Extensive phenotypic plasticity can allow the populations of natural enemies to better adapt to changes in local environmental conditions and thereby gain greater efficiency in attacking their target pests. Phenotypic plasticity includes transgenerational effects, which are known to affect natural enemies’ reproductive and developmental traits, such as diapause, survival rate, development time, and oviposition (1–4). Although not all plastic responses of natural enemies are optimal, phenotypic plasticity is particularly advantageous when the environmental conditions the progeny will face can be better predicted by the parents than by the progeny themselves. For example, cues experienced by parents can provide progeny with additional environmental information beyond their own and the parents can induce or inhibit diapause in their progeny and affect other traits depending on the environmental signals they perceive (5–8). Transgenerational effects can be broader than simply maternal effects, and mounting evidence suggests that a grandmaternal effect can also alter the fecundity and development of natural enemies (9–12). Yet controlled evaluations of grandmaternal effects are limited and our understanding of how grandmaternal effects enable the progeny to better adapt to changes in their environmental conditions is still not well explained.

The spotted lanternfly, Lycorma delicatula White (Hemiptera: Fulgoridae), is a recent invasive pest in South Korea, Japan, and North America (13–16), where it threatens commercially grown grapevines and tree fruits, in addition to plant nurseries and timber industries (17, 18). Despite intensive efforts by federal, state, and local stakeholders to stop the spread of spotted lanternfly in the USA, the populations continue to spread. Since initial detection of spotted lanternfly in Berks County, Pennsylvania, USA, in 2014, spotted lanternfly infestations have been detected in 130 counties (87 under quarantine) within Connecticut, Delaware, Indiana, Maryland, New Jersey, New York, Ohio, Virginia, and West Virginia (19). Additional control methods are needed to help in the management of spotted lanternfly populations in the USA, and we are evaluating important parasitoids of spotted lanternfly in its native range as candidate biological control agents.

The insect of the current study, the wasp Anastatus orientalis (Hymenoptera: Eupelmidae), is the most widespread native egg parasitoid of spotted lanternfly, and has been used as a biological control agent in South Korea and is being considered a candidate for biological control in the USA (20–23). Large numbers of wasps are needed for testing candidate biological control agents or implementing a biocontrol programme, but artificial rearing spotted lanternfly is difficult, and the methods are not well developed (24). Thus, for laboratory research in South Korea on A. orientalis, eggs of a substitute host Antheraea pernyi (Lepidoptera: Saturniidae) were used for rearing the parasitoid (25). For this study, owing to the challenges of rearing or acquiring non-parasitized spotted lanternfly egg masses, we are also using A. pernyi as the host for rearing. However, when using substitute hosts to rear a parasitoid, changes in the biological characteristics of the insects under different rearing conditions should be considered because non-target hosts could influence the parasitoids’ response to the target host. They could influence offspring sex or vitality and poor-quality parasitoids may yield low efficiency in reared colonies or biological control programmes (26–28). Suitable environmental conditions for both the development and fecundity of natural enemies are imperative for their mass rearing in laboratory settings. Photoperiod and temperature are widely considered the most influential external factors for the development and fertility of natural enemies (29, 30).

Evaluations of the life cycle of A. orientalis in the field in China indicate that adults had two emergence periods per year. Some individuals emerged in May and others in September, with a summer diapause in between (21). Hou (31) found that, in a laboratory setting, A. orientalis could complete seven or eight generations over a span of 7 months (reared from April to December) when reared at 25°C with a L12:D12 photoperiod, and the emergence rate could reach up to 78%. In contracts, Broadley et al. (23) observed a low emergence rate of A. orientalis exposed to 16L:8D hour long-day conditions at 25°C, with most parasitoids entering a diapause. This suggests that summer diapause in A. orientalis is induced by a long photoperiod. In another study, Seo et al. (22) reported the longevity, oviposition, and sex ratio of A. orientalis under a 16L:8D photoperiod at four different temperatures (15°C, 20°C, 25°C, and 30°C). This led those authors to speculate that photoperiod influences the reproductive and developmental traits of A. orientalis. Thus, understanding the relationship between photoperiod and natural enemies’ biological characteristics is crucial for optimizing rearing methods (32, 33).

