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Role and Functions of Spiders in Ecosystem

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Lycosidae-Wolf spiders

These groups of spiders are very active and are usually chase their prey. Lycosa pseudoannulata is considered as the most efficient predators of plant hoppers in rice ecosystem (Heong et al., 1991). Mathirajan (2001) reported that predatory potential of Lycosa pseudoannulata on BPH, WBPH, and GLH was about 7.4, 6.9, and 6 respectively.

Pardosa pseudoannulata is another common spider which preferred hoppers, yellow stem borer, collembolans and flies (Rubia et al., 1990). Samiayyan and Chandrasekharan (1998) recorded that Paradosa sp. exhibited highest preference on BPH followed by WBPH and GLH and the number of prey consumed was 14.31, 10.96 and 5.73 respectively. Lu Zhong-xian et al. (2006) also documented the prey range of P. pseudoannulata which includes leafhoppers, plant hoppers, whorl maggots, leaf folders, case worms and stem borers .

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Vegetables

Thirty species of spiders in 9 consumes Amrasca biguttula biguttula, Aphis malvae. and Bemisia tabaci and caterpillars of Spodoptera litura. They predated on Hemiptera, Lepidoptera and coleopteran pests in various vegetable crops (Manu, 2005).

Crab spider, Thomisus sp. feed on caterpillar and adults of Helicoverpa armigera in tomato fields of Bangalore (Ansari and Pawar, 1980). Argiope catenulata, O. javanus and Neoscona theisi consumes Amrasca devastans, Aphis gossipii, B. tabaci, larva of H. armigera and larva of S. litura (Mathirajan and Reghupathy, 2003).

Argiope luzona, Chrysso argyrodiformis, Hipassa pantherina, Oxyopes lineatipes, Oxyopes javanus, Peucetia viridana and Lycosa pseudoannulata are the important spiders present in the snakegourd and brinjal ecosystem which feed on variety of insects like Plusia orichalcia, Leucinodes orbonalis, Aphis gossypii, B. tabaci and Epilachna vigintioctopunctata (Sankari and Thiyagesan, 2010).

Pulses

The prevalent spiders found in redgram ecosystem were Thomisus shivajiensis, Clubiona abbotti and Hippasa haryanesis, which predated on lycaenid butterfly, Lampides boeticus (Singh and Mavi, 1984). They also observed to be control the population of H. armigera, Clavigrella sp. and moderately feed on Melanagromyza obtusa (Arrora, and Monga, 1993). Borah and Dutta (2001) reported that Oxyopes shweta, Thomisus sp. and Saliticus sp. predated on pod borer Maruca testulalis.

Oilseeds

Eighteen species of predatory spiders belonging to 16 genera in 7 families are commonly observed under oil yielding crops (Sadana and Goel, 1995). In groundnut Oxyopes salticus, and Misumenops sp. occupied 85.8 to 97.7 per cent of overall population of spiders and effectively control the population of sesame capsule borer Antigastra catalunalis and Acherontia styx (Biswas et al., 2001).

Plantation crops

In coconut, 26 species of spiders under six families were commonly observed in which Rhene sp. and Cheiracanthium sp. were reported as the predators of black-headed caterpillar Opisina arenosella (Sathiamma et al., 1987). Bhat et al. (2013) recorded that about 117 species of spiders under18 families were recorded from the cashew plantations. Telamonia dimidiata and O. shweta were recorded as the most common spiders of cashew plantation which were act as predators of tea mosquito bug Helopeltis sp. Argiope pulchella, Neoscona mukerjeri, Oxyopes sunandae, Nephila pilipes were also observed to be take part in keeping the pest population under control. In tea plantations, 85 species of spiders under 52 genera and 18 families were recognized from India. Araneus mitificus, A. pulchella, Neoscona bengalensis, Dendrolycosa gitae, Thiana bhamoensis were reported as the predominant spider fauna (Saha et al., 2016).

Fruit crops

About eleven spiders were documented from mango orchards in which Araneus singhagadensis, Cheiracanthium danieli and Stegodlyphus sarasinorum were the prevalent species act as predators of mealy bugs (Tandon and Lal, 1983). The predatory potential of Lyssomanes sikkimensis on mango hoppers was about 0.60 to 5.20 per day (Sadana and Meenakumari, 1991). O. javanus, A. pulchella and Tetragnatha sp. were also observed from mango ecosystem which feed on caterpillars of mango (Shivamoorthy, 2016).

In grapevine, 27 species of spiders belonging to 14 families were recorded and the important species documented were C. inclusum, T. dilutum, T. melanurum, Trachelas paceficus and Hololena dedra which were predated on grapevine leaf hopper Erythroneura variabilis (Costello and Daane, 1995). Most common spiders in apple orchards were Anyphaena accentuata, Clubiona brevipes, C. corticalis and C. leucaspis which were reported as effective predators of aphids and caterpillars (Marc et al., 1999).

Flower crops

The important predatory spiders found in jasmine ecosystem were Phidippus punjabensis, Salticus sp., Cheiracanthium sp., Pardosa sp. and Theridion sp which were effective against larvae of Nausinoe geometralis (Shukla and Sandhu, 1983).

Factors affecting spider population in an ecosystem

Ecological factors

  • Soil type- If the soil type is favorable for the plant growth, then there will be more plant complexity in the field which provides a habitat for the spiders.
  • Moisture content –Spiders prefer high moisture content in the environment.
  • Rain fall- If there is a reduction in rainfall, it will reduce the population of spiders in the field.
  • Temperature-Spiders cannot tolerate high temperature conditions in the field.
  • Percentage of organic matter –If there is sufficient organic matter in the soil, there will be detritivores species in the field, which act as an off season food for the spiders.

