Dahlias and Their Buddies

Dahlias are great cut flowers and it is hard to beat them for flower color, range of shapes and vase life. They do have weakness; their weakness is their friends that like to frequently hang out with them. Dahlias have their buddies but you may not care for some, or for their relationship with your flowers. Take the corn rootworm, for example. This is a bug only an environmentally-crazed entomologist would care for.
Recently some cut flower growers in the Midwest experienced the impact of corn root worm beetles turning the foliage and flowers of dahlias into Swiss cheese. One grower lamented that this pest is worse than Japanese beetles. She noted that white dahlias were especially susceptible to the feeding of this beetle.

A Corn Rootworm by Any Other Name…
If you’re located in the West, you are probably encountering Diabrotica virgifera (western corn rootworm – WCR). If you’re in the Midwest, you’ll see Diabrotica barbari (northern corn rootworm – NCR) in your cut flower fields. Both are types of leaf beetles (characteristic swollen femurs on their hind legs) and both are problems on a number of new crops.
The adult beetle feeds on foliage and flowers. The NCR adult beetle is pale to dark green.  The western corn rootworm has a different coloration – gold body with a black head and three black stripes on the elytra (wings).  Until recently, rootworm injury was mainly confined to continuous corn areas, since rootworm beetle adults deposit their eggs only in corn fields. A new biotype cropped up a couple of years ago which deposits its eggs in soybeans and other crops, as well as a few weeds and cut flower species.  I suspect that dahlia and sunflower may turn out to be host or preferred feed host plants. This new biotype is commonly called the first-year corn rootworm and it is reported to be moving eastward toward Ohio. Aren’t bugs amazing? They adapt so quickly and terrorize the poor human population keeping us on our toes.
If cornfields surround you, or are within a couple of miles of your cut flower operation, you may be SOL.  You’ll have problems unless your neighboring farmers start rotating crops   more frequently with plants not susceptible to corn rootworm. The farmer can apply an insecticide to the soil before planting or just post-planting, to reduce populations. If pressure is heavy,  insecticides may be failing to control this pest. Unfortunately, many farmers have been treating this pest with insecticides and not rotating crops like they should. Economics play into this in a big way.
The eggs hatch into larvae, which feed on corn root systems for a period of three to four weeks, during which time they pass through three growth stages commonly referred to as the first, second, and third instars. A mature third-instar larva is about a half inch in length and has a dark brown head and anal plate. At maturity, the third instar transforms into a pupa, which is inactive for a week or two. The pupae then turn into adult beetles, which emerge from the soil and start feeding on corn foliage, pollen, and silks around mid-July.
Unfortunately they will also feed on dahlia flowers. The adults are active for about 10 to 12 weeks, during which time they feed, mate, and deposit their eggs, which become the overwintering stage of the life cycle.
Back to what you can try. The problem with spraying when cut flowers are in bloom that you’ll also kill your beneficials. You can try Neem but it will have to reapplied frequently. Acephate (Orthene) will give pretty good control. I would avoid carbaryl (Sevin) since it is so hard on beneficial organisms.

Evil Weevils

One of our Maryland cut flower growers brought in a sample of adult weevil back in July. Shortly after this my technician brought in a collapsed dahlia plant with weevil larvae in the stem. The larvae were the same weevil that the grower had brought in a couple of weeks earlier. This weevil is called the ironwood weevil, Rhodobaenus tredecimpunctatus. The weevil is amber brown with black spots on its elytra.  Delight in the pictures of the adult weevil and of the larvae in the dahlia stem.
Once this weevil becomes established in your cut flower plot it may be a little difficult to get rid of it. The females overwinter in leaf litter so the best thing you can do is clean up all plant litter at the end of the season.
Acephate (Orthene) or a synthetic pyrethroid such as Bifenthrin (Talstar) should provide good control of the adult. Once the larvae are inside the stem control will not be as effective.

European corn borer, Ostrinia nubilalis, is another agronomic nuisance cut flower growers need to watch out for.  The second generation of this pest lays its eggs on dahlias and sometimes injures them seriously. This insect feeds not only on corn, but on other crops (including potatoes, peppers, and beans), many weeds, and on a variety of other herbaceous plants. The larvae tunnel in the stem in all directions, weakening them so that they break. The larva is pale white or gray with black tubercles and is not more than an inch long when fully grown. Adults have a wingspread of an inch or so and are buff to brown. There are usually two generations annually. Eggs are laid on the underside of leaves and the larvae tunnel in the stalks and pupate in the burrows. Second generation larvae and those of the single generation corn borer overwinter in the stems and pupate in the spring.
The parasitic wasp Trichogramma has been used for control. This tiny wasp attacks the egg masses of the corn borer, and the eggs of other caterpillars, too. Be sure to purchase the insects from a reputable supplier and make sure the strain you purchase is known to be well adapted to attacking corn borer. Insecticides will harm the Trichogramma wasps.

A Gift from the South

The potato leafhopper, Empoasca fabae, doesn’t overwinter in most northern states; every spring they get caught in the updraft winds and are carried north on the trade winds. They drop out and start feeding on vegetable crops, shade trees such as red maple, and unfortunately they feed on dahlias. The damage can be so heavy that it causes a burned appearance on foliage that is called “hopper burn”.
The adult is small, about 1/8” long; a slender, pale green insect that sucks plant juices and causes a blanching of the dahlia foliage.  Once they move into your plots they have multiple generations and can be a problem until fall.
Acephate or carbaryl, which are among the compounds registered for use against this pest, sprayed in mid-June and if needed, again in July and August, should give good control. August treatments may also control the European corn borer. Imidacloprid, applied as a soil drench early in the season, will provide systemic control.
Hang in there with your dahlias; they’re great cut flowers.  Don’t give up on dahlia just because it has a few buddies. They can all be taken out with a little work.