The behavior of the ladybird and its ability as a predator
Louise Vedel Olsen
The ladybird is one of the best-known predators in North America (Hoffmann & Frodsham, 1993). It is used as a biological control agent, often to control aphids. The foraging behavior of the ladybird larvae and the adult is different. Theory indicates that it is an adult’s ability to select suitable patches for oviposition that is likely to be the most important in the determining its fitness (Kindelmann & Dixon, 1993). Their tendency to stay and lay their eggs only in areas where the aphid population density is high means that they attack aphids only when these are sufficiently numerous to support a generation of larvae (Wratten, 1973). Each group of prey and predators has a particular chemical image. All stages of the ladybird have similar species-specific alkanes. The larvae have besides toxic alkaloids to protect themselves for predators (Dixon, 2000). The larvae will search for food in the area they found food last time. The foraging behavior of the larvae is hunger dependent. Hunger will change their behavior in a bad direction; it will have more difficulties to find food. There is also a difference between the male and female adults behavior. The male’s success depends on its ability to fertilize the female whereas the female’s success is its ability to reproduce (Reiss, 1989). The adult ladybird has a wide prey range, but the larvae only fed on fewer species of prey. The ladybird is attracted to the odour of their prey and stay in the area where the prey is, or has recently been. Species specificity is important to be a good biological control agent, whereas the predator’s ability to attack all development stages of their prey also is important. In general the coccidophagous species are better biological control agents than aphidophagous species because they are more species specific in their search for prey.
The ladybird is one of the best-known predators with over 450 species in North America (Hoffmann & Frodsham, 1993). It is often used in biological control of especially aphids. In this article I will look at the behavior of the ladybird and why it is used as a predator in biological control. The first and most successful case of biological control was the introduction of the Australian ladybird, Rodolia cardinalis, into California in 1888. It was introduced to control a scale insect, which was devastating the California citrus industry (Dixon, 2000). Most ladybirds are beneficial as both adults and larvae, feeding primarily on aphids and other small insects (Hoffmann & Frodsham, 1993). Only a few species are feeding on plants. Some ladybirds are more successful as predator than other, the aphidophagous species are less successful than coccidophagous species (Hodek, 1967).
To learn more about the dynamics of ladybird-prey interactions, there have been many studies on the ladybirds foraging behavior (Dixon, 2000).
The basic functional response to pray density includes three important components: (1) the time predator and prey are exposed, (2) the discovery rate of the predator (including speed of movement, range of perception and capture efficiency), (3) the time spent chasing, killing and consuming prey (Wratten, 1973). There are also three possible components of the numerical response to prey density: (1) a relatively rapid response by concentration of the predator in areas of high prey density; (2) a reproductive response through the laying of more eggs by each female; (3) a relatively rapid numerical response through improved survival of immature predators (Huffaker et al., 1968). The animal prey supply predators with nutritionally balanced diets, that makes the predator able to be very species specific in its choice of food (Malcolm, 1992). Some ladybirds do better when they feed on certain species of prey (Dixon, 2000).
The length of the life cycle varies depending upon temperature, humidity, and food supply. Usually the life cycle from egg to adult requires about three to four weeks, or up to six weeks during cooler spring months. In the spring, over wintering adults find food, then lay from fifty to three hundred eggs in her lifetime in aphid colonies. Eggs hatch in three to five days, and larvae feed on aphids or other insects for two to three weeks, then pupate. Adults emerge in seven to ten days. There may be five to six generations per year. In the autumn, adults hibernate, sometimes in large numbers, in plant refuse and crevices. Beetles are always found under leaves which protect them from cold winter temperatures (Lyon, 2001).
Most studies on foraging behavior in ladybirds have been on the larvae (Dixon, 2000). Theory indicates that it is an adult’s ability to select suitable patches for oviposition that is likely to be most important in the determining its fitness (Kindlmann & Dixon, 1993). Adults in selecting a habitat and then a patch of prey play a major role in determining the range of potential prey available to their larvae (Dixon, 2000). Both adults and larvae feed on many different soft-bodied insects with aphids being their main food source. Mealy bugs, spider mites and eggs of the Colorado potato beetle and European corn borer are other food sources (Lyon, 2001).
The ladybird have tendency to stay and lay eggs only in areas where the aphid population density is high means that they attack aphids only when these are sufficiently numerous to support a generation of larvae (Wratten, 1973).
The ladybird undergoes complete metamorphosis with egg, larval, popal, and adult stages of development (Hoffmann & Frodsham, 1993). The larvae and the adult ladybird have different foraging behavior, which will be explained in the following passages.
