Male squash blossoms, C.Quish photo.

Male squash blossoms, C.Quish photo.

Where are all my summer squash? Why do my plants have many blossoms and not squash? These are a few of the questions I hear about yellow and zucchini squashes when the squashes look like they should be setting fruit. Be patient, gardeners, squash will come.

Squash plants produce separate male and female flowers on the same plant. The male flowers contain the pollen, the male part the reproduction process.  The female flowers have the ovary at their base. The ovary looks like a very small squash. This ovary will not develop and will be aborted,(dropped off), of the plant if pollen is not moved from the male flower to female. The process is called pollination, resulting in fertilization, then the ovary will develop into the fruit, the squash. The male flowers are produced and open a few days before the female flowers open. So the males are ready before the females. (I am not going to comment on this.)

The male flowers are on a long stem with no little squash at the flower base.

Male Squash Blossom, photo by C.Quish

Male Squash Blossom, photo by C.Quish

Male squash blossoms, C.Quish photo.

Male squash blossoms, C.Quish photo.

The female shows the small squash.

Female squash ovaries at base of flower. C.Quish photo.

Female squash ovaries at base of flower. C.Quish photo.

 

Insects such as bees are the common pollinators of squash plants. They feed on the nectar in the flower, and in the process pick of pollen from the male flowers, dropping some in the female flower when the move into it. If all goes well, fertilization happens and the squash will develop.

A common pest insect of summer squash is the squash vine borer which lays eggs on the stems of the squash. The eggs hatch into a larva which tunnel into the stem to feed. Their feeding damages the inside of the stems and the water conducting vessels of the plant, causing part or entire collapse and wilt of the plant. The squash vine borer is a clear winged moth with 1/2 inch long orange abdomen with black dots. It flies during the day and rests at night. The SVB is attracted to the color yellow. A trap can be made by filling a yellow bowl with with soapy water. The SVB will fly into the bowl and drown. Place trap near squash plants. Other management options are to plant a second crop of summer squash in early July that will mature after adult borers have finished laying eggs. Pull and destroy any plants killed by squash vine borers to keep the larva from overwintering after feeding for four to six weeks. They exit the stems and burrow a few inches into the soil to pupate  where they stay until the following summer. There is only one generation per year..

I use a row cover as a physical barrier that keeps out all insects. The row cover is a poly spun fabric similar to mosquito netting placed over all of the squash plants in the bed, then held down with weights to exclude all insects. The row cover also excludes the pollinators, so I have to either hand pollinate each female blossom or remove the cover once the female blossoms appear to allow insects in to do their job of pollinating. If hand pollinating, do it in the morning as the pollen is most available then.

Row cover protecting squash from insects reaching the plant. C.Quish photo

Row cover protecting squash from insects reaching the plant. C.Quish photo

Squash vine borer adults, Photo UMN.edu

Squash vine borer adults, Photo UMN.edu

 

Squash vine borer larva and damage. Photo UMN.edu

Squash vine borer larva and damage. Photo UMN.edu

 

-Carol Quish

Squash vine borer adult, http://www.extension.entm.purdue.edu

Photo by Rob Durgy, UConn

Squash Vine Borer larva and damage, photo by Rob Durgy, UConn

Watching my summer squash’s leaves collapse  just about overnight signals a problem. Further investigation proves worthwhile when I find a small, 1/8th hole on the plant’s stem. I cut the stem lengthwise, carefully splitting apart the vine to reveal the hollow vine filled with sawdust like frass. Frass is the excrement of an insect. About 12 inches away from the original hole lurks the offending caterpillar; the larval stage of the Squash Vine Borer. Not a pretty site from where I stand! Along with the insect chewing the inside of the vine, centipedes have also entered the vine feeding on the frass. All of the these insects introduce bacteria causing rot leaving a mushy trail in its wake. This plant is too far gone to save. It is pulled out after I kill the borer making sure I got them all,  then tossed the plant in the compost.

The life cycle of the squash vine borer, Melittia satyriniformis, has one generation per year. The adult is a clear winged moth active during the day.  There are orange and black rings  on the abdomen. The front wings have greenish to black color while the hind wings are clear edged in brown.  The eggs are laid at the base of the plant on the soil. Once the eggs hatch, they burrow into the vine to begin feeding. Adults are appear late June through July and into August. Signs of an infestation are small holes in the vine and piles of frass on the vine and below on the soil. The larva can reach 1″ in length. They mature after feeding for about four weeks at which time they will leave the vine to burrow into the  soil. Once in the soil they will spin a cocoon in which to pupate. They spend the fall and winter in the pupa state until they emerge next summer as the adult moth.

Control measures are too late for my plant and should have been used much earlier in the season. Row covers made of Remay, a poly spun product reminiscent of mosquito netting, is an effective barrier, light-weight when applied over plants at the beginning of the season. The edges must be pinned down or buried in the soil to prevent the adult insects from crawling under the fabric. The idea is to exclude the adult moth from laying eggs at the base of the plant. Once the plant flowers, the Remay should be removed to ensure bees and other insects are able to reach the flowers for pollination. Turning over the soil at season’s end will expose overwintering pupae to the elements, and hopefully succumb to the cold. Another turn of the soil in the spring will further disrupt their protective site. Pull and destroy plants as soon as they finish producing to possible larvae from entering the soil. Chemical controls are either carbaryl (Sevin) or permethrin applied to the vines once per week starting the last week in June and continuing until the end of July.

If you do find frass and a hole in the vine, cut lengthwise splitting the vine until you find the boring caterpillar. Remove it then put the vine back together and mound soil on top of the damaged area. It might recover!

_ Carol

(cc) 2005, Rasbak.