Population dynamics and overwintering of a biological control beetle, Agasicles hygrophila, on a nontarget plant Alternanthera sessilis, along a latitudinal gradient Yan Wang, Mohannad Ismail, Wei Huang, Yi Wang & Jianqing Ding

Journal of Pest Science

ISSN 1612-4758

J Pest Sci DOI 10.1007/s10340-018-1031-8

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Journal of Pest Science https://doi.org/10.1007/s10340-018-1031-8

ORIGINAL PAPER

Population dynamics and overwintering of a biological control beetle, Agasicles hygrophila, on a nontarget plant Alternanthera sessilis, along a latitudinal gradient

Yan Wang1,2 · Mohannad Ismail1 · Wei Huang1 · Yi Wang1 · Jianqing Ding3

Received: 17 April 2018 / Revised: 5 July 2018 / Accepted: 24 July 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract Assessing the impact of temperature changes on insect population and overwintering on nontarget hosts is important for prediction of nontarget efects in weed biological control. Agasicles hygrophila is a beetle used for biological control of the invasive plant alligator weed, Alternanthera philoxeroides, with nontarget damage to a native plant, Alternanthera sessilis. In this study, we monitored plant growth and phenology along with beetle population and overwintering on these two hosts along a latitudinal gradient from subtropical to temperate climates (Guilin, Wuhan and Kaifeng) in China. We found only annual A. sessilis seedlings in temperate Kaifeng, but both annual seedlings and perennial ramets of A. sessilis were found in Guilin and Wuhan. However, in winter, living A. sessilis plants were found in only subtropical Guilin. Beetles at the Guilin site could successfully maintain their populations and overwinter on A. sessilis. Although the beetle could sustain its populations on A. sessilis at the other two higher-latitude sites during the growing season, it failed to overwinter on either species, indicating that the temperatures in diferent climate zones may directly and/or indirectly afect the development of insect biological control agents and their population sizes on nontarget hosts. Therefore, it is important to consider the shifting plant–insect interactions induced by climate when assessing potential nontarget efects of species introduced as biological control agents.

Keywords Biological control · Risk assessment · Host range · Plant–insect interaction

Key message We hypothesized that the population size and overwin- tering ability of insects used for weed biocontrol vary with Assessing the impact of climate on insect on host plants latitudes. is important for accurate prediction of nontarget efects in Insects successfully maintained populations and overwin- weed biological control. tered on their target weed and nontarget host at low-latitude sites but failed to overwinter at higher-latitude sites. Consideration of the shifting interactions among weeds, Communicated by M. Traugott. biocontrol insects and native plants induced by climate is critical for biological control programs. Electronic supplementary material The online version of this article (https​://doi.org/10.1007/s1034​0-018-1031-8) contains supplementary material, which is available to authorized users. Introduction * Jianqing Ding [email protected] Classical biological control of exotic weeds, based on the 1 Key Laboratory of Aquatic Botany and Watershed Ecology, introduction of specialist natural enemies from the weed’s Wuhan Botanical Garden, Wuhan 430074, Hubei, China native range, is often a long-term solution for regulating 2 University of Chinese Academy of Sciences, Beijing 100049, exotic weed populations in their introduced ranges (Müller- China Schärer and Schafner 2008). However, insect host specifc- 3 School of Life Sciences, Henan University, Kaifeng 475004, ity and potential nontarget efects on native plants remain Henan, China important concerns (McEvoy 1996; Myers and Cory 2017;