Larvae: The most important sensory input for foraging ladybird larvae is the chemical nature of the cuticular waxes of the insect they encounter. Each group of prey or predators has a particular chemical image. All stages of development of ladybirds also appear to be similarly coated with species-specific alkanes. In this case to attack something with the same alkane profile as your self could be risky as instead of being the cannibal you could become the victim. The larvae also contain toxic alkaloids to protect them self for predators like birds (Dixon, 2000). The larvae are most active during the day. The tend to be positively phototaxis and negatively geotaxis, which means they will move toward light and away from gravity (Dixon, 1959). That will lead them to the top of the plants where most of the aphids live. Encounters with prey that do not result in capture initiate a change in searching behavior. When the encounters with prey result in feeding the larvae will walk more slowly and change their direction of movement more frequently than before feeding (Hunter, 1978). They will search for food in the area where they fed last time, is there no more aphids they will increase their speed of movement. Speed will increase over time. If the larvae lose their contact with the prey, the larvae appear more intent initially on pursuit rather than trying to find another prey item. Hungry larvae have a longer handling time than the well-fed larvae (Dixon, 2000). That has a connection to the frequency of change in direction. As described above the larvae will change direction more often after feeding, which mean that it will search the particular feeding area better. The time spend feeding on each item is a simple linear function of the size of the prey relative to that of the predator. Development of an optimal foraging model in which handling time is determinated by average encounter rate, because extraction of food from prey item becomes progressively more difficult. At high encounter rates prey should be abandoned when it becomes more profitable to capture another (Cook & Cockrell, 1978). The longer handling times of hungry larvae possibly result from the larvae spending longer searching the substrate with their mouthparts to ensure that each prey items has been totally consumed. This is consistent with the increased extraction of food from large prey items in poor quality habitats predicted by the optimal foraging model. However, it is difficult to understand why handling time does not decrease through a catch sequence as hunger decreases, unless larvae measure hunger by the contents of their mid-gut and their response to prey availability.
Many of the components of the foraging behavior of ladybird larvae have been shown to be hunger dependent; this is a largely ignored in the development of optimal foraging theory. It is largely that it is by means of these hunger induced changes in foraging behavior that ladybird larvae forage in a manner similar to that predicted by optimal foraging theory and so maximize their rate of energy intake. Some species of the ladybird larvae can respond to their own tracks and in that way avoid searching the areas they have already searched (Dixon, 2000).
The ladybird larvae consume 200- 300 aphids per day as is grow (Hoffmann & Frodsham, 1993).
Adult ladybird: There are differences in the male ladybirds and female ladybirds behavior. The male’s job is to find the female and fertilize her whereas the female’s job is to reproduce, locate and ovipositing in high quality patches of nursery prey.
The adult ladybird gets all their energy to their searching behavior and reproduction from their prey. They store their energy for searching and over wintering in the fat body (Hodek & Honek, 1996). When they emerging from hibernation in the spring will they use some of their fat reserves to produce eggs. If the ladybirds offspring are to survive, the prey has to be abundant for long enough to sustain their development. The adult ladybird are known to have a wide dietary range but their larvae are only found developing on a fewer species of prey. That means the adult ladybird have to find the species that the larvae can feed on and oviposit close to these aphid colonies (Hodek, 1993). The aphidophagous ladybird matures most of the eggs it is going to lay in patch by feeding on aphids in that patch and only stays and lays eggs if its rate of capturing aphids exceeds a certain critical value (Dixon, 1959). The ladybird lay clusters of eggs; it is a kind of defense. They are more deterred from attacking clusters than single eggs. When the larvae hatch is it often seen that more than one larvae fed on the same aphid. That can happened because the larvae orientate to the alarm pheromone the aphid releases (Hemptinne et al., 2000). The coccidophagous species lay their eggs individually under the ovisac or body of a coccid. The larva that emerges from the egg first eats the contents of the ovisac and then the adult coccid. The larvae reaches en advanced stage of development on one prey item. That could also mean that the presence of ladybird larvae indicate that the aphid colony is in a late stage of development.
The ladybird mainly searches for food during the daytime. This diurnal rhythm is determined by an endogenous circadian timer, which overrides hunger as a determinant of searching activity. The ladybird does not move at random but appear to be attracted to sites where prey are or have recently been present. The ladybird is attracted by the odour of their prey. They move more slowly and turn more frequently in the presence than in absence of the odour of prey (Hattingh & Samways, 1995). The ladybird can only respond to prey over relatively short distance, but what extent olfactory and visual cues are used is unknown. Many authors have claimed that ladybirds only respond to the presence of prey after physical contact (Storch, 1976).
Males begin developing their gonads earlier in their development than females, and this has costs interning of the growth rate that males can sustain. Male’s reproductive success is a function of the rate of encountering females, as described earlier. Small males may be favored when food is limiting because they require lower absolute amounts of food and can therefore spend more time looking for females (Reiss, 1989).
