Trophic groups’ interactions among cover crops, insect herbivores, and natural enemies (predators and parasitoids) in the tea agroecosystem.
The incorporation of aromatic plant species in tea plantations mitigated pest attacks, enhanced the populations of natural enemies, and hence improved conservation biological control
[66][43]. Intercrops (
Cassia tora L. and
Chamaecrista rotundifolia Greene) in tea plantations increased the abundance of arthropod predators by providing feasible microhabitats for nests and shelter
[43,52][36][42]. Intercropping tea with the aromatic plant
Cassia tora L. helped control the critical phytophagous pest (
E. vitis), assisted by the biocontrol approach. The populations of generalist predators (ladybird beetles and lacewings) increased due to the release of repellent volatiles
[52][42]. Three prime volatiles that exerted repellent effects on leafhoppers were p-cymene, limonene, and 1,8-cineole
[52][42]. Also, tea intercropped with
C. tora showed an increased abundance of the natural enemy (dragonfly)
Sympetrum croceolum Selys 1883
[52][42]. Zhang et al.
[66][43] mentioned that intercropping tea with aromatic plants,
Cassia tora L.,
Leonurus artemisia Loureiro and Mentha haplocalyx Briq, reduced the infestation of
E. onukii Matsuda in tea plantations. Intercropping cover crops (
Lavandula pinnata Lundmark,
Cassia tora L.,
Hedyotis uncinella Hook,
Trifolium repens L.,
Vigna sinensis L.) in the tea plantation helped in reducing the pest attack of TGL (
E.vitis Gothe) with the increase in the diversity of generalist predators and parasitoids
[67,68,69][44][45][46]. The population abundances of tea geometrid moth caterpillars (
Ectropis oblique Prout and
Ectropis griseescens Warren) decreased with the intercropping of cover crops (
Chamaecrista rotundifolia Greene and
Indigofera hendecaphylla Jacq.) in tea. The population of tea geometrids decreased due to the increased abundance of predatory parasitoids, namely, Formicidae, Araneida, Mymaridae, Braconidae, and Trichogrammatidae
[43][36]. Tea plants intercropped with aromatic plants (
Ocimum basilicum L. and
Perilla frutescens L.) decreased the abundance of crucial tea pests:
Empoasca onukii Matsuda and
Apolygus lucorum Meyer-Dür. The reduction in the pest attack of tea green leaf hopper and mirid bug was due to the increased diversity of natural enemies and predators (coccinellids, lacewings, parasitoids, and spiders). Ye et al.
[70][47] mentioned that intercropping tea plants with economic crops (citrus, waxberry, and snake gourd) helped suppress key tea pests by increasing the diversity and species richness of key araneid natural enemies. Intercropping tea with
Ageratum conyzoides assists in the biological control of the tea pest
Empoasca pirisuga Matumura by increasing the diversity of predatory mites (
Anystis baccarum L.)
[71][48]. White clover (
Trifolium repens L.) as an intercrop in tea reduced the populations of key tea pests: tea geometrid (
Ectropis grisescens Warren), tea green leaf hopper (
E. vitis), and tea aphid (
Toxoptera aurantii Boyer de Fonscolombe)
[67][44]. Moreover, the population of natural enemies (Araneae, coleoptera, hymenoptera) in the tea ecosystem increased
[67][44]. Maize as an intercrop in tea plantations reduced the population of tea green leaf hoppers (
E. onukii Matsuda) and white flies (
Trialeurodes vaporariorum Westwood)
[62][38].
The predominant natural enemies on tea plantations are ladybird beetles, spiders, assassin bugs, lacewings, and praying mantises
[72][49]. The potential of intercropping to enhance arthropod biodiversity provides a gateway for biological pest control in tea plantations
[43][36]. The most abundant natural enemies (predators and parasitoids) in the tea plantations after intercropping cover crops (as seen in
Figure 2) and their ecosystem services in biological control are discussed below.
