Trap Plants: Can You Hear Them Sucking In the Insects?

Can you lure insects from your cut flowers with sexier, better-smelling and better-looking plants? There is no doubt that certain plants appear to be magnets for insects. Whether it’s the volatiles they emit or the color spectrum plants reflex or radiate there is strong preference of certain insect species for specific cut flower species. Can you use this magnetic draw to pull insects onto these crops and away from your cut flower production area?

For years we have had anecdotal reports from growers that if they planted a block of a certain cut flower away from their main growing area they could concentrate the insects on these lure or trap plants, then treat just this area with insecticide. The growers using this technique claim that they don’t need to spray the whole cut flower plot, reducing the amount of pesticide used. We decided to test out this concept to see if it worked with three key insect pests of cut flowers. Adult Japanese beetles, cucumber beetle and harlequin bugs were ideal targets for this approach.

The major elements of this season-long project were: 1) Research and demonstration to document the efficacy of using trap plants highly attractive to key cut flower insect pests to draw pests to “kill zones” of trap plants treated with systemic insecticides, and  2) Education to extend results of beneficiaries, and increase adoption of sustainable practices. Lofty but obtainable goals.

About the Insects

Japanese beetles, Popillia japonica, cause damage to several species of cut flowers including zinnias, roses, hydrangea, and willow stems. The beetles use their chewing mouthparts to feed on foliage and flowers causing a skeletonization of the plant part they feed on. In 2003 – 2006 we had large numbers of Japanese beetles feeding on cut flowers in Maryland. The large adult beetle populations numbers were influenced by conducive weather conditions for Japanese beetle larval survival in tuft areas. These large populations descended on cut flower fields throughout the state for 3 years in a row, causing major loss. Many growers had to make repeated applications of insecticides to reduce damage. The adult beetles are actively feeding on foliage from June through August. Two of the most frequency damaged popular cut flowers damaged by Japanese beetle are zinnias and dahlias.

Harlequin bugs, Murganita histrionica, damage plants by inserting a stylet mouthpart into plant parts and extracting plant sap. The plant tissue surrounding where the insect feeds discolors to a whitish color. When the insect feeds on new growth it causes distortion and twisting of growth. Harlequin bugs cause major injury on snapdragons, cut ornamental cabbage and kale, and cleome.

Spotted cucumber beetle, Diabrotica undecimpunctata, causes damage as the adults chew on flowers and foliage of host plants. Damage to cut flowers occurs when they feed on the flower petals, cutting holes and making flower unthrifty in appearance. Cucumber beetles commonly damage amaranth and sunflowers. Cucumber beetle adults are found in cut flower fields from May through frost. Eggs are laid in soil near host plants and larvae feed on roots for about 30 days. There can be up to 3 generations per season.

About the Trap Plants

Roses and hibiscus plants are highly attractive to Japanese beetles. These plants have been used as trap plants in vineyards to concentrate Japanese beetle feeding into specific sites where insecticide is applied to kill the insects.  

Harlequin bugs are highly attracted to cleome.  Candytuft and amaranth are reported by several growers to be highly attractive to cucumber beetles. We chose to use candytuft as the trap plant for cucumber beetles.

The idea of using traps plants is to attract feeding insects to a specific area where they can be treated for control. Reducing a population in an area has potential to reduce injury to desired cut flower species. By treating the trap plants (rose, cleome and candytuft)  and reducing population in the area, growers may be able to reduce damage to desired cut flowers. This avoids expensive applications to large planting areas and is a more environmentally sound method of managing pests with pesticides.
    
In previous studies, several systemic insecticides have proved effective in controlling Japanese beetles and should work on harlequin bugs. These systemic insecticides available to growers are in the class of neonicotinoids including Safari (dinotefuran) and Marathon/Merit (imidacloprid). These systemic insecticides are long lasting in plants when applied as soil applications, providing control for several weeks. These new classes of pesticides are effective but expensive, making applications to large field planting cost restrictive. By treating the trap plants, insecticide treatments are confined to a small area separate from where workers will be cutting flower stems, risking exposure to pesticide applications.
    
We evaluated this method for efficacy in controlling pests and reducing or eliminating damage to cut flowers that are highly susceptible to damage from these two pests.
    
At each site were  20-foot rows of zinnia, snapdragons, and sunflowers. Three separate plots were set up at each site. No trap plants were planted at one. This tested the amount of damage that could be expected on a typical cut flower plot. The second plot had trap plants 10 feet from the test plot, but they were not treated with any chemicals. At this plot we wanted to test if the insects would feed first on the trap plants, or remain just on the trap plants since they were so attractive as a feeding site. The third plots had trap plants 10 feet from the test plot, treated with the systemic imidacloprid. Here we wanted to attract the insects to the trap plants and kill them before they had a chance to damage the cut flowers in the test plots. Each of the plots was at least 100 meters apart at each of the sites.
    
The three Maryland sites were the Central Maryland Research and Education Center in Clarksville; the Extension office in Clinton; and the Montgomery County Extension office in Derwood. We used three sites to increase the chance that we would experience insect pressure at least one site. The main Japanese beetle pressure was at the CMREC site but there was no activity of Japanese beetles at the Prince George’s County or Montgomery County sites. We had harlequin bug activity and Cosmopepla bimaculata  activity at all three sites. The spotted cucumber beetle populations were very low at all three sites and no damage was noted on any of the sunflowers.

Discussion

It appears at least with light population pressure of Japanese beetles, trap rose plants may have the potential to attract adult Japanese beetles, if the trap plants are planted 10 foot away from the plants you want to protect. Rose plants treated with imidacloprid appear to kill beetles and there was no damage to zinnias plants growing within 10 ft of the trap plants.
    
Cleome is highly attractive to harlequin bugs and pulled harlequin bugs onto the trap plant. Cleome treated with imidacloprid at the CMREC site effectively killed harlequin bugs. The harlequin bugs never became a real problem on the snapdragon plants. Another insect, Cosmopepla bimaculata, was found on both the cleome plants and the snapdragon plants. In the snapdragon plot with untreated trap plants the C. bimaculata reached such high levels at CMREC that plant tips died back and plants did not flower. In the plots without trap plants the C. bimaculata were present on the snapdragons in fairly heavy numbers but did not keep the plants from flowering. In the plot with trap plants treated with imidacloprid the C. bimaculata were present in moderate numbers but no plant injury was observed and flowering was good.
    
The spotted cucumber beetle just did not become a problem in any of the plots at CMREC or at the other two sites and we did not record any injury to the flower petals on the sunflowers. Candytuft plants took a long time establish, and we did not observe cucumber beetle feeding on the plants. Several grower have suggested that amaranth would be a better choice of a trap plant for spotted cucumber   beetle adults.

Conclusion

Roses have a potential for being used as a trap plant for Japanese beetles, and treating them with the systemic imidacloprid can give fairly effective control, when pest  populations are low. It would be interesting to continue this work and test roses’ effectiveness as trap plants when populations were medium to high levels.
    
Cleome is effective at attracting harlequin bugs, but the major pest of snapdragons, at least in Maryland, is Cosmopepla bimaculata. Cleome is attractive to this pest, but snapdragons are more attractive in comparison. If you used the snapdragons as a trap plant to protect Cleome from C. bimaculata. it might be a feasible approach.

For future work in controlling spotted cucumber beetle we would suggest trying amaranth plants as the trap plants.
   
A special thanks for the ASCFG grant that enabled us to conduct this trial.


Contributors include Brian Clark, Chuck Schuster, Paula Shrewsbury, Suzanne Klick and Shannon Wadkins, University of Maryland Cooperative Extension.

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]