Action: Grow non-crop plants that produce chemicals that attract natural enemies
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Natural enemies: Four studies from China, Germany, India and Kenya tested the effects of growing plants that produce chemicals that attract natural enemies. Three (including one replicated, randomised, controlled trail) found higher numbers of natural enemies in plots with plants that produce attractive chemicals, and one found that the attractive plant also attracted natural enemies in lab studies. One found no effect on parasitism but the plant used was found not to be attractive to natural enemies in lab studies.
Pests: All four studies found a decrease in either pest population or pest damage in plots with plants that produce chemicals that attract natural enemies.
Yield: One replicated, randomised, controlled study found an increase in crop yield in plots with plants that produce attractive chemicals.
Crops studied were lettuce, orange, safflower and sorghum.
This intervention involves growing non-crop plants which produce volatile chemicals (quickly evaporating scents or odours) that attract natural enemies, thereby encouraging the enemies to the main crop. Non-crop plants could be grown in field margins or interspersed into the main crop. Lab studies demonstrating an attractive effect of a plant species or variety to a natural enemy are also included.
Here we present evidence from four of six studies testing this intervention.
Supporting evidence from individual studies
A controlled study in summer 2000 in Bonn, Germany (Sengonca et al. 2002) found that the abundance of four natural predators was significantly higher in lettuce Lactuca sativa plots intercropped with attractant plants (3.0-3.2 larvae and 3.2-3.6 adults per lettuce plant) than in monoculture lettuce plots (1.5 larvae and 1.7 adults). Egg and pupae abundance of the three ladybird species (Coccinellidae) and common green lacewing Chrysoperla carnea was similar between intercropped (12.5-13.0 eggs and 1.5-1.7 pupae per plant) and monoculture (11.5 eggs and 1 pupa) plots. Aphid (Aphidoidea) abundance was significantly lower in intercropped (110-125 per lettuce plant) than monoculture (160 per plant) plots. Natural predator and aphid numbers were similar between plots intercropped with wormwood Artemisia vulgaris, tansy Tanacetum vulgare or stinging nettle Urtica dioica. The attractant plants were tested separately in three blocks inside a 6 x 50 m field. Each attractant plant species was grown in nine plots of 1 x 0.3 m, placed in a 3 x 3 grid among 20 lettuce rows.
A replicated, randomised, controlled study in 1998–1999 in western Kenya (Gohole 2003) found that larval and pupal parasitism of four pest stem borer species (Crambidae, Noctuidae and Pyralidae) by four parasitoid wasp species (Hymenoptera) was similar in plots of sorghum Sorghum bicolor intercropped with molasses grass Melinis minutiflora (4.4% parasitism) and plots of sorghum monoculture (5.1% parasitism). Parasitism differed for only one of four seasons (in 1998), when pupal parasitism was higher in monoculture plots. The spotted borer Chilo partellus was less abundant in intercropped plots (2,750 individuals) than in monoculture plots (3,601). Intercropped plots contained one row of molasses grass for every three sorghum rows. Plots were 9 x 10 m with the treatment replicated in 11 blocks over three fields. Laboratory studies of odour choice found that volatiles from sorghum or maize Zea mays with molasses grass were not more attractive to the stem borer parasitoid sp. Cotesia sesamiae than maize or sorghum volatiles alone. The parasitoid Dentichasmias busseolae was repelled by molasses grass volatiles.
A controlled study in 2001-2003 in Guangzhou, China (Kong et al. 2005) found that more predatory mites Amblyseius newsami occurred in orange Citrus sinensis orchards with a tropical whiteweed Ageratum conyzoides ground cover (0.3 mites/orange tree leaf) than in control orchards (0.09 mites). The pest citrus red mite Panonychus citri was also less numerous in orchards containing tropical whiteweed (0.03 mites/leaf) than control orchards (0.18 mites). Odour choice tests in the laboratory found that Amblyseius newsami was strongly attracted to volatiles from fresh leaves (61% of choices versus a control) or essential oils (95% of choices) from tropical whiteweed. The study compared an orchard with a tropical whiteweed understorey grown for two years, and a control orchard with a groundcover of naturally growing weed species (but with tropical whiteweed removed). Mite counts took place in June 2003 using 15 randomly selected orange trees.
A replicated, randomised, controlled study in 2006-2007 in Karnataka, India (Hanumantharaya et al. 2008) found more natural predators, including lacewing (Neuroptera) eggs and ladybirds (Coccinellidae), in safflower Carthamus tinctorious intercropped with 7-13% coriander Coriandrum sativum (6.0-7.6 lacewing eggs and 1.0-1.4 ladybirds/plant) than in safflower monoculture (4.8 and 0.8, respectively). Cotton bollworm Helicoverpa armigera damage was lowest in intercropped plots of 13% and 10% coriander (16-17% of safflower capsules damaged) and greatest in safflower monoculture plots (21% damaged). Safflower yield was greater (0.92-1.1 t/ha) in intercropped plots (at all densities of coriander) than in monoculture plots (0.86 t/ha). The experiment comprised four treatments (safflower mixed with coriander at 5%, 7%, 10% and 13% of sowing seed volume) and a safflower monoculture control, replicated three times.
- Sengonca C., Kranz J. & Blaeser P. (2002) Attractiveness of three weed species to polyphagous predators and their influence on aphid populations in adjacent lettuce cultivations. Anzeiger Fur Schadlingskunde-Journal of Pest Science, 75, 161-165
- Gohole L.S. (2003) Enhancing foraging behaviour of stemborer parasitoids: role of a non-host plant, Melinis minutiflora. PhD thesis. University of Wageningen.
- Kong C.H., Hu F., Xu X.H., Zhang M.X. & Liang W.J. (2005) Volatile allelochemicals in the Ageratum conyzoides intercropped citrus orchard and their effects on mites Amblyseius newsami and Panonychus citri. Journal of Chemical Ecology, 31, 2193-2203
- Hanumantharaya L., Venkateshalu ., Kubasad V.S. & Raju S.G. (2008) Role of cropping pattern for the management of insect pests of safflower, Carthamus tinctorius L. 7th International Safflower Conference, Wagga Wagga, Australia, 1-4.