## 1 Introduction  
Plant parasitic nematodes (PPNs) are a devastating problem to a wide range of crops worldwide [1]. Within PPNs, more than 4100 species have been described. The damage caused by PPNs was estimated to be more than 100 billion dollars per year worldwide, which is mainly caused by root knot nematodes (RKNs) and cyst nematodes (CNs) [2]. Among CNs, the beet cyst nematode _Heterodera schachtii_ is a major problem and an important sedentary parasite of sugar beets, as well as more than 200 plant species belonging to 95 genera [3,4]. _H. schachtii_ is a highly specialized obligate biotrophic nematode. When conditions are favorable, the infective second stage juveniles (J2s) hatch from eggs and spread in the soil. Having reached a host root, they enter it and migrate through the cells into the vascular cylinder. There, they search for a specific cell that is suitable to establish the initial syncytial cell (ISC) [5]. From the ISC, a highly active syncytial nurse cell system develops through fusion with the neighboring cells. The hypertrophic and hypermetabolic syncyums serve as the only source of nutrients for the developing juveniles. The nematodes undergo three molts before reaching the adult stage. Males leave the root in a vermiform shape, whereas females remain at the root, grow to a lemon-like shape and, finally, rupture the root cortex. After mating, females produce eggs inside their bodies, until they eventually die. After death, their cuticles turn into brown-tanned cysts containing the eggs, which can survive for years until the conditions are again favorable [5]. For decades, the control of nematodes has heavily relied on chemical nematicides [6]. Nowadays, many of these \"old\" chemical nematicides are banned in most countries due to their environmental toxicity. Crop rotation and resistant or tolerant crop cultivars--if available for the specific crop plant and CN--have also been used and are still in use [7,8]. The effectiveness of crop rotation is limited though in some cropping systems because of the wide host range or long-term survival of CNs. Furthermore, the occurrence of virulent and resistance-breaking _H. schachtii_ populations jeopardize the efficacy of using resistant crops [9]. All these factors minimize famers' management options. Therefore, the urge for finding novel, environmentally friendly and effective management strategies to control nematodes is rising. One option is to make use of the enormous variety of plant metabolites that are available as bioactive compounds with diverse properties. The challenge is to identify their effects, modes of action and suitability for application in sustainable crop production. Plant bioactive metabolites naturally play an important role in plant development and are involved in interactions with other organisms, such as insects, pathogens and neighboring plants [10, 11, 12, 13, 14, 15, 16]. Terpenoids are the largest and most diverse family of bioactive natural products, and several have allelopathic potential and are effective against different pathogens [12, 16, 17]. Multiple studies report the activity of terpenoids against nematodes [12, 18, 19]. For example, out of 22 investigated monoterpenes, borneol, carveol, citral, geraniol and \(\alpha\)-terpineol were shown to be the most active against the RKN of _M. incognita_, as they decreased nematode hatching and survival, as well as the parasitism of _M. incognita_ on tomato plants [12]. In another study, carvacrol, thymol, nerolidol, \(\alpha\)-terpinene, geraniol, citronellol, farnesol, limonene, pseudoionone and eugenol were shown to have nematocidal activities against the nematode _Caenorhabditis elegans_[19]. Such reports demonstrate the potential of terpenoids as a source of nematicides. So far, no comparable studies were performed with a focus on _H. schachtii_. Therefore, we decided to investigate the effects of commercially available plant terpenoids against the CN _H. schachtii_.