A transgenerational effect has been observed in other Anastatus species. In addition to environmental variables, the maternal oviposition experience and age of Anastatus disparis (Hymenoptera: Eupelmidae), an egg parasitoid of Lymantria dispar (Lepidoptera: Lymantriidae), significantly influences sex allocation in their progeny (34, 35). To our knowledge, however, no study has yet reported on whether or not transgenerational experience affects the biological characteristics of A. orientalis. Moreover, the maternal photoperiodic response has not been sufficiently investigated. Hence, in the present study, we investigated the effects of transgenerational photoperiod experience on the fertility and development of A. orientalis by using A. pernyi eggs as a host. The empirical data on the effects of transgenerational photoperiod experience on insect fertility and development acquired by our study could also be used for optimizing rearing methods and storage of natural enemy parasitoids, and for better understanding A. orientalis population dynamics in the field across a geographic and climatic range.

Transgenerational photoperiod effects have been shown to influence reproductive and developmental traits of wasp progeny, thus achieving a cumulative photoperiodic effect (34). However, as far as we know, a transgenerational photoperiod influence on the fertility and development of Anastatus species progeny has not been clearly demonstrated. The present results show that photoperiod and generation significantly affect female fecundity, female ratio, adult longevity, and diapause of A. orientalis wasps. The interaction between generation and photoperiod had a significant effect on female longevity and female fecundity, but there was no significant interaction between generation and photoperiod on male longevity. When reared under the long photoperiod condition (L16:D8), the third generation of A. orientalis had greater fecundity, a larger number of progenies, and a higher proportion of females than the population of wasps reared under the other photoperiods or the other generations of wasps. These results show that transgenerational photoperiods affect the biological traits of A. orientalis.

Studies to date have shown that various factors are important in regulating the fertility and development of Anastatus species. The key role of the natal host, photoperiod, temperature, and other environmental factors in the regulation of sex allocation, fertility, and adult longevity of Anastatus has been clearly demonstrated in both field observations and experimental studies (36–38). In addition, endogenous factors, such as their maternal age, parental experience, food plant of the host prey, and individual size, can also influence the resulting biological characteristics in certain Anastatus species (39). Our results suggest that photoperiod conditions and parental experience significantly impact longevity, female fecundity, sex ratio, and diapause rate of A. orientalis progeny. Host egg nutrient contents can affect sex ratio, adult longevity, and fecundity of a parasitoid’s progeny. Bai et al. (40) showed that females of Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) from natural hosts were larger, more fecund, and lived longer than those from factitious hosts. Prior studies have found that A. pernyi eggs with higher nutrient contents are preferred and consumed, and this results in longevities reaching up to 64.3 days (25). Prior studies found that females can have a longevity of anywhere from 39 to 68 days in laboratory rearing settings (22, 23). In addition, findings show that larger parasitoids emerge when they develop in larger or more nutritious host eggs. Larger-sized parasitoid progenies show increased longevity, and larger-sized females are capable of higher oviposition rates. Together, these findings and this study show that A. pernyi is a suitable alternative host for rearing of the parasitoid.

Although temperature and photoperiod are known to induce diapause in most insect natural enemies, the results of this study indicate that the diapause in A. orientalis is induced by photoperiod alone. Seo et al. (22) tested the effect of temperature on biological characteristics of A. orientalis, but did not find any correlation between temperature and its diapause. Results by Broadley et al. (23) showed that a long photoperiod induces A. orientalis to enter diapause. Our experimental results show that diapause of A. orientalis is affected by its exposure to photoperiod, with wasps that experience a longer photoperiod showing a stronger prosperity for entering diapause. Therefore, we conclude that photoperiod is an important environmental factor for diapause induction in this parasitoid wasp. In addition, generation also had a significant effect on the diapause of A. orientalis; it increased in later generations. Only by surveying more successive generations can we verify this trend. These results provide additional information for viable long-term storage methods for A. orientalis, which in turn will improve rearing efficiency.