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Raghu et al. 2007). Many factors, especially climate and tropical to temperate regions. Its geographical distribution host plant availability, may afect insect populations and largely overlaps with the current distribution of the invasive their damage to host plants and even result in host shifts A. philoxeroides and A. hygrophila (Lu et al. 2015). Unlike from target plants to surrounding plant species (Ismail et al. A. philoxeroides, which grows in both terrestrial and aquatic 2017; Jang et al. 2015; Posledovich et al. 2015; Rasmann habitats, A. sessilis occurs mostly in terrestrial habitats (Lu et al. 2014). Studies of the efects of spatial and temporal et al. 2015; Ma 2001). variations in climate on the performance of biological con- Earlier laboratory studies of host specificity showed trol agents may help to predict their nontarget efects after that A. hygrophila has a narrow host range, being able to release (Lu et al. 2015). complete its development on only A. philoxeroides (Mad- Temperature plays an important role in the biology of dox et al. 1971; Wang et al. 1988; Wu et al. 1994). As insects; thus, the variations of temperature in diferent cli- A. hygrophila lays its eggs on leaves, and mature larvae matic zones may directly afect insect survival, population pupate in stems, previous studies have also suggested that and distribution (Ismail and Brooks 2016; Lencioni 2004; A. hygrophila might not be able to complete its life cycle Ma et al. 2006; Robinet and Roques 2010; Stoeckli et al. on the terrestrial forms of A. philoxeroides, due to the sus- 2012). Temperature may also indirectly determine insect ceptibility of the eggs to desiccation and the restrictions in performance by afecting host plant development, availabil- physical structure, such as stem diameter and tissue mass ity and quality (Jamieson et al. 2015; Rasmann et al. 2014; density, of terrestrial plants, which were thought to prevent Uelmen Jr et al. 2016; Yang and Rudolf 2010). As a conse- pupation (Coulson 1977; Ma et al. 2003; Pan et al. 2011). quence, insects tend to adapt to unfavorable conditions by Recent works, however, have demonstrated that A. hygroph- overwintering, i.e., they pass the winter season by almost ila can also feed on a co-occurring nontarget host plant, A. completely ceasing activity until climate or food conditions sessilis (Lu et al. 2012, 2015). Furthermore, Lu et al. (2015) become more favorable. Overwintering ability is a key cri- proved that the beetle could occur in terrestrial habitats and terion in determining insect population dynamics, especially could pupate inside the stems of both A. philoxeroides and at high latitudes, where average temperatures in winter are A. sessilis and that increasing temperature could increase usually low (Bale and Hayward 2010). Overwintering is par- A. hygrophila damage to A. sessilis along latitudes. As dif- ticularly important when assessing whether insect biological fering climate across latitudes can dramatically afect plant control agents can sustain populations on hosts across years. performance, climate-induced variations in A. sessilis phe- For weed biological control programs, if the target weeds, nology and food availability may afect A. hygrophila devel- insect biological control agents and nontarget hosts co-occur opment and population dynamics. However, no studies have across diferent climate zones, then insect populations, over- yet reported how geographical and climatic variation might wintering abilities and nontarget efects may vary with aver- afect the population and overwintering of this beetle on A. age temperature where climates difer (Lu et al. 2015, 2016). sessilis. Thus, monitoring insect biological control agent populations In this study, we conducted feld experiments at three dif- and overwintering on nontarget hosts at diferent latitudes is ferent latitudes representing diferent climates across China important for assessing nontarget efects in diferent climate (Fig. 1) to assess the efects of climate on beetle interactions zones. with A. sessilis. Specifcally, we asked how these diferent Native to South America, Agasicles hygrophila Selman climates directly and/or indirectly (via the phenology and and Vogt (Coleoptera: Chrysomelidae), is a specialized her- growth of the host plants) afect A. hygrophila development bivorous beetle that has been introduced for biological con- and population dynamics and, in particular, the beetle’s abil- trol of alligator weed, Alternanthera philoxeroides (Mart.) ity to overwinter on A. sessilis. We predict that A. hygrophila Griseb. (Amaranthaceae), in many countries, such as the performs better on A. sessilis in warmer climates in low- USA, Australia and parts of Asia (Coulson 1977; Julien et al. latitude regions than in colder and higher-latitude regions. 1979; Ma 2001). Alternanthera philoxeroides can grow in both aquatic and terrestrial habitats. Agasicles hygrophila has been found to prefer the aquatic form of the plant to the Materials and methods terrestrial ones, thus providing efective control of aquatic A. philoxeroides at lower latitudes in the USA, Australia Study organisms and China (but not of the terrestrial and plants in higher latitudes) (Coulson 1977; Ma 2001). However, winter tem- Alternanthera philoxeroides is a perennial herbaceous plant peratures could restrict the beetle’s range at higher latitudes able to asexually propagate from stem and root buds (i.e., (Coulson 1977; Julien et al. 1995; Lu et al. 2013). Alter- clonal ramets), while its seeds are not viable in China (Zhu nanthera sessilis (L.) R.Br. ex DC., the only native conge- et al. 2015). Alternanthera sessilis is an annual or peren- ner of A. philoxeroides in China, is widely distributed from nial herbaceous plant able to reproduce both sexually and

