Introduction

Amblyseius orientalis (Ehara) (Acari: Phytoseiidae) is a widely distributed and effective predatory mite for spider mite control in fruit production in China (Sheng et al. 2014; Zheng et al. 2008). Previously, it was used exclusively as a spider mite control agent (Zhang et al. 1992; Zheng et al. 2008); however, in the past decade, it was found to be effective against thrips and whiteflies as well (Sheng et al. 2014; Yang et al. 2018). Thus, the mass rearing and large-scale production of this native predatory mite is gradually attracting attention. Early studies of A. orientalis rearing involved host plants and spider mites. However, it was not practical for large-scale production of A. orientalis in a natural enemy factory, as it was a long and costly process (Zhang et al. 2015). Some studies have attempted to rear predatory mites on pollen, but the cost was too high and results were not satisfactory (Liao and Zhu 1985).

Carpoglyphus lactis (L.) is a fast-reproducing storage pest mite which has been widely used as an alternative prey (Knapp et al. 2018; Ramachandran et al. 2021; Wang et al. 2008; Wu et al. 2009). It can be used for successful large-scale rearing of many commercial predatory mites, such as Amblyseius swirskii (Athias-Henriot) (Nguyen et al. 2013; Zhang and Zhang 2021). Our laboratory has reported that A. orientalis feeding on C. lactis can complete development, mating, and reproduction (Sheng et al. 2014). Compared with diets of Bemisia tabaci (Zhang et al. 2015), Panonychus citri (Zhang 1990) and castor pollen (Liao and Zhu 1985), A. orientalis feeding on C. lactis had a shorter preoviposition period and higher reproduction rate (Sheng et al. 2014). Due to diet compatibility, C. lactis appears a suitable alternative diet for large-scale production and field applications of A. orientalis.

Instead of spider mites, few studies have investigated whether there is a preference of A. orientalis to developmental stages of prey C. lactis as well as their predatory capacity on C. lactis. Moreover, we notice that the density of prey to A. orientalis is influential to predator population increase during both laboratory observation and factory production (personal observation). Although mass rearing method was considered with no negative impact on predation and development of the predatory mite (Saemi et al. 2017), many studies have already revealed the critical effect of prey on predation and female fecundity of the predators (Werling et al. 2013; Osman et al. 2016; Rasmy and Abou-Elella 2002). If prey numbers were much higher than predator numbers, they could inhibit the expansion of A. orientalis populations instead (personal observation).

In this study, we investigated the predatory capacity of A. orientalis on different developmental stages of the prey C. lactis, the stage preference and the effect of prey numbers on predator reproduction. The aim of this study was to uncover the potential reason of low population growth of A. orientalis when using C. lactis as an alternative prey in large-scale production.

Materials and methods

Mite rearing

Amblyseius orientalis was collected from soybean fields in Changli Research Institute of Pomology, Hebei Academy of Agricultural and Forestry Sciences (119°09′E, 39°43′N). The colony had been maintained on C. lactis for 10 years in the Laboratory of Predatory Mites, Institute of Plant Protection, Chinese Academy of Agricultural Sciences (Yan et al. 2022). Carpoglyphus lactis feeding on yeast were provided by Shoubonong Biotechnology Company (Beijing, China). All mites were reared in an incubator (RXZ, Ningbo, China) at 25 ± 1 °C, 70 ± 5% RH and L14:D10 photoperiod.

The predatory capacity of Amblyseius orientalis

To investigate the predatory capacity of A. orientalis on C. lactis at different densities, we transferred C. lactis adults and deutonymphs into small arenas (Yan et al. 2022). Briefly, the arena was a tiny device with two tightly clipped layers (modified Munger cage): a transparent acrylic board (30 × 20 × 3 mm) with a 1 cm diameter hole in the middle sealed by nylon mesh with glue, one piece of rectangular glass (30 × 20 × 1 mm) on top. The prey densities were set at 5, 10 and 20 adults or deutonymphs per arena. Amblyseius orientalis female adults were starved for 24 h in a plastic box before being transferred separately to each arena with C. lactis adults or deutonymphs. All treatments were repeated with 30 A. orientalis adults. The consumed number of both stages of C. lactis was recorded within 24 h.