The interactive effect of temperature and photoperiod on diapause regulation has been documented in many insect species. For example, development of Chrysocharis pubicornis larvae (Hymenoptera: Eulophidae) showed that a short-day-type response affected by temperature and the percentage of individuals entering diapause increased with the temperature and photoperiod (41). In addition, results by Li et al. (42) suggest that the diapause response of Microplitis mediator (Hymenoptera: Braconidae) is determined by photoperiod and mediated by temperature. When M. mediator were exposed to 16°C and 18°C combined with a photoperiod of L10:D14, the percentages of parasitoids that entered diapause was 97.9% and 87.8%, respectively, and there was no incidence of diapause at temperatures of 22°C, 24°C, and 26°C or photoperiods of L2:D22, L14:D10, L16:D8, L18:D6, L20:D4, or L22:D2. Therefore, we are now performing experiments to study the effects of photoperiod and temperature, as well as the interaction of the two, on induction of diapause of A. orientalis.

Similar to Broadley et al. (23), but in contrast to work done by other scientists (22, 25), we were unable to produce non-diapause A. orientalis adults under the long photoperiod. This may be due to the strain of A. orientalis used between these different studies. Wu et al. (43) used molecular tools to examine the genetic composition of A. orientalis, and the results suggest a genetic component in determining the diapause behaviors of A. orientalis. In addition, Broadley et al. (44) determined that the A. orientalis used previously in their study (23), which also went into diapause when exposed to long photoperiod conditions, was a homogenous colony composed of haplotype C. Because the wasps in this study responded similarly, it is highly likely that specimens from our study are composed primarily of haplotype C. However, the different results across studies also may be due to the differences in colony rearing protocol or potentially in how transgenerational effects were expressed. Hence, several generations preceding the experiment should be kept under strictly controlled constant conditions, as this helps to evaluate such discrepancies. Similar precautions to exclude multigenerational maternal effects should be considered in experimental studies with this species and other insect species.

Storage time plays a key role in the production of Anastatus species, in that a longer storage time can provide a flexible supply of parasitoids for their timely and urgent field release in biological control programmes. The most commonly used preservation method for an egg parasitoid is cold storage (45–48). However, this method not only fails to guarantee its shelf life but also affects the quality of natural enemy products (23, 49, 50). According to our study’s results, diapause may be a better way to preserve A. orientalis, and rearing at long-day conditions can induce diapause. In addition, individuals that experienced diapause exhibited a higher sex ratio and greater fecundity, two traits that can augment its performance as a biological control agent. In other insect species, diapause has a positive effect on post-diapause adults’ fertility and development (51, 52). However, although diapause allows insects to cope with adverse environmental conditions, it also poses substantial fitness costs. Ellers et al. (53) showed that an increase in diapause length not only led to higher mortality among diapausing pupae of Asobara tabida (Hymenoptera: Braconidae), but also caused a significant decrease in egg load, fat reserves, and dry weight of the emerging adult females. Carvalho (54) results suggested that individuals experiencing diapause of Utetes anastrephae (Hymenoptera: Braconidae) have lower fecundity. Therefore, the correlation between diapause and reproductive or developmental traits varies with insect species.

Our study clearly shows that transgenerational experience can have far-reaching effects in subsequent generations. Unfortunately, it is not yet known how such effects are mediated or how grandmaternal effects in isolate of material effects would affect the outcome, but future studies are planned. Temperature conditions experienced transgenerationally may alter fertility and development of A. orientalis, but further study is needed (55). The empirical data from this study on the effects of transgenerational photoperiod experience on insect fertility and development have potential applications for better understanding A. orientalis population dynamics in the field across a geographic and climatic range, and improving insect rearing and storage methods.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

All authors conceived, facilitated, and designed the research. K-XB conducted the experiments, analyzed the data, and conducted statistical analyses. X-YW and BX wrote the manuscript. X-YW, L-MC, HB, and JG secured funding. All authors contributed to the article and approved the submitted version.