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Fig. 1 Locations of the feld experiment sites in China (a) and temperatures at each site in January 2017 (b). The geo- graphical coordinates of each site are 25.0804°N, 110.3004°E (Guilin), 30.5476°N, 114.4182°E (Wuhan) and 34.8185°N, 114.3025°E (Kaifeng), respectively. Mete- orological data for each site was obtained from www.tianq​i.com

asexually in southern China but only via seeds in the tem- Experimental design perate northern regions (Lu et al. 2015). Adults and lar- vae of A. hygrophila feed on the leaves of A. philoxeroides In October 2015, we collected A. sessilis seeds in Xianning, and A. sessilis, causing severe damage to the aboveground Hubei Province, and cultured the plants in March 2016. parts, and adults also lay eggs on the leaves. Mature larvae Alternanthera philoxeroides was collected as whole plants bore into the stems and pupate there. Upon emerging, adults in Wuhan in May 2016. To obtain similar-sized plants of chew out of the stems, leaving emergence holes (Maddox these two species, we cut their stems and propagated them et al. 1971). In tropical southern China, A. hygrophila may in a greenhouse. Similar-sized plants of both species were have 8–9 generations/year, and each generation may last then transferred to the experimental felds at each of the 1–2 months (Ma 2001). three sites on the July 5, 10 and 15, 2016, at Guilin, Wuhan and Kaifeng, respectively. To determine whether A. hygrophila could overwinter Study sites with only A. sessilis available and to monitor its popula- tion dynamics, we established 15 1.0 × 1.0 m plots (1.0 m We conducted the experiments at three sites along a latitudi- apart) at each site in July 2016. As the soils at each of the nal gradient: Guilin Botanical Garden in Guangxi Province three sites are of diferent quality, we removed the native (25.0804°N, 110.3004°E) (hereafter referred to as Guilin), soil in each plot to a depth of 30 cm and replaced it with Wuhan Botanical Garden in Hubei Province (30.5476°N, identical peat moss. Plots were randomly assigned to one 114.4182°E) (hereafter referred to as Wuhan) and Kaifeng, of three plant composition treatments, (1) mixed culture in Henan Province (34.8185°N, 114.3025°E) (hereafter (A. philoxeroides + A. sessilis), (2) A. philoxeroides mono- referred to as Kaifeng). The climate in Guilin is typically culture and (3) A. sessilis monoculture, with fve replicates subtropical, with warm winters (minimum temperatures (plots) for each treatment. For the monocultures, ten similar- in January 2017 ranged from 3 to 15 °C) and hot, humid sized stems of A. sessilis or A. philoxeroides were planted summers. The climate in Wuhan is between subtropical and in each plot. In the mixed cultures, fve A. philoxeroides and temperate, while Kaifeng is temperate and has a cold winter fve A. sessilis stems were planted in each plot, with two of (see Fig. 1). These three sites represent the geographical the same species planted in opposing quadrat corners, and range of A. philoxeroides and A. sessilis in China. While A. the remaining single A. philoxeroides and A. sessilis stems hygrophila is commonly found in Guilin and may be able to planted in the center of the plot. All plots were caged using overwinter in Wuhan if the winter is warm, it has never been nylon mesh to exclude outside herbivores or natural enemies found in Kaifeng (Lu et al. 2015). All experimental sites of the beetle. Prior to releasing the insects, we randomly were in open felds that were mowed before the experiment. sampled three individuals of each plant species from each

