochemicals in the same operation as tank-mixes allowing savings in labour, machinery and energy costs (Gooding and Davies, 1992). However, foliar nutrition may also affect the abundance of the pests. It is known that foliar N influences the development and reproduction of phytophagous arthropods (van Emden, 1966; Minkenberg and van Lenteren, 1986; Bernays and Chapman, 1994; Facknath and Lalljee, 2005; Lengetal., 2008; Pang and Dong, 2012; Zanardietal., 2015).

The citrus red mite,Panonychuscitri(Acari: Tetranychidae), is an important and widely distributed citrus pest that may have originated in Asia (Kennettetal., 1999; Midgeon and Dorkeld, 2015). It occurs in all citrus producing provinces in China (Lietal., 1997), where it became a major pest after use of synthetic pesticides against other citrus pests commenced in the 1950s (Chenetal., 1980; Lietal., 2011). Immature motile stages and adults of the citrus red mite insert their stylets through slits of the surface of the susceptible tissues. The stylets are then used to suck up cell contents. This leads to stippling of plant surfaces (Keetch, 1971). Feeding occurs primarily on citrus leaves and fruits, and occasionally on green twigs (Kennettetal., 1999). Maturing leaves are preferred, and most damage occurs on adaxial leaf surfaces (Keetch, 1971). Intense feeding leads to bronzing or silvering of leaves and green fruits (Kennettetal., 1999). Severe injury may cause defoliation, fruit drop, and twig and branch dieback (Keetch, 1971). Chinese citrus farmers used to apply pesticides more than 20 times a year with up to 50% of sprays being applied for the mite (Cenetal., 2002).

In order to reduce the impact of the Asian form of huanglongbing, the most devastating citrus disease associated with ‘CandidatusLiberibacter asiaticus’, foliar fertilizer sprays are more commonly used in orchards where the disease is present. Understanding the impacts of currently used foliar fertilizers on plant-arthropod interactions is important for effective management of pests in orchards. In this study we determined the impacts of application of urea and compound amino acid fertilizers applied separately to potted citrus (CitrusreticulataBlanco cv. Shatangju) seedlings on the development, survival, reproduction and life table parameters ofP.citriand related these impacts to plant growth as well as mineral nutrient contents in leaves, in this instance the contents of nitrogen (N), phosphorus (P2O5) and potassium (K2O).

2 MATERIALS AND METHODS

2.1 Test mites

Female adults of the citrus red mite were collected from mite-infested citrus trees and maintained in the Laboratory of Insect Ecology greenhouse at South China Agricultural University (SCAU), in Guangzhou, China. No pesticides were applied on the trees for 3 months before the study commenced and during the study.

2.2 Plants used in experiments

Sixty one-year-old 40 cm-high citrus (CitrusreticulataBlanco cv. Shatangju) seedlings were purchased from the Citrus Institute of Boluo County, Yangcun, Huizhou City, Guangdong Province. They were grown in pots (size: 230 mm×345 mm) in a 2∶1 mixture of Pincher-1 seedling substrate (potted matrix fertilizer, Denmark imported peat soil) and yellow podsol soil (pH=7), from the SCAU campus.

2.3 Foliar fertilizer treatments

Forty-eight potted citrus seedlings were randomly separated into three groups, namely, each consists of 16 seedlings. Two of the three groups of citrus seedlings were treated with foliar fertilizers urea (0.50%) and compound amino acid (0.17%) as in Table 1, respectively, and another was treated with water as the control. Each foliar fertilizer was applied twice, once 2 d before the study commenced, and again 15 d later. A compound fertilizer (N∶P2O5∶K2O=15%∶6%∶9%; Guangdong Fengkang Biotechnology Co., Ltd.) was applied to soil in each pot at the rate of 10 g per pot three months before, and seven times during the study. The seedling trees were watered weekly. No pesticides were applied to the seedling trees before or during the experiments.

Table 1 Foliar fertilizers applied to Citrus reticulata Blanco cv. Shatangju seedlings

2.4 Duration and survival of immature stages

The three treatments were made as in section 2.3 (Table 1) and each treatment consisted of 4 seedling trees. To determine the developmental duration and survival rates of immature stages, the gravid mite females were randomly sampled from infested trees maintained in the greenhouse and transferred to fully expanded immature leaves (9-12 leaves) on 12 trees of the three treatments that were maintained in separate environmental chambers at 25±3℃, 60%±10% RH and with a photoperiod of 14L∶10D. Forty females were transferred to each tree. The females were allowed to oviposit for 24 h and then removed. Eggs laid on per tree were calculated, then about 100 were retained on the tree, the extra eggs were removed, and observed daily until they hatched. Development and survival of immature stages (larva, protonymph, and deutonymph) were also assessed every 24 h until they became adults (9-12 d).

