Helianthella quinquenervis (Asteraceae), or the aspen sunflower, occurs in rocky meadows near patches of aspen (Populus tremuloides) in and around the Rocky Mountain Biological Laboratory, elevation 2871m. Flower heads of aspen sunflowers tend to emerge in early to mid-July based on elevation (Inouye 2008). Ants commonly seen foraging for extrafloral nectar on the flowers include species in four genera: Formica, Tapinoma, Myrmica, and Lasius. All four genera forage on the plants several weeks prior to flower head emergence and continue until flowering ceases and the flower heads wilt.

Research sites were located near aspen stands in the rocky meadows on the west facing hill-slopes approximately 0.6 kilometers north of the Rocky Mountain Biological Laboratory between elevations 2927-2946m. Twenty plots were randomly assigned to patches of aspen sunflowers as they occurred in the meadows.

In order to determine the effect of extrafloral nectar on ant abundance, I provided ants with an additional 15% honey in water solution. This extra resource mimics additional production of extrafloral nectar. Furthermore, a honey in water solution was chosen based on Inouye and Inouye’s (1980) findings that aspen sunflower extrafloral nectar is especially rich in amino acids.  A small hole, (0.16 cm in diameter) was drilled using a drill bit at the bottom of a two mL centrifuge tube so that ants were able to gain access to the solution.  I placed the centrifuge tube behind the flower head at the margins of the involucral bracts where extrafloral nectar is secreted to mimic an increase in extrafloral nectar availability. The centrifuge tube was attached to a 17 gauge steel wire with twist ties, which was in turn twist-tied to the stalk of plant.

 

To assess the importance of ant abundance and species composition to herbivory and pollinator visitation I also manipulated ant access to aspen sunflowers. One treatment completely restricted ant access to the sunflower through the application of Tanglefoot (Grand Rapids, MI, USA) around the stalk. A second treatment excluded the large dominant Formica sp. at the sites. This treatment consists of a plastic milk jug cap (3.5cm in diameter) with 0.80 cm holes drilled through the top. These holes are small enough to deny larger Formica sp. access while large enough to allow smaller, less aggressive ant species passage.  Tanglefoot was applied to the edge of the milk jug cap to prevent ants from climbing over the side. In order to account for the potential effect of the steel wire, all experimental stalks received steel wire attachments.  Additionally, all leaves and other material from neighboring plants that come in contact with the experimental sunflower stalk above the treatment application were removed to prevent their use as a bridge by ants.

Manipulations of extrafloral nectar and ant abundance were combined in a factorial experiment (Table 1). Each treatment was applied to individual aspen sunflowers in a randomized-block design across 20 blocks for a total of 100 plants.

 

 

Extrafloral (EFN) Treatments/Ant Treatments

EFN sugar addition + Formica exclusion

EFN sugar addition + control

Control + Formica exclusion

Control + total ant exclusion

Control + control

Table 1: Experimental Treatments

 

 

Plant growth rate, condition (e.g. flower head emergence and number, leaf number and damage, flower head wilting and number), and ant abundance were measured weekly for three weeks for each aspen sunflower plant from 9-22 July. Growth rate was measured as the change in height from the top of the terminal flower to the ground. Leaf damage was measured by counting the number of herbivore-induced holes present in the leaves.

Ant species abundance on each plant was recorded as the number of each ant species on the plant during a 30 second recording period in the morning (0800-1000h) and afternoon (1200-1600h). Counts were recorded twice a week for three weeks from 11-23 July. Voucher specimens of each ant species were collected for identification and deposited in the RMBL collection.