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Journal of Pest Science plot at all three sites and measured the main stem diameter. selected ten ramets from each monoculture plot for both The stem diameter of A. philoxeroides was bigger than that plants and ten seedlings of A. sessilis from its monoculture of A. sessilis, regardless of site and plant combination. The plots to measure ramet length and annual seedling height. diameter of monoculture A. sessilis was similar among the three sites (Supplementary Table 1). Statistical analysis On August 26 and 30 and September 2, 2016, we released four pairs of newly mated A. hygrophila into each plot in To examine the impacts of latitude and host on A. hygrophila Guilin, Wuhan and Kaifeng. The beetles were collected in overwintering, the number of adults was analyzed using the a feld in Wuhan. After releasing the beetles, we counted generalized linear model (GLM) with a Poisson distribution. A. hygrophila adults in each plot every 2 weeks until early The model included experiment site (Guilin, Wuhan and November, after which we counted insects every 4 weeks Kaifeng) as a fxed factor and time (n = 12) as a covariate. between November 2016 and February 2017 corresponding Each host plant combination (A. philoxeroides monoculture, with plant depletion and insect removal (see details below); A. sessilis monoculture, and A. philoxeroides and A. sessilis then, we counted insects every 3 weeks after February 2017 mixture) was analyzed separately. To examine the impact of corresponding with the growth of plants. At the Guilin site, host on the performance of A. hygrophila in Guilin, adult to avoid heavy defoliation that could kill all the plants and weight was analyzed using a linear model (LM) with plant cause the insect population to collapse, we removed all species (A. philoxeroides and A. sessilis monocultures) as adults and mature larvae from the A. philoxeroides monocul- a fxed factor and time (n = 12) as a covariate. Females and ture and mixed culture plots after each survey between Octo- males were analyzed separately. To examine the growth of ber 23, 2016 and February 18, 2017. Similarly, all adults the two plant species in their monoculture treatments, the and mature larvae were removed from the monoculture A. number of ramets and ramet length were analyzed using sessilis plots in Guilin on December 26, 2016 and February GLM and LM, respectively. The model included experiment 18, 2017. We also removed half of all adults after each sur- site and plant species as fxed factors and time as a covariate. vey in the monoculture A. philoxeroides and mixed culture To examine the growth of A. sessilis annual seedlings in the plots between March 10 and April 19, 2017. A previous monoculture treatment, the number of seedlings and their study had shown that such removal is an efective approach heights were analyzed using GLM and LM, respectively. for long-term monitoring of insect populations in host pref- The model included experimental site as a fxed factor and erence/performance tests (Ding et al. 2007). We removed time as a covariate. All data were analyzed using R 3.2.0 insects at only the Guilin site; no insects were removed at (R Foundation for Statistical Computing, Vienna, Austria). other two sites due to very low populations. Multiple comparisons were carried out using least square To examine the efects of host plants on insect develop- mean post hoc tests (LSM), and p values were adjusted using ment, we used the fresh weight of the beetles as an index of false discovery rate (Benjamini and Hochberg 1995). the body size of adults, as it was easily and rapidly measured during the frequent experimental periods (Knapp and Knap- pová 2013), and data were collected from Guilin, which had Results the longest survey period. At each survey date, female and male adults were randomly collected from each monocul- Population dynamics and overwintering of A. ture plot of A. philoxeroides and A. sessilis, and their sizes hygrophila were measured. The number of adults varied among the plant composition treatments over time (5–74 females and Beetle numbers decreased with increasing latitude (from 9–54 males on A. sessilis, 15–81 females and 16–81 males Guilin to Kaifeng) in all combinations (A. philoxeroides 2 on A. philoxeroides) depending on insect availability during monoculture: 2 = 9345.3, P < 0.001; A. sessilis mono- 2 2 the experiment, with low numbers at earlier stages and high culture: 2 = 5207.4, P < 0.001; mixture: 2 = 7509.8, numbers at later stages. All weight measurements were taken P < 0.001). There were also significant effects of time using an electronic balance (Sartorius BS 110 S, Germany). and its interaction with site on beetle numbers (time in 2 To investigate the efects of the winter and spring seasons A. philoxeroides monoculture: 1 = 529.8, P < 0.001; time 2 on plant growth and/or food availability, which might afect in A. sessilis monoculture: 1 = 139.3, P < 0.001; time in 2 insect development, we made phenology measurements of mixture: 1 = 157.6, P < 0.001; site × time in A. philox- 2 the plants. We began noting greening of stem buds (peren- eroides monoculture: 2 = 490.5, P < 0.001; site × time in 2 nial or biennial ramets) in both species and seed germina- A. sessilis monoculture: 2 = 323.2, P < 0.001; site × time 2 tion (annual seedlings) of A. sessilis in January 2017. We in mixture: 2 = 163.2, P < 0.001). From late September to counted the number of perennial or biennial ramets of each mid-November, there were always more adults in Guilin species and the annual seedlings of A. sessilis. We randomly and fewer adults in Kaifeng in every host combination,

