Assessing impacts on non-target aphids

Aphis monardae is an aphid species under study for potential non-target impacts. In prairie habitats, A. monardae populations aggregate in flower heads of their host plant, Monarda fistulosa

When introducing new natural enemies for long-term (permanent) biological control, it is essential to minimize potential unwanted affects. As part of the NCSRP Soybean Aphid Biological Control Project, we have been assessing the potential for negative impacts of those Asian natural enemies that we are considering for release to control the aphid.

As part of this effort we have worked to identify a group of native U.S. aphids for field sampling that complements laboratory host specificity testing and other studies. We followed a stepwise procedure to define a group of aphids for study. We first compiled a list of the 400 aphid species in Illinois, under the assumption that we would find many, if not most, throughout the Midwest. We then applied a set of three "filters" to pare the list to a manageable size.

Filter A – Aphid phylogeny. We have focused on aphids most closely related to the soybean aphid. Other aphids, more distantly related, are found in places we felt soybean aphid natural enemies would not be typically searching for hosts to attack. This left about 300 aphid species.

Filter B – Aphid origin. We decided to focus only on native aphids, as these species represent our natural U.S. fauna. This left about 200 species.

Filter C – Host plants. We then identified host plants that are shared between at least two aphid species. This was done to increase the chance that we would encounter aphids in our sampling.

By applying the filters, we arrived at 84 aphid species on 32 plant taxa. Most plants are found in prairies, grasslands or old fields. Others are wetland plants or are found in wooded areas.

We use transect sampling and visual observations to locate aphids, to measure their relative densities (and those of their host plants), and to collect to rear their parasitoids. We also use sweep-net sampling to collect predators and get a measure of other aphids in the habitat. We hope to describe the relative abundance and distribution of the aphids, and have an estimate of the “natural enemy load” the aphid species are carrying (e.g., relative parasitism rates and densities of predators).

We will use these data to assess the relative risks of non-target impacts, along with study in Asia, the suction trap network, previous aphid biocontrol programs, and host specificity testing. We will conduct a risk analysis to find a balance of benefits of controlling the soybean aphid and potential harm via non-target impacts on native aphid species.

Host specificity testing

Host specificity testing of Binodoxys communis strains from both China and Japan indicate an intermediate level of host specificity. Soybean aphid is the most suitable host, with Aphis monardae showing slightly lower suitability on vegetative growth of the native prairie plant Monarda fistulosa.

Seven other species of aphids showed low suitability and a further 6 species were not used by B. communis at all. Aphis monardae is a native non-pest aphid that inhabits native prairie habitats and is therefore a non-target species that may be at risk if B. communis is released.

To evaluate this risk further, we have made observation of A. monardae in native prairie settings throughout Southern Minnesota. We found it abundant in both undisturbed prairie remnants and parkland meadows. During these observations, we also noticed that (i) this aphid tends to cluster in flower heads of its host plant, and that (ii) many of these aphid colonies are tended by ants (Lasius sp.). Both of these factors may decrease the risk of introduced B. communis to the native A. monardae. Results of this work were recently published as Physical and ant-mediated refuges from parasitism: implications for non-target effects in biological control. Wyckhuys, K. A. G., R. L. Koch, and G. E. Heimpel. 2007. Biological Control 40: 306-313. Read abstract»

2007/2008 Update
We again surveyed native aphids in 3 states (IA, IN, WI) to provide information on potential non-target effects of introduced biological control agents on native aphid species.

As in 2006, our survey efforts were focused on prairie habitats. A majority of native aphids species in the genus Aphis, the genus which contains the soybean aphid, are found on prairie plants. In 2007, we monitored 26 prairie sites in central Iowa, 6 sites in IN, and 4 sites in WI. Monitoring of aphids was conducted by visually inspecting plants along three 25 by 2 m transects per sampling date.

Data from the North Central Regional Soybean Aphid Suction Trap Network can be found on their website. The suction traps run from May through October and collect migratory aphids as they are flying.

Parasitoid wasps of the aphids were reared from collected aphid colonies. We also used sweep net sampling to survey additional natural enemies adjacent to each transect. All aphids, their host plants and parasitoids are in the process of being identified to species, and predators are being identified to at least the family level.

K. Wyckhuys traveled to Lang Fang China in summer 2007 to begin experiments and supervise studies on habitat specialization of B. communis. Aphids were collected from over 30 host plants and examined for parasitism to determine whether B. communis was attacking aphids other than soybean aphids and cotton aphids (two primary known hosts). Data are still being analyzed from the 2007 field study, but analysis from a similar study in 2006 revealed that B. communis was attacking only cotton and soybean aphids.

North Central Regional Soybean Aphid Suction Trap Network
In 2007, there were 42 suction traps operating in 10 Midwestern states. In addition to identifying all the aphids of agricultural interest, people in D. Voegtlin’s lab also identified native aphids that were encountered.

Of particular interest are the Aphis species because they are the non-target aphids that are most likely subject to parasitism by natural enemies released for control of the soybean aphid. An online key to the genus is being developed.

Publications

Wyckhuys, K. A. G., R. L. Koch, and G. E. Heimpel. 2007. Physical and ant-mediated refuges from parasitism: implications for non-target effects in biological control. Biological Control 40: 306-313. Read abstract»

Wyckhuys, K. A. G., and G. E. Heimpel. 2007. Response of the soybean aphid parasitoid Binodoxys communis to olfactory cues from target and non-target host-plant complexes. Entomologia Experimentalis et Applicata
Volume 123 Issue 2, Pages 149 - 158.  Read abstract»

Chacon, J. M., D. A. Landis, and G. E. Heimpel. 2008. Biotic interference of a classical biological control agent of the soybean aphid. Biological Control 46: 216-225. Read abstract»