Why does pumpkin squash ( kabocha/kombucha, Japanese pumpkin) grow into a nice vine, but many of the young fruit turn yellow and drop off?
This sounds like a case of an insect pest called the pickleworm( Diaphania nitidalis). This moth was discovered in O`ahu in 2003, and by 2005 had spread to Kauai, Maui and the Big Island. It attacks many cucurbits such as cucumber, squash, pumpkin, zucchini and cantaloupe. Squash seems to be the most susceptible. The caterpillars like to feed on the blossoms and afterward will burrow into the developing fruit. The adult moths are colored with yellow and brown and have a purplish sheen.
Some insecticides are registered on cucurbits and have been successful, with diligent spraying, in commercial crops. One organic insecticides that may be effective on the pickleworm is known as Bt, (Bacillus thuringiensis). It is actually a bacterium which is deadly to the moth larvae, but not to humans. One problem is coverage; the spray adheres to the surface of the leaves and blooms, while the caterpillar is on the inside of the bloom and young fruit.
It has been difficult for the homeowner to control the pickleworm. One method is exclusion. Place screening material over the plants, or individual blooms. This will keep the moth out and prevent her from laying eggs. The problem however, is that the cover would also exclude honeybees. Honeybees are important for pollinating many of the cucurbit crops. A lack of pollination can be a cause of blossom drop. Because the moth is a night flyer, you can cover the crop at dusk and uncover it during the day. This will allow honeybees to visit the plant during the daylight hours, but keep the moths out at night. Admittedly this is quite a tedious process.
Alternatively, since the moth is relatively large, a wingspread of about one inch, ¾ inch bird netting can be used to build a permanent structure over the plants. This will exclude the moth while allowing bees to freely move through the netting.
There are a few squash varieties which show some resistance to the moth. They are Butternut 23, Summer Crookneck, Early Prolific Straightneck and Early Yellow Summer Crookneck. There have also been reports from other areas of the country, that the early crops have less damage.
Recently a company named High Mowing Organic Seeds has released a new variety of zucchini called Partenon. It is a parthenocarpic variety which means that fruit develops without pollination. Thus this variety can be grown in places that exclude pollinators (honeybees) such as a greenhouse or netted structure. Partenon is also tolerant to powdery mildew. (www.highmowingseeds.com )