2.1.1. Ladybird Beetles
Coleopterans are distinct members of food webs in tea plantations, with high species richness and abundance
[65][41]. Tea plants intercropped with herbaceous perennial legumes (
Chamaecrista rotundifolia Greene and
Trifolium repens L.) had high beetle abundance and species richness, while tea plants intercropped with
Paspalum notatum Flugge had enhanced biomass and species richness
[65,67][41][44]. The most abundant predatory beetles were
Serangium japonicum Chapin and
Pharoscymnus taoi Sasaji in the tea plantations intercropped with C. rotundifolia and
P. notatum [65][41].
Propylea japonica Thunberg,
Harmonia axyridis Pallas, and
Coccinella septempunctata L. were the predominant species of coccinellids in the tea plantations intercropped with an aromatic plant (
Cassia tora L.)
[66][43]. Cover crops (round leaf cassia, white clover, cassia) intercropped in tea plantations provided a feasible habitat and food source for the ladybird beetles, which helped reduce the attack of tea pests (
A. lucorum and
E. onukii).
2.1.2. Spiders
Spiders are predators of insect pests in cultivated ecosystems, and the maintenance of a thick layer of leaves within the tea canopy is essential for sustaining spider communities
[73][50]. Intercropping cover crops can profoundly affect the spider communities in tea plantations due to the favorable habitat and food source provided by diverse cover crops. The Araneida number was high on the plots where tea was intercropped with
C. rotundifolia and
I. hendecaphylla, where they helped to check the population of
Empoasca onukii Matsuda, Thysanoptera, and Geometridae caterpillars
[43][36].
A well-maintained tea canopy, where
C. rotundifolia and
P. notatum were used as cover crops, saw
Coleosoma octomacutatum Bösenberg & Strand,
Telamonia bifurcilinea Bösenberg & Strand, and
Erigone sp. as the dominant predatory spider species
[54][29]. Moreover, the populations of
Erigone sp. were higher in tea intercropped with
P. notatum, while the absolute abundance of spiders was greater in tea intercropped with
C. rotundifolia [54][29]. In tea plantations intercropped with
C. tora, the predominant predatory jumping spider species were
Evarcha albaria L. Koch and
Plexippus paykulli Audouin, which preyed upon tea green leaf hopper (
E. onukii Matsuda)
[66][43]. Intercropping tea plants with citrus, waxberry, and snake gourd helped suppress tea green leafhopper (
E. vitis Gothe), with the increased diversity of key araneid natural enemies
[70][47]. Furthermore, population density and spatial distribution patterns of the leafhopper and the Orb-weaver spiders (araneids) were regulated, thereby checking the population abundance of tea leaf hopper (
Empoasca vitis Gothe)
[70][47].
2.1.3. Mites
Diversifying tea agroecosystems using cover crops can strengthen predatory mite densities, thereby providing a viable biocontrol tool and promoting the environmentally benign production of tea products
[43,74][36][51]. In tea plantations, mites constitute one of the most critical and complex communities of arthropods, where intercropping cover crops assist in augmenting the densities of the predatory Whirligig mite (
Anystis baccarum L.)
[75][52]. Tea plantations, where
P. notatum and
C. rotundifolia are used as intercrops, exhibited a high number of individuals (N), higher species richness (S), effective diversity index (eH’), and absolute abundance (n) of predatory
Anystis baccarum L.
[76][53]. Liu
[71][48] mentioned that tea intercropped with
A. conyzoides and
C. tora saw a decrease in tea green leaf hopper (
E.pirisuga) abundance due to the augmented density of
A. baccarum. Furthermore, intercropping cover crops like
P. notatum and
C. rotundifolia helped to enhance the densities (per unit abundance) of the predatory mite,
A. baccarum, in the tea canopies and may reduce the populations of the leafhopper pest,
E. onukii [77][54].
2.1.4. Lacewings
Lacewings, voracious predators, are predominantly seen in tea plantations and assist in biological control. Intercropping diverse cover crops in tea plantations increased the abundance of green lacewings, stimulating the biological control of tea pests. The diversity of predatory lacewings (
Chrysopa septempunctata Wesmae1) was increased when
C. rotundifolia and
I. hendecaphylla were used as cover crops in tea plantations, which helped in the control of green leaf hoppers and tea geometrids
[43][36]. Moreover, tea plants intercropped with
C. tora demonstrated a marked increase in
Chrysopa sinica Tjeder
[66][43].
