It is parasitoids.They are host specific and death is ensured. Egg parasitoids and early instar larval parasitoids are most promising.

But for big colonies of pests (scale bugs or aphids for example), predators are capable of eating a lot of them, no matter of the stage, when most of the parasitoids will avoid the area because of the honeydew that is produced : a lot of them prefer to parasit isolated individuals. 

I think the best strategie is combination of both.

Yes,I agee generalist predators have that advantage. In planned IPM programmes (preferably areawide) predators will not be economical as their cost of mass culturing is huge.

In my mind, it depends of the context and of the target!

I think you have to think about your final strategy of crop protection regarding the complete pathosystem and regarding the crop production system.

In greenhouses you have to select beneficial, often, with tolerance to high humidity and high temperature. This would not be the case for the same pest but outside.

Other example : regarding aphids control under greenhouses. If you detect too late the presence of aphids, parasitoids will not be efficient but predators yes. But if you detect the first presence of aphids (few individuals or less than 2 foci), I think that in this case parasitoids are better.

Use Beneficial Control Agent as tools to build your own and specific stategy. 

If you have to select a good candidate as BCA : look for the situation, for the abiotic conditions and for the potential BCA available and why they are enought/efficient. This allows you to define the specifications of the new BAC you need: and so the choice between parasitoids or predator.

It does indeed depend on the context and the target. For example, in the case of the Pink Hibiscus mealybug in the Caribbean a combination of both coccinellid predator and a parasitoid gave excellent control in a relatively short time. The coccinellid was used to reduce high mealybug populations rapidly and the parasitoid then searched more intensively for mealybugs when the population was lower. Using either natural enemy alone may not have reduced the pest population as rapidly and efficiently as when both were released.   

Moreover, efficiency of a beneficial agent also depend of the good timing of its use regarding the:

- date(s) of the release of the beneficial agent in the crops,

- stage of the targeted pest  

Of course if you release an egg parasitoid on a larval population or if you release a predators of larvae on adults you will have bad results even with a good beneficial agents. That why, a good evaluation of the sanitary state of the crop is for me the 1st step of a complete IPM or Biocontrol strategy.

I think that when you ask which of them, predator or parasitoid, have more efficiency, it shows you refer to biological control and want to have the best candidate for using in this method.

But do nor forget, the biological control means, use the suitable organism on the suitable plant in suitable time in suitable accumulation, all of these are important for use biological method, some times the predator is conform with those factors and other times parasitoid is perfect.

 Deciding on which natural enemy is the best choice to control a pest, i.e., predator vs parasitoid. There is no standard answer for that question. The answer to that depends on each particular case, depending on the insect target (pest). The question could be extended to which is a better option, Predator vs parasitoid vs (predator + parasitoid) vs (predator + predator) vs (parasitoid 1 + parasitoid 2), etc. It becomes more difficult  when we try to include more than two natural enemies to control a pest, because we have to know or find out what will be the interactions between, lets say between the parasitoid and the predator, e.g., do they attack the same life stage of the pest? Will there be a synergistic effect (being able to coexist and feed on different times and life stages of the pest) or an antagonistic effect,e.g., intraguild predation. We have to understand the ecological niche before making decisions. Compatibility among NE is  important. Deciding on which is a better option, predator or parasitoid is not an easy question. What insect pest are you trying to control? and which natural enemies, predators and parasitoids are available that you could use to control the insect target? If we narrow the question to a specific case, perhaps we could discuss that particular case, by looking into the literature and experiences by Researchgate members.

yI believe that parasitiods have more important as a biocontrol agents  for wide area becase these are  easy to mass reared.shorter life period d  stored in labs at large scles while storage as well as rearning of predators is bit difficult due to the presences of canabolism .so,according to my personal experience parasitiodsaremost economical 

Depends on the cropping system and the herbivorous species. For example, parasitoids are more effectively in tomatoes at suppressing noctuid larvae and pentatomids and parasitoids appear to be most important for aphid suppression, at least at high aphid densities. However, if we consider mortality from all causes, the tomato plant via its glanular trichomes efforts the greatest mortality to noctuid larvae and to flower thrips.