Similarly, we investigated the predatory capacity of A. orientalis adults and larvae on eggs of C. lactis at different densities. We collected sufficient C. lactis eggs to set three densities of 30, 45 and 60 per arena. Amblyseius orientalis females were starved for 24 h and then separated into the arena with different densities of prey eggs. The same process was repeated, with one newly hatched A. orientalis larva per arena, and three prey densities of C. lactis were set at 10, 20 and 30 eggs per arena.

Stage preference of Amblyseius orientalis to prey

We studied the developmental prey stage preference when A. orientalis was exposed to egg, nymph, and adult in the same arena. We collected one starved A. orientalis female adult and transferred it to the arena with 10 eggs, 10 deutonymphs and 10 adults of C. lactis. The predation number of A. orientalis on each developmental stage of C. lactis was recorded after 6, 12 and 24 h. The experiment was replicated 30×.

Effect of developmental stage and density of Carpoglyphus lactis on predator fecundity

We moved one gravid A. orientalis adult into the arena with densities of 30, 45 and 60 eggs or 5, 10 and 20 deutonymphs of C. lactis. Each density was repeated 30×. These arenas were changed daily with the same number of eggs or deutonymphs of C. lactis, respectively. We then recorded the number of consumed eggs or deutonymphs of C. lactis daily and the oviposition of each A. orientalis for 7 consecutive days.

Statistical analysis

Kruskal-Wallis tests with pairwise comparisons were used to examine the effects of different C. lactis densities on predatory capacity, prey preference and fecundity of A. orientalis (α = 0.05). All analyses were run in SPSS v.25.0 and results were visualized in GraphPad v.8.02.

In the preference experiment, the prey preference index (Ci) of predators to various stages was expressed as Ci = (Qi-Fi) / (Qi+Fi) (Ivlev et al. 1962; Zhou and Chen  1987), where Fi is the proportion of the ith prey type in the environment, and Qi is the proportion of predation of the ith prey type by the predator. If Ci = 0, the predator had no preference for the ith prey. If 0 < Ci < 1, the predator had a positive preference for the ith prey, and if -1 < Ci < 0, the predator had a negative preference for the ith prey.

Results

The predatory capacity of Amblyseius orientalis on Carpoglyphus lactis

Prey densities significantly affected predation but in different trends (Fig. 1). The maximum number of C. lactis adults preyed upon by A. orientalis females was 2.21 at the lowest density of five mites per arena (χ2 = 12.436, df = 2, p = 0.002; Fig. 1A). In contrast, only 0.80 killed prey was observed at the density of 20 C. lactis adults per arena. The opposite tendency was observed when A. orientalis fed on deutonymphs: at the highest density of 20 mites per arena the maximum predation number was 6.44, which was significantly more than 4.07 in the group of five prey mites per arena (χ2 = 22.414, df = 2, p < 0.001; Fig. 1B). When testing A. orientalis adults and larvae on C. lactis eggs, the consumption number increased with prey densities. The maximum predation number of A. orientalis adults was 45.07 at the highest density of 60 eggs per arena, in contrast to the predation of 25.30 on 30 eggs per arena (χ2 = 44.869, df = 2, p < 0.001; Fig. 1C). Similarly, the maximum predation number of A. orientalis larvae was 13.08 at the highest density of 30 eggs per arena, which was more than the 7.62 in an arena with 10 prey eggs (χ2 = 24.536, df = 2, p < 0.001; Fig. 1D). In total, taking C. lactis eggs and deutonymphs as preys, the predation of A. orientalis adults tended to increase with prey density, whereas the predation number tended to decrease when feeding on high density of C. lactis adults (χ2 = 26.336, df = 1, p < 0.001; Fig. 2).