2.1.5. Parasitoids
Parasitoids are essential ecosystem service providers renowned for their role as biocontrol agents in sustainable pest-management strategies
[76][53]. Parasitoids can be used as biological control agents to mitigate pest attacks in tea agroecosystems. Zhang et al.
[66][43] revealed that the critical parasitoid species in aromatic plants (
Leonurus Artemisia Lour. and
C. tora) intercropped in a tea garden were
Ephedrus plagiator (Nees),
Amitus hesperidum Silvestri,
Aphelinus mali Haldeman, and
Apanteles adoxophyesi Minamikawa. The most dominating parasitoids included families belonging to Eulophidae, Encyrtidae, Mymaridae, and Scelionidae, while abundant parasitoids included families belonging to Braconidae, Aphelinidae, and Trichogrammatidae in the tea plantation intercropped with cover crops (
C. rotundifolia,
I. hendecaphylla, and
V. sinensis)
[43][36]. The cover crops (
I. hendecaphylla and
C. rotundifolia) intercropped with tea constituted a higher species richness of parasitoids.
I. hendecaphylla harbored more significant numbers of Braconidae, Eulophidae, and Aphelinidae parasitoids, while
C. rotundifolia harbored Trichogrammatidae parasitoids
[43][36]. Furthermore, Chen et al.
[43][36] reported that the incorporation of
V. sinensis as a cover crop in tea plantations increased the abundance of Eulophidae parasitoids.
3. Cover Crops, Tea Yield, and Biological Control
The incorporation of leguminous cover crops (hairy vetch, soybeans, sunn hemp, and winter peas) can generate the following substantial input cost savings for cash crop. The leguminous covers help add and retrieve nutrients by providing (45–224 kg ha
−a) of available N for cash crop production
[77][54]. The cover crops promote long-term sustainability and higher cash-crop yields, with reduced fertilizer, herbicide, and pesticide applications
[78][55]. The combined utilization of leguminous and grass cover crops has been reported for multiple ecosystem services (biological N
2 fixation, weed control, attracting pollinators, and increasing SOM)
[79][56]. Moreover, leguminous cover crops remarkably increased subsequent primary crop yields by 9.7% on average compared to fallow across China
[80][57].
Soybean and tea cover cropping improved secondary metabolites with enhanced N uptake, encouraging the reduced application of synthetic nitrogen fertilizers
[81][58]. Tea plants intercropped with white clovers (
T. repens) saw an increase in tea yield by 32.6% and a decrease in the ratio of polyphenol to amino acid (TP/FAA) in spring tea and autumn tea (17.10% and 30.90%)
[67][44]. Intercropping tea with
Vulpia myuros L. improved tea root activity with accelerated nutrition uptake and increased functional tea quality components (free amino acids, polyphenols, and caffeine)
[82][59]. Cover crops also have potential in pest management, as they can break pest cycles
[83][60]. Intercropping cover crops in tea rows outranks tea rows clearance and natural weed cover, where enhanced tea production and biological pest control were considered the additional benefits of cover crops
[67,84][44][61].
Litsea cubeba Lour, a vegetative branch inter-row cover, has been suggested as an important biological control agent in sporadic planting or branch coverage in tea plantations
[85][62].
Despite the broad potential in agricultural pest management, cover crops are still not considered a silver bullet due to the risks associated with environmental stewardship
[86][63]. The wide adoption of cover crops has not been achieved yet due to the economic, biological, and farm operational factors and uncertainties in pest dynamics
[87][64]. The excessive use of pesticides might provide increased yield for a few years, but in the long run it will depopulate the beneficial arthropods
[88][65]. Hence, cover crops will quantify ecosystem services, including increased yield, quality, and biological pest control in the tea agroecosystem (as seen in
Figure 3).
Figure 3. Ecosystem services provided by cover crop intercropping in the tea agroecosystem.