1. Introduction
The Asian citrus psyllid
D. citri Kuwayama (Hemiptera: Psyllidae), is a well-known significant citrus pest worldwide. Huanglongbing (HLB) is one of the most devastating diseases of
Citrus spp worldwide that limits commercial production [
1]. The HLB-infected trees show off-season bloom, twig dieback, stunted growth, early fruit dropping and ultimately, death of the whole plant may occur [
2,
3]. However, no cure for HLB has been reported [
4], and controlling
D. citri is the only effective way to manage HLB [
5]. The primary control measures of
D. citri largely depend on synthetic insecticides [
6,
7].
Similarly, Florida's citrus industry has been devastated by HLB and, since 2005, lost 74% of production [
8]. Insecticidal suppression of
D. citri has played a disproportionally significant role in HLB management. Eight to 12 insecticide applications are commonly used in one cropping season in China, Florida, Brazil, Mexico and other major citrus-growing countries worldwide [
9]. Under such selection pressure, the resistance to organophosphate, pyrethroid, neonicotinoid and carbamate insecticides in
D. citri has been reported [
9,
10]. Similarly, the massive use of these synthetic insecticides also negatively affects the environment, mammals, natural enemies, pest resurgence and development of resistance [
11]. Therefore, further effort is needed to identify a greater diversity of more sustainable tools to improve HLB management.
Botanical insecticides have long been used as alternatives to synthetic chemical insecticides for pest management because botanicals cause no or minimal threat to the environment or human health [
12]. Among the botanical pesticides, essential oils (EOs) are gaining the attention of scientists due to their ability to interfere with the primary metabolic, biochemical, physiological and behavioural functions of insects [
13,
14].
Artemisia absinthium L. (Asteraceae), known as wormwood, is an aromatic and medicinal herb of ethnopharmacological interest [
15]. In addition, it has been reported to have toxicity against
Callosobruchus maculatus and
Bruchus rufimanus [
16], repellent and larvicidal activity against mosquitos (Benelli, 2015; Govindarajan and Benelli, 2016), ingestion toxicity against
Sitona lineatus [
17] and
Drosophila melanogaster [
18], and fumigant toxicity against
Tetranychus urticae and
Bemisia tabasi [
19].
This study aimed to investigate the toxicity and behavioral effect of Artemisia absinthium essential oil against D. citri. Furthermore, the effect of Essential oil on the fitness of D. citri was also assessed using two-sex life table tools.
4. Discussion
Botanicals insecticides are plant-derived materials and include pyrethrin, azadiractin and neem oil, garlic, capsaicin, and vegetable oil. Botanicals are generally short-lived in the environment, as they are broken down rapidly in the presence of light and air [
34]. These include plant extracts and essential oils, which are eco-friendly, biodegradable, and nontoxic to mammals [
15]. Essential oils from various plants are reported having antifeedant, repellent, and toxic against many insect pests [
35]. These comprise various compounds but are dominated mainly by monoterpenes and
sesquiterpenes [
36].
These monoterpenes and sesquiterpenes can be lethal against insects in different ways. For example, limonene decreased oviposition in mite
Oligonychus ununguis [
37], carvacrol and thymol showed contact toxicity against
Pochazia shantungensis [
38], α-pinene, (-)-limonene and (-)-carvone showed fumigant and antifeedant activities against
Solenopsis invicta and Meloidogyne incognita [
39], thymol showed substantial contact toxicity against
Blattella germanica (Yeom et al., 2012), and 1,8-cineole showed both contact and fumigant toxicities against
Sitophilus oryzae and
Ectomyelois ceratoniae [
40]. Carvacrol showed contact toxicity against
D. citri [
15].