Aphid life cycle and defense:
The aphids over winter in the egg stage. The egg hatches in the spring into a female called "stem mother". This female give live birth to female aphids. Asexual reproduction continues with both wingless and winged forms being produced. In late summer or fall, the asexual reproducing females produce sexual, males and females. These sexual form mate and lay eggs for overwintring (http://www.treehelp.com/, 2001)
Many aphid exhibit a very strong dropping response in the presence of foliar-foraging predators, often involving the use of alarm pheromones (McConnell and Kring, 1990).
Ladybird as biological control agent:
To be effective in biological control, an agent must be host specific so that they do not affect nontarget organisms.
Some of the desirable characteristics of natural enemies are:
Good searching ability—ability to locate the target host
Host specificity – attacks only the target pest
High reproduction rate -- high rate of increase
Short life cycle – shorter life cycle than host
Adaptability – adapts well to host habitats
Host synchrony – well adapted to different stages of life cycle of target host
Sustainability – able to maintain itself after reducing host population
(Kok and Kok, 2001).
What is a predator? A living organism that feeds upon other organisms that are smaller and weaker than itself.
Specificity is seen to as an important feature of a biological control agent an candidate species are intensively screened and the rile of their attacking non target species assessed before their use in biological control programs. There is some evidence that biological control can be favorable for conservation where a pest is causing damage in a nature reserve (Samways, 1994). Some of the attributes and reasons, such as abundance of the predator and ephemeral nature of aphid populations are a consequence of the operation of other processes. Problems created by a mismatch on the temperature thresholds of predator and prey.
Another important ability of the predator is if they attack all stages of prey. Most biological practitioners consider searching ability/capacity to be one of the most important attributes of an effective natural enemy. You can make three important attributes. 1: specificity. Some insect predators are monophagous, which could be an important attribute of a biological control agent. The Coccidophagous ladybirds are more prey specific than the aphidophagous ladybird. 2: voracity. Is indeed considered an important feature of an effective biological control predator. The faster a prey reproduces, the more voracious must a predator be. 3: attack rate. The number of prey attacked per predator per day. That number can you find by using formula 1.
Formula 1: Na = a N / ( 1 + a Th N)
Na is the number of prey attacked per predator per day; a is the instantaneous attack rate; N is the density of prey; Th is the handling time associated with prey attacked.
It is also possible to use the consumption rate, but it may not be so correct because prey may die after predator attack without being eaten.
The beetles released in the summer do not disperse any great distance, but remain for the most part in the areas where they were released. Their feeding habits are not normal. They will drink water, but have no appetite since they apparently are able to exist on their stored fat. Some feeding and reproduction occur, but these activities are much slower then those occurring in natural populations (Lyon, 2001). If prey is plentiful, the beetles will stay, lay eggs and become effective aphid predators.
The development of coccidophagous species is slower than that of aphidophagous species and the efficiencies of converting prey biomass into ladybird biomass are very similar in these two groups of ladybirds. Coccidophagous ladybirds have lower lifetime fecundity that aphidophagous ladybird. Coccidophagous species generally do everything more slowly than aphidophagous species. The Coccidophagous is more successful as a biological control agent (Dixon, 2000).
There are three types of biological control, they are described in the following phages
Augmentation is the increase of native agents for control of native or exotic pests. Usually there is a lack of synchrony where the pest occurs early in the season and the natural enemies are absent. Thus, releasing the natural enemy early in the season will help to ensure that there is no scarcity of natural enemies when the pests first appear. This process is called augmentation
Classical biological control is the use of exotic biological control agents imported from its native home into the target area against exotic pests that have arrived without their natural enemies. In the absence of their natural enemies, the exotic pests multiply rapidly and spread to become a major pest. Thus, classical biological control attempts to introduce the natural enemies also into the new area so that they will reestablish equilibrium with the pest and keep it under control. This is referred to as an old association because the natural enemy and pest were in the same place or ecosystem.
Neoclassical biological control is the use of exotic biological control agents against a native pest. Since they were not in the same area before, this is referred to as a new association.
(Kok and Kok, 2001).
The body of theory regarding the stability of predator – prey interactions, however, largely ignores the possible effects of predator interactions. The finding that the synergistic suppression of pea aphid populations intensifies at high prey densities suggests that predator – predator synergisms may stabilize aphid population dynamics and deter outbreaks. If predator – predator interactions in a community are largely additive or synergistic, predator complexes should promote stability and deter outbreaks of prey populations (Losey, 1998).
Natural enemies work quietly, often unnoticed and so their importance is often overlooked. Frequently their effectiveness as biological control agents becomes noticeable only after they have been eliminated through the wide use of pesticides that kill not only the target pests, but these natural enemies as well. The importance of natural enemies is now widely recognized among pest management specialists, and current pest control practices emphasize the preservation among pest management specialists, and current pest control practices emphasize the preservation of natural enemies (Kok and Kok, 2001).
The ladybird is a good predator and is also widely used as biological control agent. The ladybird need some time to get established, but is very effective when it is.
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