A balancing act
It’s been a while! I got busy and then it was winter, a less inspiring time for insect photography. Recently, however, I got asked to give an insect related talk at a conference, and it’s motivated me to get this up and going again. Also my wood sorrel just got aphids again.
How to control insect pests while getting most of what you want is an age-old problem in both agriculture and horticulture. Looking at it through an ecological lens, there are really two main approaches; we call them top-down and bottom-up. Top-down is exerting control from ‘above’ the plant. This includes natural ways, such as using predatory insects, and chemical ways, mainly insecticides. This area gets a lot of focus. What insecticides work best? When should the beneficial insects be released and how many do you need?? I think about questions like these a lot, both professionally and in my own garden. And I like to take pictures of the process, like this lady beetle eating an aphid (providing top-down control).
There is a lot of advise out there about how to manage top-down control. But top-down is not the only way to control insect (or non-insect) plant pests.
Bottom-up control refers to mechanisms for control that go through the plant. There are many variations on this. One you may have heard of is resistance. Some plants are resistant to certain pests or diseases. For example, some tomatoes are resistance to a disease called fusarium wilt. Resistant tomatoes will not suffer from the disease like susceptible tomatoes even though they are treated the same. You can see this clearly in this photo from the Missour Botanical Garden (http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/cankers/fusarium-wilt-of-tomato.aspx)
Some plants deter pests from eating them by tasting bad to particular pests. This is a whole field in ecology, but for our purposed today, we are just going to say that this is a form of bottom-up control.
The type of bottom-up control that I want to talk about has to do with the inputs a plant gets (how much fertilizer, water, sun, etc) and how those affect both pest and beneficial insects. Just like a child’s diet impacts how various aspects of their development, a plant’s access to nutrients, water, and sunlight changes want the plant will look like as it grows. It also changes how the plant responds to insects and how insects respond the plant.
The most basic version of this is that giving a plant more nutrients can increase the number of pest insects attacking the plant. This is common for roses. If you over-fertilize your roses, you are very likely to have lots and lots of aphids on those plants, especially if the roses also have lots of water. The rose pictured below, for example, was covered in aphids.
Aphids get their nutrients from the plants they feed on. So when a plant has more nutrients, more of them get passed to the aphids. Then the well-nourished aphids can grow and reproduce faster. This leads to larger populations of aphids on the well-fed and well-watered roses. As long as you are not giving them WAY too much water and fertilizer, these roses will usually make more flowers for you to look at as well, which is why people feed and water their roses in the first place!
But the interaction can get even more complicated when you start to look at beneficial insects too (adding in top-down control). Sticking with the roses example, let’s say that lacewings and parasitoid wasps (how have I not done a post on those yet?!) are attacking the aphids. Juvenile lacewings, like the one shown below, eat the aphids.
The well nourished aphids on the well nourished plants are bigger, meaning one aphid is a larger meal for a lacewing. This can lead to the lacewings actually eating fewer aphids, since each individual aphid is more food. In this situation, the lacewings are providing less top-down control than they would on a plant with less-nourished, smaller aphids. This is doubly bad for the plant; it has more aphids because they are growing and reproducing faster AND fewer of these aphids are being eat by lacewings.
But what about the wasps? First, some parasitoid wasp biology. 1) The wasps lay one egg inside of each aphid they attack. 2) Only the females do this. 3) Wasps can determine the sex of the egg they lay (another future post). With that covered, I’ll continue on.
Many species of parasitoids will choose to lay female eggs in larger aphids (like those larger aphids on the well-nourished roses). This leads to an increased in the proportion of female wasps in that population, which in turn leads to more aphids being attacked since only females attack them. So this can mean that while a well-nourished rose bush is getting less top-down control from lacewings, it is getting more from parasitoids. And parasitoids are almost always more important in top-down control than lacewings.
Striking the balance between top-down control and bottom-up control to achieve the biggest yield (roses in our example, but it could easily be soybeans), is a tricky business!