## 1 Introduction  
The herbaceous peony (_Paeonia lactiflora_ [PERSON]) is a kind of traditional plant in Asia that has existed for more than 1200 years [1]. The species is currently largely distributed and cultivated in many regions all over the world, such as New Zealand, Europe, and North America [2, 3]. _Paeonia lactiflora_ is well known as an ornamental plant and it is used in traditional medicine [4, 5]. Furthermore, the oilseed of _P. lactiflora_ is of high nutritional value including linoleic acid, linoleic acid, oleic acid, calcium, magnesium, and other elements [6, 7]. Peony oil has a high proportion of polyunsaturated fatty acids (PUFAs) which can help reduce the risk of cardiovascular disease in human [8, 9]. Therefore, the development of _P. lactiflora_ cultivation is of great significance to improving the supply of high-quality edible oil. However, low yield is a key factor limiting the application of _P. lactiflora_. Efficient pollination could commendably improve seed formation [10].  
Insect pollination plays a vital role in the balance and manipulation of both natural and agricultural ecosystems by providing efficient pollination and contributing to improving crop yields and nutrition [11, 12, 13, 14]. Recently, insect pollinators have been widely used in agriculture including crops, vegetables, fruits, and flowers [15, 16, 17, 18, 19, 20].  
A great variety of insects can provide pollination services for plants, including Hypernoptera, Diptera, Coleoptera, and Lepidoptera insects. Among various insect pollinators, bees from Hypernoptera represent an important group. In view of their high pollinating ability and adaptation for artificial breeding, pollinating bees are the dominant pollinators in agricultural systems. Bee pollination has been recognized as an indispensable measure for crop production safety. Honeybees, such as _Apis mellifera_ L. and _Apis cernana cernana_, are the main species used for pollination in agriculture.  
The behavior and efficiency of insects pollinating different crops differ a lot. By analyzing pollination behavior and efficiency, _A. mellifera_ has been characterized as the most suitable pollinator for _Citrus maxima_[21], whereas for _Paeonia ostia_, the species _A. mellifera_, _A. cerana cerana_, and _Bombus flavescens_ are candidate pollinators [22]. Recently, many kinds of insects were deemed to pollinate _P. lactiflora_[23].  
It is necessary to evaluate the pollination efficiency of insects. DNA barcodes in pollen samples from corbiculae (pollen baskets) of bees have been a useful tool to analyze bee pollination [24]. DNA barcodes refer to a DNA-sequence-based identification system and they have become an important tool in taxonomy [25; 26; 27]. The plastid _rbcl_ gene is one of the frequently used DNA barcodes and it has been applied as a species-level discriminator [28; 29]. Researchers used the _rbcl_ gene in a pollen mixture to identify plant species [24].  
Against the background that the pollination behavior and efficiency of _A. mellifera_ in a _P. lactiflora_ field are still not fully understood, we selected _A. mellifera_ as the candidate pollininator for _P. lactiflora_. To evaluate the pollination efficiency of _A. mellifera_ in _P. lactiflora_ field, we (1) analyzed the pollination behaviors of _A. mellifera_ in a _P. lactiflora_ field; (2) identified the pollen that adhered to _A. mellifera_ legs through molecular markers; and (3) evaluated the number and weight of _P. lactiflora_ seeds.