1. Which Animals Hunt Soft-Bodied Aphids?
Aphids have a soft outer shell and live in groups. Because of this, many animals are thought to eat them [1]. Common predators include the seven-spotted ladybird and the harlequin ladybird, as well as the larvae of lacewings and hoverflies. The long-tailed bushtit also eats aphids [1]. Parasitic wasps, such as aphid wasps, are also counted as natural enemies because they lay eggs inside aphids rather than eating them directly [1].
Even though they target the same aphids, their methods differ greatly. Some eat both adult and young aphids; others eat only young aphids, while some consume aphids from the inside out. Let us start with the most familiar predator: the ladybird.
2. Do Ladybirds Eat Aphids as Both Larvae and Adults?
The harlequin ladybird is a carnivorous insect that eats aphids in both its larval and adult stages [2]. If food runs out, they may even cannibalize other larvae or pupae [2]. They are described as generalists because they eat many types of aphids without being picky [3].
Farmers noticed this appetite. The harlequin ladybird eats large numbers of aphids and is easier to breed than other ladybirds, so it has been used in "biological control" to reduce pests using natural enemies [5]. The National Agriculture and Food Research Organization (NARO) developed a strain called the "non-flying harlequin ladybird." This helps them stay in greenhouses instead of flying away to the ceiling [5].
In a test by the Mie Prefectural Agricultural Experiment Station, harlequin ladybird larvae were released into strawberry fields infested with cotton aphids in early spring. The effect of reducing aphids lasted for about one week [4]. In this test, both harlequin and seven-spotted ladybird larvae preferred to eat the aphids located in the flower clusters [4]. Although adult ladybirds are memorable, it was the larvae that showed effectiveness in this field trial. Next, we look at an insect often seen on flowers.
3. Do Hoverflies Eat Aphids as Adults?
The hoverfly is a member of the Syrphidae family. It has yellow patterns on a black body and looks very much like a wasp [6]. This appearance is called "mimicry," imitating a wasp [6].
Adult hoverflies fly from flower to flower, eating nectar and pollen. In doing so, they help with pollination. It is not the adults that eat aphids, but the larvae [6]. The same insect performs completely different jobs as a child and an adult.
The power of these larvae is also tested in agriculture. In a test example from Tokushima Prefecture, placing plants like sorghum that had produced hoverfly and gall midge larvae inside eggplant greenhouses showed control effects similar to releasing the aphid parasitoid wasp [7]. Next, we look at the lacewing, whose eating habits are recorded in encyclopedias.
4. What Is Unusual About Lacewing Eggs and Larvae?
Lacewing larvae are carnivorous insects that catch and eat aphids and spider mites. According to the World Encyclopedia, a lacewing larva eats 300–600 aphids during its larval stage [8]. This number is an estimate that varies by species and conditions.
Most adult lacewings seem to eat pollen, flower nectar, and the sweet honeydew produced by aphids. However, some species are carnivorous even as adults [8].
Their eggs are even more unusual. The eggs are attached to the tips of long, thin stalks, laid in groups of about ten or more [9]. The stalks look like grass and have long been called "udon-ge" (noodle flower) in Japan [9].
Some lacewing larvae hook plant fragments or the corpses of their prey onto hooked hairs on their backs [9]. The shells of what they have eaten become a coat that helps them hide. So far, all these predators "eat" aphids. But there is a natural enemy that reduces aphid numbers without eating them.
5. How Do Parasitic Wasps Reduce Aphid Numbers Without Eating Them?
Female aphid wasps insert an ovipositor into the aphid's body and lay eggs inside [10]. The hatched larvae grow by eating the tissues inside the aphid. Eventually, the aphid becomes a dry, hard shell called a "mummy" [10]. The larva pupates inside this mummy, and the adult wasp emerges through a round hole [10]. Part of this process is also explained in NARO's manual for using the *Aphidius* wasp [11].
In other words, the aphid is not eaten up but turned into a "growing place" for the wasp's child. This is the opposite of how previous predators ate from the outside.
This trait is used in agriculture. Aphid wasps are considered one of the most widely used biological pesticides for controlling aphids on greenhouse vegetables [11]. Species such as *Aphidius colemani* and the native Japanese *Aphidius gifuensis* are used [11]. However, releasing them after aphids have already increased may not be effective enough, so it is recommended to release them in the early stages of infestation [11].
6. How Do Ants Help Aphids Defend Themselves Against Predators?
Some aphid species are protected by ants. Ants gather for the honeydew that aphids produce, and in return, they drive away natural enemies like ladybirds [1]. However, not all aphids live with ants; the relationship differs by species.
Some aphids defend themselves with help from their group. In a study by the University of Tokyo (published in 2013), researchers examined an aphid that forms galls on plants [12]. When a hole opens in the gall, predators like ladybird larvae enter. Then, newly hatched first-instar larvae and old adults who have finished reproducing gather around the hole to defend it. Individuals of intermediate age flee deeper inside to focus on reproduction [12].
The defending first-instar larvae reportedly stab predators with their mouthparts, while the old adults stick to enemies with a liquid secreted from their abdomens to stop their movement [12]. Different age groups take different roles in defense. After reading this, does the underside of leaves look more lively? Next, we will summarize clues for finding these natural enemies.
7. How Can You Find Signs of Aphid Enemies Under Leaves?
If you find plants with aphids, gently peek at the underside of the leaves. You might see ladybird larvae, hoverfly larvae, lacewing eggs, or hard, swollen mummies. A magnifying glass is useful for small things.
Look only; do not touch. If there are wasps or caterpillars, do not approach them. Wash your hands after you finish looking. Avoid areas where pesticides have been sprayed.
If you record the insects you find by type of natural enemy in a notebook, you will gradually see "which predators come to leaves with many aphids." If you want to know more, NARO and prefectural agricultural experiment stations publish materials on using natural enemies. These can help you learn which insects are used and when in farming fields.