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Journal of Pest Science despite the removal of insects from the cages in Guilin The performance of A. hygrophila to prevent insect outbreaks from causing plant collapse (see Supplementary Table 2). Beetle numbers decreased Plant species had no signifcant efect on the weight of to zero in Kaifeng and Wuhan in mid-November and female or male adults (female: F1, 838 = 1.862, P = 0.173; early December 2016, respectively (Fig. 2). However, male: F1, 791 = 2.614, P = 0. 106) at the Guilin site, but time A. hygrophila successfully overwintered on A. sessilis and its interaction with host species had signifcant efects in Guilin, although the populations in the monoculture on beetle weight (time in female: F1, 838 = 3.857, P = 0.050; A. sessilis plots decreased from January to April 2017 time in male: F1, 791 = 11.657, P < 0.001; species × time in (Fig. 2). female: F1, 838 = 19.280, P < 0.001; species × time in male: F1, 791 = 8.910, P = 0.003) (Fig. 3). Adult weights varied sig- nifcantly with time. Females that fed on A. philoxeroides were signifcantly larger than those that fed on A. sessilis

Fig. 2 Population of Agasicles Kaifeng Wuhan Guilin hygrophila adults in mono- culture Alternanthera philox- a eroides 400 plots (a), monoculture only A. philoxeroides Alternanthera sessilis plots (b) and mixed plots (c) in three 300 sites from September 2016 to April 2017. M, L and E labels 200 on the X-axis refer to ‘Middle’, ‘Late’ and ‘Early’, respectively. Arrows indicate the dates of 100 insect removal at Guilin. Data are the mean ± SE 0 adults b 400 only A. sessilis

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Fig. 3 Fresh weights of Agasi- 12 a cles hygrophila female (a) and male adults (b) from September * 2016 to April 2017, collected * from Alternanthera sessilis and * 10 Alternanthera philoxeroides * monoculture plots in Guilin. Data are the mean ± SE

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2 on November 7, 2016 and signifcantly smaller on March P < 0.001), and time (ramet number: 1 = 75,984, P < 0.001; A. philox- 10 and 30 and April 19, 2017. Males that fed on ramet length: F1, 138 = 96.207, P < 0.001) (Fig. 4). The eroides were signifcantly larger than those that fed on A. interactions among site, species and time significantly 2 sessilis on September 25 and November 7, 2016 and sig- afected ramet number (site × species: 2 = 7204, P < 0.001; 2 2 nifcantly smaller on October 23, 2016 and February 18 and site × time: 2 = 11, P = 0.004; species × time: 1 = 199, 2 April 19, 2017. P < 0.001; site × species × time: 2 = 219, P < 0.001). The interaction between site and time (F2, 138 = 4.249, Plant phenology P = 0.016) and the interaction between species and time (F1, 138 = 16.083, P < 0.001) signifcantly afected ramet Annual A. sessilis seedlings (seed origin) were found in length. Kaifeng, but both annual seedlings and ramets of A. sessilis Both the number and the height of annual A. sessilis were found in Guilin and Wuhan (Figs. 4, 5). The annual plants were signifcantly afected by site and time (num- 2 seedlings of A. sessilis started to grow on October 23, 2016 ber by site: 2 = 126,717, P < 0.001; number by time: 2 and April 12 and 18, 2017 in Guilin, Wuhan and Kaifeng, 1 = 59,335, P < 0.001; height by site: F2, 69 = 291.152, A. sessilis respectively. Meanwhile, the ramets of started P < 0.001; height by time: F1, 69 = 27.507, P < 0.001), and to grow on February 27 and March 15, 2017 in Guilin and the interaction between site and time had a signifcant efect 2 Wuhan, respectively, and the ramets of A. philoxeroides on seedling number ( 2 = 7197, P < 0.001) and height started to grow on the March 1 and 9, 2017 in Wuhan and (F2, 69 = 10.531, P < 0.001) (Fig. 5). Kaifeng, respectively. Both A. philoxeroides and A. sessilis survived above ground in Guilin during winter, and new seeds of A. sessilis and stem buds of A. philoxeroides were sprouting during winter 2016. Discussion Ramet number and ramet length were significantly 2 afected by site (ramet number: 2 = 1633, P < 0.001; ramet Monitoring insect populations and examining the efects length: F2, 138 = 33. 831, P < 0.001), species (ramet num- of climate on insect performance are critical for assess- 2 ber: 1 = 6175, P < 0.001; ramet length: F1, 138 = 37.241, ing nontarget efects in current and future weed biological

