The effects of larval food intake can be obvious (death vs. survival), however they can also be subtle, altering growth and development rates, size, and reproductive performance. Additionally, they can be expressed in the short term (immature stages), the intermediate term (adult stage), or in a later generation. This post will cover:
- Effects of Food Intake on Larvae
- Effects of Food Intake on Reproduction
- Prey Specifity
Effects of Food Intake on Larval Development and Survival
The type and variety of prey is very important to lacewing development. For example, Ceraeochrysa cubana that were fed 5 different types of arthropods had development times ranging between 25-47 days, and mortality rates between 2% and 80%. Chrysoperla rufilabris showed similar variation, as did C. carnea. It is notable that some species do not show this, C. externa can eat a wide spectrum of prey and be generally unaffected. The type of prey also effects larval weight gain. C. carnea cocoons varied between 9-12 mg in one study.
The nutrient quality of prey can vary among species, as well as between members of the same species. They may have a higher or lower nutritional value, and could be toxic, depending on the host plant.
Chrysopid larvae are resilient, and can compensate for low prey levels during some parts of their life cycles. Food deprivation in early life means longer development time, but dry weight and food to body mass conversion weren’t affected. However, high intake during early instars or by adults won’t make up for low intake during the third instar, likely because most of the weight gain happens during said instar. This could vary depending on the species, as well.
Interactions with Host Plant of Prey
Plants influence the way predators behave in a number of ways. They are a primary food source for non-predatory adults, secrete volatile chemicals, as well as structural and morphological features that may affect larval search.
Surface structure on a plant can promote or hinder larval movement. Typically, the biggest issue is the presence, type, and density of trichomes on the plant surface. The speed of larval movement is inversely proportional to the density of glandular trichomes. This means that hairier plants will be more problematic for lacewings; Cannabis sativa, for example, would likely be unsuitable. C. carnea searches must faster on cotton plants compared to hairier tobacco leaves, and C. rufilabris is affected similarly. The first instar is more affected than the second, so it likely has a lot to do with developmental stage and the size of the larvae in comparison to the trichomes. Epidermal wax also alters larval movement. a ‘bloom’ in epidermal wax can slow down larvae notably. The final consideration is the complexity of the architecture of the plant, which could have some effect.
Effects of Food Intake on Reproduction
The quantity and quality of food available to larvae greatly affects their reproductive potential later in life. This can be noted in several traits:
- Ability to mate
- Length of preoviposition and oviposition periods
- Daily rate of oviposition
- Fecundity
- Fertility
Females generally require a large amount of energy for oogensis and sustained oviposition. In some cases, males require food before mating. Egg formation may begin before female emergence, while sperm production occurs during the larval stage. Inadequate prey can lead to sterility in both males and females. Generally, small size and lower fecundity are a result of not enough sustenance during the larval stage, and this is unlikely to be compensated for by any adult diet.
Only some metabolites from the larval stage can be transferred to adults. Because of this, females are highly reliant on external nutrient sources. These vary between species, but all of them require a protein source to continually produce eggs. C. carnea is autogenous, and can lay some eggs without postemergence feeding, however they require pollen and honeydew to sustain a high level of egg production. C. externa oviposit a negligible amount of eggs when fed a carbohydrate only diet versus when they are given protein. Chrysopa species have a wide range of variation in their diets, some are autogenous, while others are anautogenous, requiring protein (such as aphids) to begin to lay eggs. Additionally, some species can lay eggs on synthetic diets, while others need prey to do so.
Chrysopa slossonae specializes on one species of aphid. They will consume other species, but lay infertile eggs. When they are given their specific prey, they begin laying fertile eggs a day later.
Interestingly, geographical variation appears to play some role in this. Ch. oculata taken from western North America could produce fertile eggs on a synthetic diet, while those from the eastern side couldn’t.
Prey Specificity
As mentioned, the species Ch. slossonae specializes on one species of aphid (Prociphilus tessellatus). Its sister species, Ch. quadripunctata, is an aphid generalist. In nature, they are reproductively isolated, however they will hybridize in the lab, which can help demonstrate why stable prey association occurs and how it evolved in lacewings. While many herbivore specialists have been proved to have extensive traits that cause them to behave how they do, this study demonstrated that this is true for predators as well. That is, many behavioral, psychological, phenological, morphological, and physiological characteristics influence what type of prey a lacewing will feed on. This can affect reproduction, mouthparts, larval photoaxis and defensive behavior, morphology, and phenology (life cycles).
Most of these traits have a genetic basis. Variation was identified, however these traits seem to be very stable, and evolutionary change in relation to these traits seems to come with costs and benefits. It’s notable that the hybrids performed poorly compared to both the specialist and the generalist, however in nature they do not reproduce.
Conclusions
Much of what I have covered here is still largely unknown. There is data available for some species, covering development times and nutrient requirements, however a significant amount of this lacks research. In my next write up, I will discuss some of the ideas the book outlines as well as my own.
I believe the key here is learning which species in our local area prefer which pest, and to attract a variety of different lacewing generalists. Nectaries and hedgerows are critical for this – the more nectar, the more likely it is that lacewings will oviposit nearby.
The specificity of lacewing diets is also promising because it limits the damage they can do to non-pest species from evolutionary dietary deviation, which is certainly one of the problems of insect release. Additionally, there are a wide variety of native species here. For example, the photo Nothochrysa californica was taken in the Evergreen woods, and I have spotted several Neuroptera there, now that I’ve started looking for them.