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Cabbage and other Cole crops are Important parts d many New England vegetable farms Among the pests affecting these cross arc several foliage feeding caterpillars, especially the imported cabbageworm, Pieris rapae. The larvae of the pest are the familiar "green worms" that spoil the heads of cabbage or fall out of broccoli during preparation or cooking.
One of the projects of the University d Massachusetts (UMASS) Biological Control Program, working with the UMASS IPM Program, is to Improve the biological control of the pest by importing new, more effective species of natural enemies. The pest is not native to North America, but rather to Europe and Asia. During colonial days it invaded North America where, free of many of its parasites from its homeland, it became a widespread pest of cole crops
The first step in the biological control of this species was made in 1883 when one parasite, Apanteles glomeratus was collected in England and shipped to North America where it was released and became established. This parasite attacks young larvae, laying 20-60 eggs per caterpillar, and emerging from fully grown larvae just before they are ready to stop feeding and pupate. This parasite common and can attack 40-80 % of caterpillars in unsprayed plots. This helps slow the growth rate of the pest population in any local area by reducing the number of caterpillars that produce butterflies. The adult of this pest is the common white butterfly with black spots on the wing that is familiar to most gardeners and farmers.)
Apanteles glomeratus however, is not an ideal parasite because it does not kill the larva, the feeding stage, until the parasitized caterpillar has eaten as much as a healthy caterpillar and completed its larval life. Thus, while parasitism helps reduce the local pest population, it does not protect individual crop fields.
What is needed is a new parasite that kilts the caterępillar before they eat very much. Fortunately, such a parasite exists. It is a related wasp called Apanteles rubecula. This wasp attacks young larvae, but instead of killing its host at the very end of the larval life, it kills half grown larvae. By eliminating the last larval stage, when the caterpillars are largest, parasitism by the species reduces the feeding by a larva by over 70%. Unlike A.glomeratus, A. rubecula lays only one egg in its host, producing single cocoons on plant foliate instead of the pile of 40-80 cocoons produced by A.glomeratus
Work to import and establish A. rubecula in Massachusetts began in my laboratory in 1988. With the generous help of entomologists in China, Apanteles rubecula was collected in China near Beijing and sent to a USDA quarantine laboratory in Newark, Delaware. After clearing quarantine (a procedure to ensure that no unwanted or mis-identified organisms are introduced with the shipment), the wasps were sent to UMASS in July of 1988 and were used to establish a population of wasps on land belonging to the University.
Since 1988 work has continued manually on the wasp in conjunction with Ruth Hazzard, the program leader for the Vegetables IPM Project. The parasite has demonstrated its ability to survive Massachusetts Winters and to synchronize its life cycle to local seasons. Levels of parasitism in the first pest generation have been typically in the 40-50% range at the release site. The wasp appears to be especially good at finding larvae when pest numbers are low (as few as one larva per 10 plants).
Currently, we are working to establish as many populations of this wasp in Massachusetts as possible, to ensure its survival in the region. Methods of crop rotation have been developed and tested that allow the wasp to survive from year to year on individual farms. The keys to success are (1) to protect the overwintering supply of wasp cocoons and (2) to ensure a continuous supply of hosts for the wasps to sting.
Wasps overwinter in cocoons that are fixed to crop foliage. To protect these cocoons, the non-harvested parts of the cole crops plants should be left in place over the winter and not plowed down until one month after next year's early cole crops have been planted. This allows the wasp to emerge in late April and to encounter nearby cole crops on which butterflies have laid eggs, which in turn have hatched to produce the small (1/l6-1/18 inch)larvae which the wasps sting. By late May or early June wasps will have established in the spring planting and the old plants from the previous year can to plowed under the plot used to plant a later crop such as corn or tomatoes. Failure to conserve crop residues, either plowing them under in the fall or doing so too early in the spring leads to complete loss of the parasite.
Ensuring a continuous supply of pest larvae for the wasp to attack is also critical. Wasps are prone to disperse if a period occurs in which there are no larvae in the field to sting. Under these conditions wasps are likely to fly out of the field in search of larvae elsewhere. A continuous host supply requires that both a spring and mid-summer cole crop of some sort be planted. It is important that the spring crop by planted as early as possible because it must be in the ground early enough for the first flight of the butterfly to lay it eggs, If this does not happen, wasps that emerge from the overwintered cocoons will have nowhere to lay their eggs and they will disperse away from the plot in search of hosts.
Sites al which we have been successful in establishing Apanteles rubecula include refuge plantings (not sprayed and left unplowed in the fall) on conventional farms, organic farms (that plant both early and late crops in reasonable quantity) and community gardens. The last of these sites is our newest attempt to discover suitable circumstances for colonizing the parasite. Community garde offer a substantial number of cole crops plantings that are de-synchronized and thus fairly stable in terms of always having some young plants with young larvae for the wasp to attack
A laboratory colony of the wasp is kept in our laboratory and has been used to supply wasps to other areas interested in colonizing the wasp (Rhode Islands Connecticut, Ontario, South Carolina). Anyone who grows 800 or more cole crop plants, both early and late, every year and does not use pesticides on the crop is potentially a release site, provided they are willing to practice the crop rotation needed to conserve the parasite.
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