Biological and Cultural methods to control imported cabbageworms
The cabbage butterfly is commonly called as imported cabbageworm, Artogeia rapae (Pieris rapae). This is one of the most important pests of many Cole crops including broccoli, cabbage, collard greens, cauliflower, kale and turnip. Butterflies are easy to identify as they have whitish colored fore-and hind-wings with one and two black spots on the top of each of fore wings of males and females, respectively. Also, both males and females have a black spot on the outer front margin of each hind wing. Females lay singly yellow colored and oblong eggs on the either side of the leaves and depending on the temperature eggs hatch within a week. Mature larvae are velvety green in color with a narrow orange stripe down the middle of the back and a yellowish stripe along each side of the body (Fig. 1.) The pupae are green to light brown in color, attached to bottom leaves and adults generally emerge from these pupae within 2 weeks of pupation. Cabbage butterflies overwinter as pupae in previous crop plant debris in the garden.
Fig. 1. Severe damage caused by Imported Cabbageworms near growing point of a collard green plant[/caption]
Generally cabbage butterfly larvae feed voraciously near to the growing point of the host plants (Fig. 2) but they can also feed indiscriminately by chewing large irregular holes on both young and mature leaves of different host plants including broccoli, cabbage, cauliflower, collard greens, kale and turnip (Figs. 1 and 2).
Since chemical insecticides cannot be used in organic vegetable gardens, growers have to rely on the cultural and biological methods to manage populations of imported cabbageworm.
For small or large vegetable gardens, best cultural practice is hand picking and killing of all the larval stages of imported cabbageworm. Although this practice is laborious and time consuming, it works and reduces the damage caused by this economically important insect pest. Also, at the end of the fall season remove all the previous crop plant debris so that there will be less protected areas available for overwintering imported cabbage worms, which in turn will reduce the populations of adults in the next spring. This low number of adult emergence means there will be less numbers of eggs to hatch into larvae meaning there will be less larval incidence to cause the damage to the crop in the spring.
Biological methods include use of natural enemies/biological control agents to control cabbage butterflies. Three well known biological control agents including Bacillus thuringiensis (Bt), entomopathogenic nematodes and wasps have a potential to manage imported cabbageworm population in the vegetable gardens.
This bacterium is recognized as a bacterial insecticide but it is not harmful to the humans, animals or the environment. This is a very effective biopesticide on young larval stages as compared to the mature larval stages of cabbage butterflies. This microbial biocontrol agent is commercially available and can be applied using traditional sprayers. For the effective control of imported cabbageworms, Bacillus thuringiensis kurstaki should be applied at every seven day interval after noticing the first incidence of pest.
Currently, entomopathogenic nematodes are used as effective biological control agents against many different kinds of soil-dwelling insect pests of many economically important crops and turfgrasses. These nematodes are commercially available and are not harmful to humans, animals and even beneficial insects like honeybees. Canadian researchers have demonstrated that the entomopathogenic nematodes including Steinernema carpocapsae, S. feltiae and S. riobrave can cause 76 to 100% mortality of imported cabbageworms Artogeia rapae if applied at temperatures ranging from 25 to 30 °C and their LC50 values were ranged from 4 to 18 infective juveniles (Bélair et al., 2003). Mahar et al (2005) also reported that in addition to the above stated species of entomopathogenic nematodes, Heterorhabditis bacteriophora and H. indica nematodes can infect and kill both larvae and pupae of cabbage butterflies. Recently, another insect-parasitic nematode, Rhabditis blumi also been shown to be effective against imported cabbageworm (Park et al., 2012).
Following four species of parasitic wasps can serves as effective biological control agents against imported cabbage worm.
- The egg parasitic wasp, Trichogramma spp.: This is a very tiny parasitic wasp known for parasitizing eggs of imported cabbageworms. These wasps are commercially available and can be mass released when lots of adult butterflies are present in the garden and already started laying eggs on the leaves. This will prevent the hatching of eggs into larvae thus preventing damage caused by imported cabbageworm larvae to Cole crops (Oatman et al., 1968).
- The brachonid wasp, Cotesia glomerata: This gregarious wasp parasitizes the larvae of the imported cabbageworms. This wasp species is not commercially available but it can naturally occur (Herlihy et al., 2012) and capable of suppressing the populations of cabbage butterflies in the vegetable gardens and fields. This wasp lays eggs inside the young caterpillars of imported cabbageworms. The eggs hatch and the larvae develop inside the developing imported cabbageworm larvae, then emerge as mature larvae and pupate in yellow silken cocoons outside the host, which dies during the process of the emergence of wasp larvae. If this wasp is present in the fields, which are infested with imported cabbageworms or other insect hosts, it can parasitize and kill over 60% of their insect host larvae.
- The solitary wasp, Cotesia rubecula: This naturally occurring parasitic wasp is known to its specificity to the members of genus Pieris especially imported cabbageworms. Although C. rubecula wasp parasitizes all the stages of imported cabbageworms, it prefers last instar of imported cabbageworms, which is the most damaging stage. This is the most studied parasitic wasp of imported cabbageworms and found to be distributed throughout the US (Herlihy et al., 2012).
- The pteromalid wasp, Pteromalus puparum: This tiny wasp specifically parasitizes pupae of imported cabbageworms and other lepidopterous insects. Since this wasp parasitoid kills only pupae of its insect host, it does not reduce the larval feeding damage caused before pupation but it certainly reduces the emergence of the next generation of adults. This means there are less number of egg laying females that results in the less number of eggs and therefore, less larval incidence to cause severe damage to the crop.
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Cameron, P.J. and Walker, G.P. 1997. Host specificity of Cotesia rubecula and Cotesia plutellae, parasitoids of white butterfly and diamondback moth. Proceedings of 50th N.Z. Plant Protection Conference: 236-241
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Mahar, A.N., Jan, N.D., Chachar, Q.I., Markhand, G.S., Munir M. and Mahar, A.Q. 2005. Production and infectivity of some entomopathogenic nematodes against larvae and pupae of Cabbage Butterfly, Pieris brassicae L. (Lepidoptera:Pieridae). Journal of Entomology 2: 86-91.
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Park, H.W., Kim, H.H., Youn, S.H., Shin, T.S., Bilgrami, A.L., Cho, M.R. and Shin, C.S. 2012. Biological control potentials of insect-parasitic nematode Rhabditis blumi (Nematoda: Rhabditida) for major cruciferous vegetable insect pests. Applied Entomology and Zoology 47: 389-397.