Direct and Indirect effect of Chemicals

  • Herbicide treatments lead to destruction of habitat of spiders than by direct toxic effects (Raatikainen and Huhta,1968)
  • Chemical application leads to reduced prey availability
  • Repellant effect of chemicals

Effect of insecticides, botanicals and microbials on spiders

Rather than direct toxicity, chemicals lead to the destruction of the habitat which negatively affects the population of spiders in agro ecosystems. Besides, pesticide application resulted in reduction of prey availability along with consumption of poisoned prey which ultimately resulted in death of spiders (Marc et al., 1999).

Size of spray droplets also plays an important role in determining the spider population. Samu et al. (1992) proved that finer spray droplets were more adhered to the webs than that of coarse droplets. Usually spiders consume their web before constructing a new one resulted in the entry of these chemicals in to the spider body.

Experiments conducted to determine the relative toxicity of chemicals to spiders revealed that a combination of thiomethoxam and imidacloprid at a rate of 25 g ai/ ha reduced the spider population in absolute numbers Krishnaiah et al. (2003). But Anis (2007) reported that imidacloprid 0.005% was safer to spider fauna in rice ecosystem and synthetic insecticides like acephate 0.05% and quinalphos 0.05% were proved to be toxic to the spiders.

Broad-spectrum chemicals like dimethoate exhibited high toxicity to spiders which resulted in cent percent mortality to the lycosid Trochosa ruricola at concentrations below the field rates (Birnie et al., 1998). In vegetable ecosystems, avermectin was recorded as the highly toxic chemical to spiders (Cheng et al., 2000). Manu (2005) identified that dimethoate at 0.05% was highly toxic to T. mandibulata and N. mukerjei caused cent percent mortality whereas imidacloprid 0.002% was less toxic to spiders.

Use of botanicals like neem oil, neem seed kernel extract and custard apple oil were comparatively safe to the spider population in agroecosystrems (Rajendran, 1987). Application of neem oil and Azadirachtin were recorded as less toxic to spiders like Distina sp., Marpissa sp. and Oxyopes sp. (Chakraborti, 2001).

Bt formulations Biotox applied at the rate of 1 kg/ha were found to be safe to spiders of Okra (Mishra and Mishra, 2002).

Fungi Safety to spiders Important spiders Reference

Metarhizium anisopliae Safe

Spiders in vegetable ecosystem (Manu, 2005)

Paecilomyces liilacinus Safe

Fusarium pallidoroseum Safe Paradosa sp., Tetragnatha maxilosa, Oxyopes sp. (Reji rani, 2001)

Beauveria bassiana Pathogenic Tetragnatha mandibulata,

C. danieli (Manu, 2005)

Conservation and augmentation of spiders

  • Establishment of favorable habitats for the spiders along with the artificial release of spiders in to the field. In China, straw beds were placed in fields which act as a shelter for spiders and these shelters along with spiders were transported to other places for the management of pest population in that area. This practice decreased the use of pesticides by 50 to 60 per cent (Zhao, 1993).• Artificial Introduction of spider egg sacs in to the field along with drosophila flies. In Japan, Drosophila sp. were artificially released into rice fields in order to act as a food source for spiders in dearth periods (Kobayashi, 1975).
  • Maintaining ground cover crops in orchard which provide a habitat for spiders (Riechert, and Lockely, 1984).
  • Maintenance of weeds and annual flowers in the field will conserve the spiders and also maintain the micro climate in the field. e.g. The aquatic weed, Pistia straitoides, in rice sheltered more spider and spiderlings of Oxyopes sp. and P. pseudoannulata, which provides a suitable atmosphere for the maintenance of spiders (Rajendran, 1987).
  • Addition of organic manure in to the field. It leads to the abundance of soil arthropods which act as supplementary food source for spiders (Settle et al., 1996).
  • Minimum tillage will conserve the spider population (Rajeswaran et al., 2005).
  • Botanicals and bio pesticides are less toxic to the spider population in the field. Use of botanicals like neem oil, neem seed kernel extract, neem seed biters, chinaberry oil and custard apple oil almost conserved the natural spider population (Rajendran, 1987).
  • If there is a severe pest attack, we can depend upon some spider friendly insecticides.

Spider friendly insecticides

  • Insecticide Dosage Agro ecosystem Reference
  • Imidacloprid 17.8 SL 0.02% Vegetable (Manu, 2005)
  • Flubendiamide 480 SC 0.02% Rice (Sekh et al., 2007)
  • Chlorantraniliprole 20 SC 40 g a. i. / ha Rice (Shanwei et al., 2009)
  • Indoxacarb 14.5 SC 72.50 g a.i./ ha Rice (Karthick et al., 2015)
  • Chemical application should be judicious and limited to the periods when the spiders are inactive and in sheltered locations.

Conclusion

Spiders, the generalist predators in the agro ecosystems play an important role in the management of pest population and reduce the pesticide use pattern which eventually leads to sustainable agriculture. Besides, over wintering nature of the adults make it easier for the spiders to invade the ecosystems early in the season, so that they can subdue the pest population in a better way than other natural enemies. The increased inclination towards pesticide free products and environmental stability in the current decades led to the projection of spiders as a boon to natural pest control mechanism.

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