Insect pest populations are known to be regulated, at least in part, by naturally-occurring natural enemies such as pathogens, predators, and parasitoids — the latter living at the expense of their host, like parasites, but eventually killing it. Natural enemies have the advantage to exert a prolonged pressure on pest populations. They maintain pest densities at low levels, thereby preventing insecticide applications. Properly quantifying the impact of pest management strategies is of utmost importance in agriculture to ensure they are both economically advantageous as well as reliable. However, assessing the impact of natural enemies on insect pests is usually much more complex than for insecticides.
This article focuses on aphid control by parasitoids. Aphids belong to a large group of small, sap-sucking, insects that reproduce fast and includes important pests of various crops. Aphid parasitoids are tiny wasps often used as biological control agents. Adult females lay a single egg inside their host and the emerging larvae feed on the aphid, which remains alive until the parasitoid enters the last stage of its immature development. The aphid is then killed and its cuticle expands, hardens, and becomes beige or black. The swollen aphid, called a mummy, protects the pupating parasitoid until its emergence as an adult.
From an experimental perspective, mummies represent a record of the parasitoid-induced aphid mortality and, therefore, facilitate biological control studies. The proportion of parasitized aphids (mummies) is often used directly to quantify the capacity of parasitoids to control aphid populations. However, although this approach provides instantaneous measures of mortality, it does not consider the consequences on subsequent aphid generations and, thereby, the long-term impact of natural enemies.
The Propagated Mortality Analysis
In the first part of the article, we developed an inferential method, designated as the propagated mortality analysis (PMA), to assess the impact that parasitoids have on aphid populations in open fields. More specifically, the PMA allows translating observed mortality rates into their impact on forthcoming aphid populations and into estimates of control that are useful in the decision to intervene.
The method relies on the fact that many aphid species have a single peak of abundance in annual crops during the summer. This population dynamics holds even in absence of natural enemies and tend to be driven by bottom-up effects such as the plant quality, which changes throughout the season. We modified an existing model describing this population dynamics to incorporate the influence of parasitoid-induced mortality on aphid populations. Being able to compare an aphid population in the presence versus the absence of parasitoids allows us to estimate their impact at any time during the season.
To facilitate interpreting the impact of parasitoids, we provided two estimates of control. We first evaluated the decrease in aphid populations due to parasitoids at peak aphid density, because producers and agronomists often want to know if pest populations will reach some intervention thresholds. We next calculated the decrease in aphid populations, associated to the parasitoid, in terms of cumulative aphid densities (CAD) over the period during which the crop is susceptible to aphids. The latter estimate of control was considered because yield loss is often calculated from CAD. The economic value of naturally occurring parasitoids can, therefore, be assessed in terms of crop protection benefits.
The method can be used for any aphid system and only requires measuring aphid density and the proportion of parasitized aphids in the field, as well as knowledge of parasitoid developmental time. We encourage the use of PMA by making the R code available for its implementation.
The Case Study
In the second part of the article, we used PMA to assess the impact of a naturally occurring parasitoid species, Aphelinus sp., on the soybean aphid in Québec soybean fields. The method showed both the peak aphid density and the CAD to be reduced only between 1-7% by the parasitoid and to translate into crop protection values of less than 1%. Low control was associated to the late arrival of the parasitoid in the field, compared to the soybean aphid, while low crop protection value arises from that the soybean aphid only slightly reduces soybean yields at usual densities in Québec.
Perspectives
The strength of PMA relies on its ability to assess parasitoid impact independently of its seasonal abundance. Therefore, the method opens the path to compare the efficacy of natural enemies that exhibit different patterns of colonization and foraging strategies. Given the accessibility of the method, we hope PMA will be used to quantify ecosystem services as well as to strengthen the predictability of control of mass-reared parasitoids and the establishment of release strategies.
These findings are described in the article entitled Estimating parasitoid impact on aphid populations in the field, recently published in the journal Biological Control. This work was conducted by Alexandre Leblanc and Jacques Brodeur from the Université de Montréal.