2.5 Adult-stage life-cycle parameters

Ninety fully expanded mite-free citrus leaves (30 from each treatment) were picked from trees of the 3 treatments in section 2.3 and put in glass Petri dishes (one per dish). The dishes are 90 mm in diameter, with moist cotton wool in the bottom. The petioles of each leaf were also wrapped in moist cotton wool and the cotton wool was kept moist during the experiment.

To determine pre-oviposition and oviposition periods, female fecundity, and female and male longevity, 90 pairs (male and female) of newly emerged adults developed from experiment described in section 2.4 were transferred to each leaf as above (30 pairs from each treatment and they were transferred into the same treatment leaves). Narrow (10 mm) strips of cotton wool were used to encircle and contain each pair of mites within an arena of approximately 10±0.5 mm2on each leaf during the bioassay. Eggs and dead mites in each arena were counted and removed every 24 h until all adult mites died. All experiments were conducted in the laboratory at 25±3℃, 60%±10% RH and a photoperiod of 14L∶10D.

2.6 Growth of citrus seedlings

When the leaves were fully mature, the growth parameters of citrus seedlings treated with different foliar fertilizers were measured. All seedlings were selected for each treatment to measure each index. Plant height, stem length, leaf length and width, and length of fully-grown new shoots were measured with a tape measure. Leaf area was measured using the Coordinate Paper Method (Zhangetal., 1998).

2.7 Nutrient contents in citrus leaves

Immediately after the end of the experiment, pooled leaf samples of 9 to 12 mature leaves were taken from each treatment, dried at 80-90℃ for 15-30 min, cooled to 65-75℃ for 24 h, and sent for analysis by the Test Center of Guangdong Province to determine dry-weight content of nitrogen (N), phosphorus (P2O5) and potassium oxide (K2O). Leaf numbers on each tree were not sufficient for per replicate assessments. The total nitrogen was determined according to the Kjeldahl digestion (Barker and Volk, 1964), the phosphorus by H2SO4-H2O2Mo-Sb colorimetry (Biggeretal., 1953), and the potassium by flame photometry (Zvara and Jurga, 1966).

2.8 Life table parameters and population trend index

Life table parameters of the mite were calculated based on Chi (1988) and Pangetal. (1981). Population trend index calculation was based on Pangetal. (1981).

2.9 Data analysis

All analyses were conducted by SPSS 20.0. The single factor variance and Duncan’s test were used to test the difference of biological characteristics of the citrus red mite among different fertilization treatments.

3 RESULTS

3.1 Duration and development of the immature stage

No significant differences were observed among treatments for the developmental duration of egg, larva, protonymph, and deutonymph, and for the total developmental duration of the immature stages ofP.citri(Fig. 1). The survival rate of deutonymphs reared on citrus seedlings treated with the urea fertilizer was the highest (95.40%), which was significantly higher than those on citrus seedlings treated with the compound amino acid (75.61%) and on the control citrus seedlings (78.26%). However, although the impacts of the two fertilizer treatments on the duration of egg, larval and protonymph stages, and the total generation time ofP.citridid not differ significantly, mites in urea treatment had the highest survival rate, followed by those in the compound amino acid treatment, and those in the control (Fig. 2).

Fig. 1 Duration of immature stages of Panonychus citri reared on Citrus reticulata Blanco cv. Shatangju seedlingstreated separately with two types of foliar fertilizerFoliar fertilizers were applied twice to citrus seedlings, once 2 d before the study commenced, and again 15 d later. Forty female adults were transferred to each treated tree. The females were allowed to oviposit for 24 h and then removed. Eggs laid on per tree were calculated, then about 100 were retained on the tree, the extra eggs were removed, and observed daily until they hatched. Each treatment consisted of 4 trees. Development and survival of immature stages (larva, protonymph, and deutonymph) were assessed every 24 h until they became adults. Data in the figure are mean±SE. Bars with the same letter are not significantly different (P>0.05) while those with different letters are significantly different (P≤0.05) by Duncan’s multiple range test. The same for Fig. 2.