Fig. 1
figure 1

Predatory capacity (number of prey items consumed) of Amblyseius orientalis on Carpoglyphus lactis at different densities. Predation of A A. orientalis adults on C. lactis adults, B A. orientalis adults on C. lactis deutonymphs, C A. orientalis adults on C. lactis eggs, and D A. orientalis larvae on C. lactis eggs. Each dot represents an individual. Horizontal lines indicate the mean (± SD) of biological replicates. Means capped with different letters are significantly different (Kruskal–Wallis test: p < 0.05)

Full size image
Fig. 2
figure 2

Predatory capacity (mean [± SD] no. prey items consumed) of Amblyseius orientalis adults on various life stages of Carpoglyphus lactis at different densities. Asterisks indicated significant differences between densities (Kruskal–Wallis test: p < 0.05)

Full size image

Stage preference

Amblyseius orientalis significantly preferred eggs to deutonymphs and adults of C. lactis (Table 1). Of the C. lactis eggs 81.8% was consumed, the largest percentage. The positive index Ci of 0.431 indicated a preference of A. orientalis for C. lactis eggs. On the contrary, C. lactis adults were the least chosen with only 1.3%. The negative index Ci of −0.925 suggested that A. orientalis have a strong aversion towards adult prey.

Table 1 Consumption by Amblyseius orientalis adults of three Carpoglyphus lactis life stages
Full size table

In the 24 h preference experiment, the numbers of consumed C. lactis eggs in the three time periods were 3.87, 5.87, and 8.43, which were significantly higher than the respective adult numbers of 0, 0.04, and 0.09 (Fig. 3). Apparently, A. orientalis preying on eggs of C. lactis continued at each time period.

Fig. 3
figure 3

Developmental stage preference (no. prey items killed) of Amblyseius orientalis adult females after 6, 12 and 24 h, when offered 10 eggs, 10 deutonymphs and 10 adults of Carpoglyphus lactis simultaneously. Each dot represents an individual. Horizontal lines indicate the mean (± SD) of replicates

Full size image

Effect of prey developmental stage and density on Amblyseius orientalis fecundity

The above experiment revealed different preferences of A. orientalis to eggs and deutonymphs of C. lactis, so we further tested the effects of eggs and deutonymphs at different densities on A. orientalis fecundity. The maximum predation was 4.15 killed prey at the highest density of 20 deutonymphs per day (χ2 = 8.107, df = 2, p = 0.017; Fig. 4A). However, the highest reproduction with 2.13 eggs per day occurred at the density of five prey deutonymphs (χ2 = 21.487, df = 2, p < 0.001; Fig. 4C). When preyed on eggs, 38.62 eggs were consumed at the highest density of 60 eggs per arena, significantly more than 31.50 and 26.94 eggs at the other two density groups (χ2 = 32.833, df = 2, p < 0.001; Fig. 4B). The corresponding maximum reproduction was 1.90 eggs at the density of 45 prey items (χ2 = 14.869, df = 2, p = 0.001; Fig. 4D).

Fig. 4
figure 4

Predatory capacity (no. prey items consumed) and fecundity (no. eggs laid) of Amblyseius orientalis adult females on eggs and deutonymphs of Carpoglyphus lactis offered at three densities. Predation on C. lactis A deutonymphs and B eggs. Fecundity on C. lactis C deutonymphs and D eggs. Each dot represents an individual. Horizontal lines indicate the mean (± SD) of biological replicates. Means capped with different letters are significantly different (Kruskal−Wallis test: p < 0.05)

Full size image

Predation rate and reproduction of A. orientalis were only correlated when prey density was high (Fig. 5). When prey number was lower, although not correlated, it did not decrease reproduction.