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Fig. 4 Numbers of Alternanthera philoxeroides and Alternanthera sessilis ramets and average ramet lengths in monoculture plots in Guilin, Wuhan and Kaifeng in 2017. Plant ramets refer to ramets grown from stem buds. Bars represent the mean ± SE control programs (Lu et al. 2015). Many previous studies Previous studies have shown that low temperature can have examined insect nontarget efects by assessing host restrict the distribution of A. hygrophila at high latitudes specifcity in laboratory and common garden experiments (Julien et al. 1995) and that low temperatures in winter can (Myers and Cory 2017; Raghu et al. 2007); however, studies directly reduce the activity of A. hygrophila, leading to less examining how climate afects nontarget efects are rare (but feeding and lower fecundity (Guo et al. 2012; Zhao et al. see Lu et al. 2015). To the best of our knowledge, our study 2015). In this study, we found that A. hygrophila success- is the frst to monitor the populations and overwintering of fully overwintered in Guilin, but all beetles died in Decem- insect biological control agents on nontarget hosts at difer- ber in Wuhan and Kaifeng, suggesting that the lack of cold ent latitudes simultaneously. We found that A. hygrophila resistance of A. hygrophila could lead to the mortality of could successfully sustain populations on native A. sessilis populations in cold regions. Thus, low overwintering abil- during the growing season at all three latitudinal sites with ity is a determining factor that restricts the distribution of diferent climates, but it was able to overwinter on A. sessilis A. hygrophila. Furthermore, our monoculture experiments only in subtropical Guilin, not in the colder northern areas showed that A. sessilis could support the beetle’s develop- of Wuhan and Kaifeng. ment under natural conditions through the winter in Guilin

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(Fig. 2), and this result was similar with that of a previous the distribution of A. hygrophila and indirectly suppress study (Lu et al. 2015). these insects by afecting the survival of the two host plants. Temperature also afects A. hygrophila populations indi- Our results showed that the fresh weight of A. hygroph- rectly, through its impact on the phenology and growth of ila was similar when feeding on A. philoxeroides or A. host plants. As the beetle does not undergo winter diapause, sessilis, suggesting that the variations in plant resources A. hygrophila was unable to survive in the absence of hosts between these two hosts may not afect the adult growth of at low temperatures (Julien et al. 1995). Lu et al. (2015) A. hygrophila. Ma et al. (2003) argued that plant physical found that increasing temperature favored the overwinter- structures such as stem diameter, rather than nutrients, are ing of the beetle on A. philoxeroides. In this study, both A. key factors afecting beetle development. Moreover, it has philoxeroides and A. sessilis in Guilin survived aboveground been demonstrated that A. sessilis had a negative impact all year, including ramets of A. philoxeroides and seedlings on the preoviposition period and fecundity of A. hygrophila of A. sessilis, providing the beetle with food and refuge (Lu et al. 2010), which might explain the low populations in against cold weather. Agasicles hygrophila could success- the A. sessilis monoculture plots relative to those in the A. fully overwinter in Guilin by taking advantage of A. sessilis, philoxeroides monoculture plots and the mixed plots in our a suitable host at low latitudes. In contrast, the low tempera- study. Thus, our experiments with mixed plots further con- tures in winter in Wuhan and Kaifeng could directly restrict frmed that the beetle prefers the invasive A. philoxeroides

