Are spiders the perfect predator?

7/30/2019 Are spiders the perfect predator?

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piders are the most abundant preda-

tors in agricultural systems. They

make up 50 p er cent of the predators

in unsprayed cotton crops in Australia and

up to 80 per cent of cotton crop predators

in China. Despite their prominence, spidershave received little attention in Australian

cotton and their role in helping to manage

pests has generally been overlooked.

Research being undertaken as part of

the Australian Cotton Cooperative

Research Centre in Narrabri and CSIRO

Entomology aims to help growers and con-

sultants understand spiders and their true

value in crop, so they can begin to incor-

porate them into IPM strategies.

When people discuss beneficial species

in crops they usually ignore spiders and

talk only about predatory insects. Until

recently, the general feeling was that spi-

ders are too difficult to exploit in IPM

because most are generalist predators and

not specific to one pest species.

Although some spiders are actually spe-

cialists (for example, the striped lynx spider

is responsible for 80 per cent of the pre-

dation on the cotton fleahopper,

Pseudatom oscelis seriatus) many spiders

capture a wide range of species, and socould be called generalist predators. This

can be an advantage in IPM strategies.

N OW HERE TO HIDE

It has been shown that the best way to

control a multi-prey complex (such as that

seen in cotton fields) is to use a multi-pred-

ator complex which reduces ‘enemy free

space’ for the pests. In other words, a

number of generalist predators living on

different parts of the crop control pests

well because there is nowhere on the crop

which is safe for any pests.

Such an approach uses all the p redators

in the invertebrate community to control

pests as opposed to the use of one spe-

cialised predator to attack a single pest

species. The latter is the more common

approach when biological control agents

are used to control agricultural pests.

But a complex of general predators is

more likely to be effective as they are not

restricted to one prey species. This meansthe numbers of predators present is inde-

pendent of the numbers of any one prey

species.

Because the predators are eating a

range of prey, their numbers will remain

high even if the numbers of one pest

species drop. This system means that the

numbers of predators in the system will not

fluctuate greatly.

Control by generalist spiders has p roven

to be effective. When spider numbers were

enhanced in a garden system by increasing

its structural complexity (by mulching), pest

numbers were significantly reduced. By

feeding on a range of prey, spiders are both

buffered against a drop in specific pest

30 — TH E A U STRA LIA N C O TTO N G RO W ER N O V EM B ER– D EC EM B ER, 2 0 0 1

Are spiders the perfect predator?

By M ary W hitehouse and Louise Law rence, CSIRO Entomology

S

FIGURE 1 : Relative composition of the diets of various spider groups (identified by family name)

8 0

8 0

8 0

8 0

8 0

0

smal l Oxyop id

(Small lynx spider)

Thomisid

(crab spider)

l a r ge O x y o p i d

( large green lynx spider)

Salticid(jumping spider)

Lycosid

(wolf spider)

A p h

i d s

e t c .

f l i e s

b e e

s /w a s p s

/ a n t

s

s p r i n

g t a i l s

b e e t l e

s

b u g s

m o t h s

s p i d

e r s

o t h e

r s

8 0

8 0

8 0

8 0

8 0

0

Linyphiid

money spiders/ sheet web w eavers)

Tetragnathid

( long jawed spiders)

Arane id

(orb w eavers)

Dictynid

(mesh web spiders)

Theridiid( space web / redbacks)

Hunters

Araneid (garden

o rb weave r )

Thomisid

(crab spider)

Ad apt ed f rom Ny f f e ler 1 999

W eb bui lders

3 2



species numbers, and are ‘on hand’ to take

advantage o f outbreaks of particular p ests.

Although most spiders capture a range

of prey, different species of spiders cap ture

different types of prey (Figure 1). For

example, in a recent literature survey,

Araneids (garden orb weavers) were found

to take only aphids, flies and beetles.

Theridiids (red-blacks, tangle webs) took

aphids, flies, beetles, wasps, bees and ants.

Linyphiids (money spiders) took aphids,

flies and springtails. Hunting spiders were

found to attack a greater range of prey

than the web-builders mentioned above,

and their diet included a high proportion of

other spiders.