Fig. 5
figure 5

Linear relationship between predatory capacity (no. prey items consumed in 7 days) and fecundity (no. eggs laid in 7 days) of Amblyseius orientalis on Carpoglyphus lactis A deutonymphs and B eggs

Full size image

Discussion

The results of our study showed that A. orientalis female adults had strong predatory capacities on C. lactis eggs and deutonymphs, but not on adults. In the preference experiment, the Ci of 0.42 to C. lactis eggs showed a preference, whereas the Ci of −0.92 to adults indicated aversion. This is consistent with what we observed during practical rearing. Some studies also revealed predatory mites preferred eggs as prey (Furuichi et al. 2005; Ganjisaffar and Perring 2015; Moghadasi et al. 2013; Naeem et al. 2017; Song et al. 1995). Aversion of adult prey by the predators might be based on semiochemical compounds – an interesting study showed that Suidasia medanensis after attack could release defense volatiles that deter subsequent attack from the predatory mite A. swirskii (Midthassel et al. 2016). Whether such chemical compounds are also involved in the interaction of C. lactis and A. orientalis, or whether perhaps a physical barrier is in play in the developmental stage preference, remains to be elucidated.

Body size and mobility affects A. orientalis predation. In the experiment, the starved A. orientalis can prey on C. lactis adults and deutonymphs, both of similar size as the predator; however, the predation time was much longer than when preying on eggs. Besides body size, other factors might also affect predation, such as behavioral contact with the prey. Eggs obviously do not move, but predation of A. orientalis on mobile stages of C. lactis is relatively time-consuming, as the prey try to escape and avoid predation. Catching same size C. lactis adults was difficult for A. orientalis, as it was observed trying to grasp and prey on C. lactis. Apparently, the comparatively large body size, strong mobility and relatively thick tegument of C. lactis adults result in predation difficulty. Compared with C. lactis adults, the thin egg shell is easy to be pierced by the mouthparts of A. orientalis.

The development and reproduction of predators require crucial nutrients (Harvey et al. 2012; Williams and Roane 2007; Woods et al. 2020). Compared with high-protein prey, high-lipid prey had negative effects on the survival and reproduction of the web-building spider Hylyphantes graminicola (Wen et al. 2020). The predatory mite Parasitus consanguineus (Parasitidae), separately feeding on two prey species, the mushroom flies Megaselia halterata and Lycoriella ingenua, significantly differed in survival, female longevity and fecundity (Szlendak and Lewandowski 2009). When providing additive proteins or saccharides to prey mites in mass rearings, the phytoseiid predatory mite Neoseiulus barkeri presented higher fitness with shorter developmental durations and higher fecundity than those fed on basic diet (Huang et al. 2013). These results indicate that the food of the prey mites has a major impact on the nutritional value of the prey mites for predatory mites. In our study, C. lactis eggs could provide sufficient nutrition for A. orientalis reproduction, suggesting the developmental stage preference was not risky to predators in a long run. Our results are also consistent with a previous study that the eggs are sufficiently nutritious to support the predatory mite to complete reproduction and egg prey preference does not pose a survival risk to predators (Moghadasi et al. 2013). Neoseiulus californicus has been reported to prey eggs of five spider mite species which deposited on a variety of plants with similar performance (Gotoh et al. 2006). All this evidence indicates prey eggs are sufficient to support growth and development of the predatory mites.

In the current study, we found that fecundity of A. orientalis could be lower when providing higher density of C. lactis adults. It means that the ratio of prey and predator is critical during predatory mite rearing. As discussed above, we hypothesize that the energy supplied by the prey may be less than the energy consumed by A. orientalis during predation. Other explanations might be that the predators are deterred to prey C. lactis by an alarm pheromone or physical barrier. In mass-production, we should quantify prey density and strictly control the ratio of C. lactis to A. orientalis in inoculation to ensure the population growth of predatory mites. In short, our study elucidated the sensitive characteristics of A. orientalis to prey density, and provided a solution for stable population growth of predatory mites in mass rearing.