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Plants in the family Brassicaceae (formerly Cruciferae) have a fairly small number of insect pests, mostly specialists that have developed a taste for mustard oil and related chemical compounds. These sulfur and nitrogen containing compounds tend to discourage most generalist insects from eating brassicas, but are attractants to imported cabbageworms butterflies, cabbage maggot flies, and caterpillars of the diamondback moth, helping them to find their preferred hosts. These compounds also determine the distinctive flavor and odors that brassica bring to the tables of human beings, and certain compounds in this group give the brassica their reputation as cancer-fighting foods.
The insect pests of brassicas I study are: 1) the imported cabbageworm , with the familiar white butterflies and funny green caterpillar 2)the diamondback moth, a small gray moth, frequently seen around the plants in the day time, whose wings in the resting position form a series of white diamonds down the back, and whose caterpillar without fuzz 3) the cabbage aphid, an aphid that coats itself with a grayish-white layer of wax, and whose feeding causes alterations in the growth of the plant such as leaves curling, 4) two species of flea beetles , one with curving ' white stripes on the back and one solid black, who make little shot-holes in the leaves and whose tine larvae also feed on the brassicas.
Some of these insects in addition to being pests here in the Northeast, are also major pests of brassicas elsewhere in the world. The rate of growth of diamondback moth populations is limited here by temperature, death during the winter, and natural enemies especially parasites . But the diamondback thrives year round in hotter climates, producing more than 20 overlapping generations per year, and its natural enemies have not adapted well to hot climates and can not keep pace.
Thus in Southeast Asia, Latin America and even in the southern US and Hawaii, brassica growers have been on a pesticide tread mill, spraying so many times a year that even newly invented types of insecticides lose their effectiveness in less than one year. As you may have read, some populations of the diamondback have also become resistant to Bacillus Thuringiensis in Hawaii and in the South. In the mid 1970's, Dr. Mike Dikson, a plant breeder from the New York State Agricultural Experimental Station, while working on a sabbatical in Australia, collected a glossy cauliflower line from a seed company there. Glossiness is the result of occasional spontaneous mutations that every brassica breeder has seen (I even found one myself in a flat of seedlings from a packed of purchased seeds). The seed company in Australia was interested in glossy cauliflower because it is difficult to get sprays to stick to normal brassica leaves. On normal brassica leaves, the wax layer covers the leaf surface with microscopic tubules, standing up much like threads in a carpet, giving the leaves a hazy, bluish appearance and causing water to bead up and drop off without sticking. Glossy plants also have a layer of wax (as do all leaves), but there is less wax and it is in flat plates on the leaves, rather than standing up in tubules. As a result, the leaves look much greener and shinier, and water stick to them. Mike Dikson grew this line back in New York, and found that it had much lower numbers of caterpillars of all kinds. He began the slow process of transferring the resistance to locally-adapted, horticulturally acceptable cauliflower and cabbage varieties,
The problem of resistance to insecticides in the diamondback moth and the general interest in reducing insecticide use through integrated pest management has revived interest in plant resistance to insects in brassicas in the last few years, and especially is the glossy, insect resistant line. I started my work in this area by finding many glossy lines in several brassica crops (cauliflower, broccoli, collards, and kale ) and comparing their insect resistance to varieties with normal wax. I tested them under natural infestations , simply by planting them and counting the number of insects and the amount of the damage, and also under artificial infestation , collecting eggs from laboratory colonies of imported cabbageworm and diamondback moth and attaching the eggs to the leaf and looking at the survival of the caterpillars.
It is important to test resistance under artificial as well as natural infestations, because often under natural infestation all that is being tested is differences in preferences. For example, imported cabbageworm attack red cabbage less than green cabbage when both are grown in the same garden because the butterflies prefer to lay their eggs on green cabbage. But if no green cabbage is available or if the green cabbage becomes less attractive because the plant is smaller or more heavily damaged, the butterflies will go ahead and lay their eggs on the red cabbage and the caterpillars hatching from those eggs will do very well. Artificial infestation with a stage of insects that does not move from plant to plant measures how well the plant will resist insects regardless of the other choices available. A sample of the results of these tests is shown in Table 1.
As you can see from the table there is an impressive level of resistance to imported cabbageworm in several different glossy lines. The results has been repeated in both spring and fall plantings, in Hamden CT and at another farm in Windsor, CT, with both natural and artificial infestation for four years. Not all glossy lines are as resistant to diamondback moth but a few are strongly resistant. A colleague, Dr. Sanford Eigenbrode, has studied the behavior of newly-hatched diamondback moth caterpillars on glossy and normal leaves, and found that the larvae wander around on the glossy leaves, unable to settle down and feed. Perhaps, the difference in the wax layer interferes with the ability of the caterpillar to recognize the leaf as belonging to its host plant
Although I do not have laboratory colonies available to artificial infestations, I have looked at the numbers of other insects under natural infestation on glossy lines, Cabbage aphids, though present in large numbers only occasionally here, are commonly reduced by ,99% on the glossy lines. Flea beetles, however, are a problem. In some tests, the two specie's of flea beetles have been twice as abundant on the glossy lines as on normal varieties In other tests, there have been no differences in numbers, but flea beetle damage is much more apparent on glossy lines. I have looked at flea beetles behavior on glossy and normal ones and found that while flea beetles tend to nibble at the edges of normal brassica leaves, perhaps because the wax is thinner there, they are much more likely to bite holes all over the glossy leaves which make the damage look much worse. Mike Dikson has continued working on glossiness as a source of insect resistance, including genes for glossiness from other sources besides his original cauliflower. He released one of cabbage and cauliflower to the seed companies a few years ago, and I have read that he expects hybrid varieties of glossy cauliflower, cabbage, and broccoli soon. Because there is no general form of resistance to all insects, and because the complex of insects pests varies from one place to another and even from year to year, the plant breeder has to make judgements about which resistance are most important and which resistance to combine with other characteristics to make a variety suitable for a certain region. When glossy varieties become available , I will test them particularly their susceptibility to flea beetles to see if they are suitable for growing here in Connecticut. Even if there are not they may be useful in other parts of the nation and the world where caterpillars pests are a much more serious problem than flea beetles.
Table 1 : Total number of imported cabbageworm larvae per plant on seven dates . Hamden CT, fall 1989
|Crop and variety||Surface||Artificially infested||Naturally infested|
|South carolina Glaze||Glossy||7,0||0,0|
|White's Green glaze||Glossy||5,0||1,7|
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