Amy Stone (OSU Extension, Lucas County) posted a BYGL Alert on August 8, 2023, titled, “Sneak A Zucchini on Your Neighbor's Porch Day.” ‘Tis the season. Unfortunately, omnipresent porch cams mean stealthily delivered veggie bundles may receive the “return to sender” treatment.
Squash Vine Borers (SVB) (Melittia cucurbitae) are highly effective in squelching squash providing another form of relief from excessive Cucurbita. I’m speaking from personal experience.
For the first time in our many years of marriage, my wife shared my joy in finding a highly damaging insect pest in the Boggscape. Of course, our perspectives were a bit different. She had run out of recipes for preparing yellow squash; I was happy to finally get pictures of SVB caterpillars and damage.
Of course, my perspective on SVB providing relief from an overabundance of squash is a bit tongue-in-cheek. Amy’s BYGL Alert posting illustrated one method of sharing the bounty.
Another is to seek out a local food pantry. Fresh vegetables are always appreciated, and gardeners can take pride in sharing their harvest. Aside from providing a good home for a lot of squash and zucchini, a food pantry is a great option when you discover that you inadvertently planted “determinate” tomatoes, speaking from personal past experience.
Beautiful Moth; Ugly Caterpillar
SVB is a native moth belonging to the family Sesiidae, the Clearwing Moths. This family of moths is so named because many species lack scales on their wings. They mimic wasps both in the membranous appearance of their wings and the shape of their bodies.
However, SVB moths only slightly resemble wasps. Their front wings are covered in dark metallic-green scales, and only their hind wings are transparent. The moth's thorax is greenish-black, and its abdomen is a flamboyant reddish-orange topped with a dorsal row of black dots. The moths fly during the day and may be seen hovering over cucurbit victims.
As with all moths, SVB develops from eggs to adults through “complete” metamorphosis. The developmental stages are eggs, larvae (= caterpillars), pupae, and adults. The “complete” in the name comes from the complete change in form that occurs during the pupal stage.
SVB females lay individual eggs on leaf stalks, vines, or at the base of plants. The oval-shaped flattened eggs are reddish-brown and so small they are difficult to spot. Each female is capable of laying around 40 eggs.
The cream-colored SVB caterpillars immediately bore into plant stalks upon hatching. The caterpillars have short thoracic legs; however, their abdominal prolegs are almost non-existent. As shown in the image below, each proleg is marked by “crochets” which are hook-like structures that provide traction.
The caterpillars develop through 4 instar stages. All stages are highly efficient at converting healthy inner-stem tissue into pulpy, macerated mush. Of course, as the caterpillars become larger, their damage escalates.
Early plant symptoms include leaf yellowing and wilting. Indeed, gardeners may conclude that plants need watering, or they are suffering from a nutrient deficiency. Of course, as the inner workings of the stems are destroyed, no amount of water or fertilizer will cause the plants to recover.
As the infested plants hurdle toward certain doom, the affected stems commonly split, and sections of the plant or the entire plant may fall over. A slight tug will cause the plant stems to break at the soil surface and yellowish goo may cause gardeners to make a disease diagnosis. Of course, cutting open the stems will reveal the true culprit.
There is typically only one generation of this moth per season in Ohio; however, occasionally a partial second generation may occur in the southern part of the state. Adults fly during the day and may be observed from early June through early August.
It’s obviously too late to prevent stem-boring by SVB caterpillars this season. Various online sources describe killing caterpillars by inserting a stiff wire into the stems to skewer the caterpillars or using a sharp knife to cut open the stems longitudinally to expose and directly dispatch the caterpillars. The split stems can be tied shut with coarse twine.
However, by the time plant symptoms become obvious, the caterpillars have usually caused so much damage, the plants fail to recover even if caterpillars are killed. This is particularly true with heavily infested plants.
On the other hand, other actions taken now can reduce an escalation of the borers in vegetable gardens next season. Once SVB caterpillars complete their development, they exit plants and crawl into the soil to pupate. Removing plants from a garden once they start to collapse will prevent the caterpillars from pupating in the soil.
All is not lost if you wait too long to remove chlorotic, wilting plants. Tilling the soil this fall can bring the pupae to the surface where they can be gobbled up by birds or freeze during the winter.
The stem-boring destruction of SVB may be observed on many members of the Cucurbitaceae family including summer squash from yellow squash to zucchini as well as winter squash, pumpkins, and gourds. Although rare, caterpillars may occasionally be found in cucumbers or melons.
Squash Bugs Add to the Squished Squash Garden Party
Squash Bugs (Anasa tristis, family Coreidae) may also add to the decline of cucurbits although not usually as dramatically as with SVB. The bugs will readily attack summer and winter squash as well as pumpkins with cucumbers occasionally on the bug menu.
Squash bugs develop from eggs to adults through “incomplete” metamorphosis. The developmental stages are eggs, nymphs, and adults. The nymphs feed alongside the adults to produce the same type of damage. I’ve never liked the term “incomplete” because it sounds like something can go horribly wrong such as bugs only developing legs on one side, so they run around in circles.
Squash bugs have one generation per season in Ohio. The adults overwinter in protected locations such as under debris beneath host plants. The females find their way onto host plants sometime in June to lay oval-shaped, yellowish-colored eggs that become reddish-bronze as they mature. The eggs are usually deposited in clusters on the underside of leaves; however, they may occasionally be laid on the upper leaf surface.
The nymphs change their color motif as they mature through 5 instar stages. They have long spindly legs causing them to sometimes be mistaken for spiders as they scurry around on the leaves and stems of their host plant.
The adults are large, flat insects that measure 5/8" long and approximately 1/3" wide. They range in color from dark gray to dark brown. The edges of their abdomen have alternating orange and brown stripes.
Squash bug adults and nymphs suck the sap out of the leaves with their piercing-sucking mouthparts. Their feeding causes yellow spots on the leaves that eventually turn brown. Heavy feeding on the stems can disrupt the flow of water and nutrients causing wilting. However, the wilting progresses slowly and does not usually involve dramatic yellowing of the leaves. Young plants may be killed, and older plants damaged to the point of disrupting fruit development.
Although squash bugs by themselves are not nearly as damaging as SVB, it’s important to keep in mind that the bugs may transmit the bacterial pathogen, Serratia marcescens. The hitchhiking bacterium produces the potentially highly destructive disease called Cucurbit Yellow Vine Decline (CYVD) on squash, pumpkins, and other cucurbits.
This is a now you see it, now you don’t disease with infections being more prevalent in some years compared to others. However, CYVD may be suspected if plants that are heavily infested with squash bugs turn bright yellow and then rapidly wilt.
Squash bugs may be managed mechanically by crushing eggs, collecting and crushing the adults, and knocking off nymphs and killing them in a bucket of soapy water. Squash bugs may also be trapped under cardboard placed beneath host plants. The bugs will congregate under the cardboard at night allowing them to be collected and destroyed the next day. Doing the “squash bug smash” dance is highly effective and so far, no bugs have become resistant.
Good sanitation is also recommended. Plant debris should be continuously removed throughout the growing season to eliminate bug hiding places. Likewise, plants and plant debris should be removed and destroyed at the end of the season to reduce adult overwintering sites.
Although there are online recommendations for using insecticides to manage squash bugs, it’s important to keep in mind that spraying blooming plants presents a high risk to the pollinators responsible for moving pollen from male to female flowers. The online recommendations focus on spraying early in the morning or late at night when there’s minimal pollinator activity. However, the insecticides available to home gardeners are more effective against nymphs and much less effective against adults. Of course, as with all pesticides, it’s critical to read and follow product label directions.