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Journal of Pest Science over the native host, A. sessilis. Furthermore, the changes in might be more critical when assessing nontarget efects adult size over time in Guilin (Fig. 3) might partly depend under the scenario of climate change (Lu et al. 2015, 2016), on the availability of suitable stems for pupation. Due to the which may alter plant physiology. Importantly, the results gradual withering and death of plant aboveground portions of our study may also extend to other weed biological con- with increasing feeding by the beetle population, a lack of trol programs for risk assessment of nontarget efects. Many suitable stems might account for the smaller sizes of some weeds, potential nontarget plants and insect biological con- adults. trol agents overlap in varying climate zones, likely leading Although A. hygrophila prefers the aquatic form of A. to diferent efects of the insects on nontarget hosts. Further- philoxeroides to the terrestrial ones (Julien et al. 1995; more, global climate warming may expand the geographi- Maddox et al. 1971), recent experimental studies and feld cal distributions of many invasive plants, biological control surveys have found that the beetle can also thrive on the agents and potential nontarget native species (Lu et al. 2015; terrestrial forms of both the invasive A. philoxeroides and McEvoy et al. 2012), creating novel biotic interactions. the native A. sessilis (Lu et al. 2015, 2016). In this study, we In summary, we found that A. hygrophila, a biological conducted our experiment in terrestrial habitats, where this control agent of A. philoxeroides, could sustain populations beetle could sustain populations and overwinter on native on the nontarget native plant A. sessilis during the grow- A. sessilis in the subtropical Guilin area. The lack of hollow ing season. When A. sessilis was the only host, the beetle stems in the terrestrial forms of A. philoxeroides was previ- could overwinter in subtropical regions but not in colder ously thought to explain the pupation failure of A. hygroph- areas. While previous laboratory tests and feld surveys ila (Coulson 1977; Ma et al. 2003). However, our results have reported only its ability to damage A. sessilis, our did not support this assertion. Since environmental hetero- study reveals how variations in climate may afect beetle geneity can afect plant morphology and physical structure, use of this nontarget plant under natural conditions. We rec- and since A. philoxeroides and A. sessilis occur over large ommend considering the shifting interactions of invasive geographical regions, variations in stem diameter and tissue plants, biological control insects and native plants induced mass density among terrestrial plant populations may afect by climate when assessing the nontarget efects of biocontrol beetle pupation on these hosts. In this study, we focused on agents. the efects of climate on A. hygrophila population and its overwintering on A. sessilis at three latitude sites; however, we only measured the stem diameter before releasing the Author contribution statement beetle but did not compare the diameter and tissue densities of plants at these three sites during the experiment, and these YW and JD designed the experiment; YW conducted the variables may deserve further study. We also acknowledge experiment; WH and MI analyzed the data; YW wrote the that our tests were conducted in cages and lasted for only manuscript; all authors revised and approved the manuscript. 1 year; thus, we cannot rule out the efects of other biotic and abiotic factors on the hosts and beetles. Under natural Acknowledgements We would like to thank Saichun Tang, Yumei Pan, condition, the beetle may disperse among latitudes during Chunqiang Wei, Baoliang Tian, Xiao Sun, Xiangqin Li, Li Xiao, Jia summer. However, if they disperse to higher latitudes where Liu, Zhenzhen Yu, Zhen Liu and Shunliang Feng for feld and labora- tory assistance. Edits and comments by Roy Van Driesche, Emmet the winter is too cold, then they may die during the winter, Van Driesche, Springer Nature Author Services as well as the journal as indicated by our fndings. The results of a recent warming editor and reviewers improved earlier versions of this manuscript. This experiment showed that increasing temperature increased work was supported by the National Key Research and Development the overwintering of A. hygrophila on A. sessilis, and living Program (2017 YFC 1200100). pupae have been found in feld A. sessilis in subtropical areas Compliance with ethical standards but not in temperate areas in China (Lu et al. 2015, 2016), A. hygrophila supporting our conclusions that can sustain Conflict of interest A. sessilis The authors declare that they have no confict of populations on terrestrial and that whether it can interest. overwinter on this native host depends on climate. Our results also have important implications for under- Ethical approval This article does not contain any studies with human standing the efects of climate on insect host performance participants or animals performed by any of the authors. and assessing nontarget efects in weed biological control. Diferences in A. hygrophila overwintering among latitudi- nal sites suggest that testing the safety of biological control References agents should consider variations in climatic and geographi- cal factors, which infuence the key life history period that Bale JS, Hayward SAL (2010) Insect overwintering in a climate change. restricts insect development and population. Such studies J Exp Biol 213:980–994. https​://doi.org/10.1242/jeb.03791​1

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