Plant volatile plays a crucial role in herbivores' host location and recognition [
41]. Odors and plant colors mediate how the herbivores' insects find and recognize their potential host [
42]. The citrus psyllid mainly relies on its olfactory and visual cues to locate and evaluates its host plants [
43]. The volatile compounds emitted from non-host plants mask the host plant odor perceived by the phytophagous insects, which results in avoidance and non-preference of host plants[
44]. Regarding the repellent activity of ABEO against
D. citri, results indicated a concentration-dependent effect. The adults strongly preferred settling on the control
C. sinensis seedlings to the treated seedlings. The psyllids settling was not significantly reduced 24 h after release compared to the control. However, after 48 and 72 h of release, only a few adults were observed on the treated plant compared to the control as
D. citri were able to identify host plants volatiles from the mixture of non-host volatiles in the open atmosphere in a short period, approximately nine h [
45]. A literature report indicated that the host finding and recognition ability of
D. citri were reduced when non-host plant semiochemicals were used [
44,
46]. Many non-host plants have shown repellent activities against
D. citri, including Guava [
3,
24,
47],
Allium spp [
48,
49]. HLB bacteria can only multiply in the body of the eukaryotic host [
50]. The transmission of HLB bacterium from infected to uninfected tree primarily takes by nymphs and adults of
D. citri [
51]. Here we found that ABEO reduces the feeding activity of
D. citri, measured as the number of purple spots on the treated leaf disc. ABEO at 20 and 20 and 30 mg/mL reduced 72.86 and 85.5% honeydew secretion compared to control. To get more accurate data on the antifeedant activity of ABEO against
D. citri, further investigation should be conducted using EPG (Electrical penetration graph) technology.
Essential oils are effective against several insect species. They act as growth inhibitors, toxins, deterrents, repellents, and toxicants [
52]. Essential oil of azadirachtin and
Piper aduncum against nymph and adults of
D. citri caused 90-100% mortality in nymph and below 80% in adults having the edge of nontoxic to ectoparasitoid
Tamarixia radiata (Hymenoptera: Eulophidae) [
53]. Similarly, the EOs from
Syzygium aromaticum, Eucalyptus obliqua , Tithonia diversifolia and Citrus limoniaI showed considerable toxicity and repellent effects against
D. citri [
54,
55]. Primarily
D. citri management was prodigiously focused on controlling adult psyllids. For this, many classes of insecticides have been utilized [
56]. Limited literature is available regarding the developmental and ovicidal products against
D. citri. The current investigation showed that potted
C. sinensis seedlings treated with ABEO give a concentration-dependent ovicidal activity. Only 11.75% and 30.44 eggs were able to hatch into adults when confined with the dry residue of ABEO at 20 and 30 mg/mL, while in control and 10 mg/mL concentration, the hatching percentages were 93.45 and 93.78%, respectively. The result showed that ABEO has ovicidal activity against
D. citri. Neurotoxicity is the possible mode of action of EOs against insects [
57,
58]. The EOs are generally composed of complex mixtures of monoterpenes, biogenetically related phenols, and sesquiterpenes, which have a wide range of hydrophilic-hydrophobic properties which are able to easily penetrate insect cuticles and interfere with their physiological functions [
59,
60].
Despite the most promising properties of EOs as a natural insecticide, many technical issues are raised for their broader application due to their rapid volatility, and poor water solubility [
13]. There are many challenges and constraints related to the commercialization of essential oils, including strict legislation [
61], low persistence of effects, and lack of quality and insufficient quantities of raw materials for affordable prices [
62]. Their rapid degradability and low persistence may significantly reduce the toxicity of EOs [
62]. However, the efficacy and persistence of EOs can be enhanced by encapsulation, nanoparticles and nano gel formulation, and cyclodextrins (CDs) [
63]. Overall, EOs have the potential to develop eco-friendly candidates for novel pest management, which should be a top priority to preserve ecosystems from contamination.
Essential oils and plant extracts are safer for the environment, humans, and non-targeted organisms than synthetic insecticides [
64]. In our current investigation, the toxicity ABEO was evaluated against
A. mellifera. The result indicated that EOshave lower toxicity, as the LC
50 value was higher than 31.5 mg/mL compared to the LC
50 value of 5.20 µg/insect against
D. citri.