One perceived downside of spiders is

that they kill many beneficial insects,

including other spiders. Although this‘hyperpredation’ can reduce the effective-

ness of a particular predator species, it

enhances the overall diversity of predators

available by not allowing one predator to

take over.

Body size also affects prey choice in spi-

ders. Laboratory and field experiments

showed that optimal prey length ranges

from 50–80 per cent of the spider’s own

length. As small spiders, including many

immatures, dominate in agricultural sites,

most spider prey will be small (less than

four mm long).

SPIDER GUILDS

In o rder to understand sp ider communi-

ties better, spiders are now being classified

into guilds (groups) based on the way they

forage (Figure 2).

Classifying spiders into foraging guilds

enabled researchers to compare the spider

communities in eight different types of

crops (Figure 3). They found that the

32 — TH E A U STRA LIA N C O TTO N G RO W ER N O V EM B ER– D EC EM B ER, 2 0 0 1

FIGURE 3 : Proportional comparison of the spider guild composition of

selected crops. The dendrog ram show s the rela tionship betw een the

spider communities in the different crops. The bar graph represents

the relative abundance of individuals in different spider guilds (based

on num erous published reports)

Stalkers

Ambushers

O t h e r

P e a n u t

A l f a l f a

S o y b e a n s

R i c e

C o r n

C o t t o n

S u g a r

S o r g h u m

Space web-builders

Foliage runners

Ground runners

Sheet web builders

W and er ing sheet w eb-bui ldersO r b w e a v e r s

FIGURE 2 : Dendrogram of spider foraging guilds illustrating the diversity of foraging methods used by

spiders (adapted from Uetz et al 1999 )

7 0 7 5 8 0 8 5 9 0 9 5 1 0 0

Similarity %

Anyphaenidae

Clubionidae

Sparassidae

Lycos idae

Dy s der i dae

Gnaphos idaeMimet idaeO x y o p i d a e

Saltic idaeThomisidae

Philodromidae

Pisaurida eA gelenidae

Amaurobi idaeHahniidae

Filistatida eLinyphiidae

Mi cryphantidaeA r ane idae

Tetragnathidae

Ulobor idaeTheridi idaeDictynidae

Pholcidae

Hunting

W e b -building

Sheet webs

W ander ing sheet/ tanglew e a v e r s

Sheet webbuilders

Stalkers/ambushers

Ambushers

Stalkers

Groundrunners

Foliagerunners

Runningspiders

Or b we a v e r s

Space web builders

Aer ia l webs

Space web spider(Photos: D McClenaghan)

Long-jawed spider

Lynx spider

Yellow nig ht stalker(Photo: C Mares)

W olf spider

3 0 … PE RF EC T P RE D A TO R



crops fell into two distinct groups of struc-

turally similar spider communities: group

one (peanut, alfalfa, soybean, rice) whose

spider fauna was dominated by ground

runners and wandering sheet web-

builders; and group two (cotton, corn,

sugar, sorghum) whose spider fauna was

more complex because it also included

many orb weavers and stalkers.

The most likely reason why crops such

as cotton support more types of spiders is

because these crops are more structurally

complex than crops such as rice. This

complexity provides spiders with more

habitats and so allows cotton to support a

more diverse range of spiders. This in turn

reduces ‘enemy free space’ for the pests.

Consequently, it is in agricultural systems

such as cotton where spiders may best be

used to he lp suppress pests.

Research has shown there is a wide

range of spiders in Australian cotton fields

and, with more and more cotton growers

now using IPM and conserving beneficials,

these spider populations are likely to be

conserved.

But there still needs to be research direct-

ed specifically at spiders in order to identify

which species are the most effective and

what conditions enhance their activity. This

information would allow spiders to become

an integral part of IPM strategies.

For more information contact Mary

W hitehouse, CSIRO Entomology, Na rrabri, ph:

0 2 6 7 9 9 1 5 3 8 , f a x : 0 2 6 7 9 3 1 1 8 6 , e m a il:

M ar y.Whitehouse@ csiro.au

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Are spiders the perfect predator?