Trap cropping for management of Harlequin bug in cole crops
A survey was conducted in 2009-2010 of vegetable farmers representing 17 counties/cities in Virginia. A questionnaire was distributed and email and telephone interviews were conducted as well as face to face interviews at farmer’s markets, grower meetings and at local farms. Although none of the farmers surveyed were federally certified “Organic,” many follow the “organic philosophy” and either did not use any chemical insecticides or used only those products approved for use for certified organic growers. A very broad range of brassicaceous crops are grown for vegetable markets, mainly in the spring and fall months, including arugula, bok choi, broccoli, Brussel sprouts, cabbage, cauliflower, collards, daikon, kale, kohlrabi, horseradish, mustard greens, rapini, radishes and turnips. Two growers in the central part of the commonwealth reported a summer crop of collard greens. Anecdotally, suggestions for potential trap crop varieties included komatsuna, cleome and horseradish. Several growers leave mustard in the field to overwinter for greens the following spring as well as early season nectar sources. A method of cover cropping with overwintering oilseed radish is being implemented in southeastern parts of the commonwealth; the purpose of this practice is to break through hard pans and to add organic matter to the soil rather than to act as a commercial vegetable crop. Of the 42 growers surveyed, all but five were familiar with HB and reported its presence in the field on a yearly basis, however only two identified harlequin bug as a key pest that required chemical management. Three growers reported using a trap crop to divert harlequin bug and all were satisfied with its effects.
Knowing the varieties of plants preferred by HB adults will aid in selecting the best trap crop for any given cash crop. To determine what species of plants are preferred by harlequin bug (HB), field-collected HB adults were offered plants of five Brassicaceous species (mustard ‘Southern Giant Curled,’ rapeseed ‘Athena,’ collard ‘Champion,’ rapini ‘Ruvo’ and arugula) and one non-brassica species (snap bean) in both field-cage choice tests. Six plants of each variety were planted (randomized by row) in each of 5 walk-in mesh field cages (3 x 3 x 2m) at the Eastern Shore Agricultural Research and Extension Center (ESAREC) in Painter, VA. Insects were introduced to each cage at five evenly spaced release points (30-50 adults), when plants were 10 weeks old and again at 14 weeks. Mustard, rapini and bean had begun flowering during 14 week observations. Four observations of HB adult location and presence of eggs masses were made every 1-2 days after introduction of insects. ANOVA and Tukey’s HSD was carried out using JMP (SAS Institutute, Cary, NC) to determine if there was a significant effect of plant species on numbers of adults and egg masses observed on each plant and to separate means, as well as to rule out date of observation, row/plant position as sources of variability (? = 0.05).
Significantly more HB adults were observed on mustard than any other plant in 10 week (n = 120, p < 0.0001) and 14 week field-cage choice tests (n = 90, p < 0.0001). Interestingly, significantly more HB adults were observed on 14 week old rapini plants than rapeseed, arugula, collard or bean plants, while this was not the case for 10 week old plants. Very little oviposition occurred within each experimental time period. In general the number of egg masses observed corresponded to the number of HB adults present on plants. There was no significant difference in HB adults observed on plants according to plant location within cages or date of observation (? = 0.05).
Mustard was clearly the preferred species within the group of plants evaluated in this study. Rapeseed and rapini were also found to attract more HB adults than collard and may offer insect pest managers alternative trap crop varieties. This experiment will be repeated in the 2010 growing season as well as a similar experiment with a new set of plant varieties and another designed to evaluate the role of plant maturity in HB attractiveness.
The ultimate goal of this study is to understand what plants are preferred by HB adults in order to manipulate their behavior in the field, thereby managing the damage they cause. This objective aims to investigate why these plants are more attractive and whether or not volatile organic compounds (VOCs) from the plant are used by HB in its host plant searching and/or selection.
a. Olfactometer to evaluate insect response to whole plant VOCs
To test the attractiveness of vegetable odors/VOCs the “flying-T” olfactometer, developed by J. Dickens, was used to test preference of HB adults for vegetable olfactory cues versus clean air. Adult HB were field collected from collards in August 2009 and each was isolated to a Petri dish with a water wick, starved and light deprived for 24-48 hours before bioassay. Insects were introduced to base of apparatus and a light source above acts to attract the insect up to the cross of the “T” where it encounters two plumes of scents (clean air or torn 6-10 week old vegetable leaves). A “choice” was made if the insect stopped, antennated and/or changed directions before climbing the arm of one side of the “T.” Each bout consisted of 20 males and 20 females. Each pair with two or more bouts was tested for significance by binomial proportions tests using JMP (SAS Institute, Cary, NC).
Both male and female HB adults respond in significant numbers to the olfactory cues of ripped vegetable leaves (mustard, arugula and collard) over clean air. This work will be continued in the 2010 growing season.
b. Identifying VOCs from whole plants
Collard greens were trimmed to fit into VOC collection jars (one leaf per jar, roughly 16 cm in length) and the cut end was wrapped in wet paper towel and aluminum foil. Baby arugula leaves (about 6 leaves per jar, each roughly 5 cm in length) were trimmed and cut ends put into small beakers of distilled water. Closed-loop headspace VOC collection was conducted using Super-Q absorbent filters (ARS, Florida) of intact leaves for 16 then 4 hours, after which leaves were torn in half and another 4 hour collection was taken. Another collection was taken from collard greens using closed-loop activated charcoal filters for 24 hours. Each blank collection was conducted on empty jars that were cleaned with ethanol or acetone. Super-Q filters were eluted with 200 µL of dichloromethane and cleaned with dichlomethane and allowed to dry over night prior to a new collection. Charcoal filters were eluted with 40 µL of dichloromethane and cleaned with a series of organic solvents (methanol:dichloromethane, 3:1, dichloromethane, acetone) and left to dry over night before they were used for a new collection.
Several compounds were consistently identified, which could be distinguished from background contaminants in each set of collections. These compounds included the green leafy volatile hexenyl acetate and several terpenoids (e.g. sabinene, ?-myrcene). For each species, mechanical damage of leaves resulted in higher emissions of hexenyl acetate than those observed from intact leaves. This protocol will be repeated with whole plants and liquid samples will be saved and used for electroantennagram work in the winter of 2010.
Impacts and Contributions/Outcomes
Graduate Research Assistant
Department of Entomology
216 Price Hall
Blacksburg, VA 2406-0319
Office Phone: 5402309902