Fig. 2 Survival of immature stages of Panonychus citri reared on Citrus reticulata Blanco cv. Shatangju seedlingstreated separately with two types of foliar fertilizer

3.2 Reproduction and longevity of adults

No significant differences were found among treatments for impacts of the two fertilizers on the pre-oviposition and oviposition period of adult female mites, or on the sex ratio, expressed as percent females (Table 2). However, significant differences were observed in the fecundity and longevity. Fecundity (number of eggs laid per female) in the urea treatment (42.1) was significantly higher than that in the control (33.1). The longevity of females in compound amino acid treatment (19.5 d) was significantly longer than that in the urea treatment (14.8 d) and the control (14.5 d), and the longevity of males in the compound amino acid treatment (17.6 d) was significantly longer than that in the control (13.1 d). The results showed that the highest fecundity of females was found in the urea treatment, and the longest longevity of female and males in the compound amino acid treatment (Table 2).

3.3 Life table parameters and population trend index

The estimated life table parameters demonstrated that the net reproductive rate (R0) and population trend index (I) of the citrus red mite were significantly higher in the urea treatment than in the control, and intermediate in the compound amino acid treatment. The mean generation time (T) of the citrus red mite was the shortest in the urea treatment, intermediate in the compound amino acid treatment, and the longest in the control. Intrinsic rates of increase (rm) and finite rates of increase (λ) are the greatest in the urea treatment, intermediate in the compound amino acid treatment, and the lowest in the control. However, differences between the two fertilizer treatments for the intrinsic rate of increase (rm), finite rate of increase (λ), and mean generation time (T) were not significant (P>0.05)(Table 3).

Fertilizer treatmentsPre-ovipositionperiod (d)Ovipositionperiod (d)Fecundity(eggs laid per female)Longevity (d)FemaleMaleSex ratio(% females)Urea (0.50%)1.68±0.13 a11.80±0.76 a42.07±3.96 a14.79±0.62 b16.50±1.48 ab59.33±5.33 aCompound amino acid (0.17%)1.63±0.15 a13.60±0.95 a37.79±1.81 ab19.50±0.91 a17.56±1.06 a58.52±0.21 aControl (Water)1.70±0.14 a11.50±0.68 a33.13±3.99 b14.45±1.17 b13.11±1.40 b55.60±2.17 a

Table 3 Estimates of life table parameters for Panonychus citri reared on Citrus reticulata Blanco cv. Shatangjuseedlings treated separately with two types of foliar fertilizer

3.4 Nutrient content in citrus leaves

The effect of treatments on the percentage of N, P and K in leaf tissues is summarized in Table 4. The total contents of N, P, and K, and the ratio of N/K of leaves in the urea and compound amino acid treatments were similar, and greater than that in the control. The K content in the urea treatment was higher than that in the compound amino acid treatment and that in the control was the lowest (Table 4).

3.5 Growth of citrus seedlings

Leaf area, and leaf length and width of citrus seedlings in the urea and compound amino acid treatments were significantly higher than those in the control, but the length-width ratio of leaf, stem length, plant height, and number and length of new shoot leaves did not significantly differ among the three treatments (P>0.05)(Table 5). The results indicated that the urea and compound amino acid foliar fertilizers promoted plant growth, particularly the size of leaves.

Table 4 N, P and K contents in Citrus reticulata Blanco cv. Shatangju leaves following application of foliar fertilizers

Table 5 Growth parameters of Citrus reticulata Blanco cv. Shatangju seedlings following application of foliar fertilizers

4 DISCUSSION

This study demonstrated that foliar application of urea fertilizers was more favorable for the development and reproduction of the citrus red mite on citrus than foliar application of compound amino acid, but the latter increased the life span of adults. Both fertilizers increased the N, P, and K contents, and ratios of N to K in citrus leaves, and promoted citrus seedling growth, particularly leaf size. Our results related to the impacts of foliar application of urea fertilizer on the citrus red mite were similar as those reported by Yingetal. (1992), who found that populations of the mite increased 3.3-fold following application of 0.5% urea to citrus leaves over 28 d.

The balance between production increases brought about by fertilization and offset losses caused by pest infestation was a critical field yield factor. We assumed that the economic benefits of urea foliar fertilizer spraying on citrus can offset the losses caused by mites. By contrast, compound amino acid foliar fertilizer can not only promote citrus growth, but also will not increase the population growth rate of the citrus red mite. Pang and Dong (2013) found that compound amino acid foliar fertilizer did not facilitateBemisiatabaci(Hemiptera: Aleyrodidae) population growth and improved nutritional conditions in tomato. It can be concluded that compound amino acid fertilizer has a good application prospect in production, although its popularization effect in the field needs further study to prove. In addition, the concentration of the foliage fertilizers we tested is only the one commonly used by farmers due to the time consumption of life table study, the dose-effect relations also need to be further explored.