In contrast, a complex of generalist predators in cotton is more effective at suppressing noctuid larvae, mirids, and spider mites than are parasitoids, although epizotics of fungal pathogens have been observed to cause rapid declines in spider mite populations. This same complex of generalist predators can consume large numbers of western flower thrips in cotton, but the massive number of thrips and the rapidness with which they move into cotton prevents the predators from exerting effective control of flower thrips, which is rarely needed. In contrast, the western flower thrips can have a highly significant impact on spider mite mortality due to this omnivorous feeding behavior. A very different conclusion regarding the relative importance of predators and parasitoids might be reached were we talking about aphids or whiteflies in cotton. 

A systematic analysis of each complex of herbivores and their set of natural enemies within a specific host plant system is required to determine the degree with which population regulation/suppression occurs under different edaphic and climatic conditions.

Very soon there will be an article on the history of biological control in Canadian forests  available in The Canadian Entomologist. In this article we show that much more parasitoids than predators were released against pest insects in Canadian forests and that with only one exception, all successful biological control programs involved parasitoids. The paper is accepted and will be available online after the proof reading process.

Parasitoids have short life cycle than predators, but whether parasitoids or predators,if narrow host range,then one can expect good control over targeted pest.

personal evelauation I suggest that parasitoids have upper hand in controlling insect pests due to short life span ,good searching ability and host specific

For some pests either predators or parasitoids are successful depending on what is  in the market.

But generally, predators acts faster and can bring down a growing pest population faster unlike parasitoids infection where most pests will continue causing damage even after infection

Some predators are big in size like Orius spp. and they  can sometimes kill even more than they need to feed on

Some predators also feed on (multiple) or even all life stages of the pests --even killing more juvenile stages than the adults while parasitoids are more specific on the size, quality of their host

Lastly, generalist-omnivores  feed on a variety of food thus maintaining the population longer than the host specific parasitoid which ceases to reproduce once all suitable hosts have been utilized

Yes definitely, predator play an important role in controlling pest population as they feed variety of juvenile as well as adult insects in greenhouse or in field condition while parasitoids limits is minimum compare to predators. So, predators are more efficient in biological control compare to parasitoids in terms of crop range.

I still think is a case by case basis. For example in scale insects, parasitoids are generally efficient natural enemies. Because of the generalist nature of "many" predators, often they will feed on non-target insects and thus not always be as efficient. However, there are also some very efficient predators. We recent publish a paper on how the coccinellid Anovia punica Gordon (Coleoptera: Coccinellidae: Anoviini) was able to efficiently control the Colombian fluted scale Crypticerya multicicacrices (Hemiptera: Monophlebidae) in just 8 months after it was first sighted on San Andres Island, Colombia. This coccinellid is apparently specialized on scale insects of the tribe Iceryini, On the other hand, on the same island, the pink hibiscus mealybug, Maconellicoccus hirsutus is under control by two species of parasitoids, Anagyrus kamali and Gyranusoidea indica (Hymenoptera: Encyrtidae). In scale insects, often as a tactic, predators may be used in the initial phase while the parasitoids become established, so I recommend a combination of both predators and parasitoids. Once the parasitoids become established, they are usually capable of maintaining the populations under control, while the ecosystem is in equilibrium. Are we talking about cotton pests? Pests in greenhouses? What is the target pest?

I agree with Ahmad.

I look forward to reading Lucas's paper that he mentioned earlier in this thread and concur with his comments.

Historically, I think that parasitoids are "better" at controlling/regulatiing pest populations. They tend to be more host specific and more highly adapted to their hosts. For that reason you will see less of the boom-and-bust life cycles often associated with predator/prey cycles. Parasitoids, in general, can maintain pest populations at a lower density. I think a "problem" with many predators is that they prey on a wide range of organisms, including conspecifics. The greatest successes with predators are those that are highly specialized to their prey as Kondo-sama mentioned in his post. Even then, one weakness of predators is that they often require more prey than parasitoids require hosts. That makes it difficult to maintain pest species at low levels and often results in the boom-and-bust cycles I mentioned before. Depending on the economic injury level (EIL) of the pest a predator may not be as successful at regulating and maintaining the pest below EIL.

Another issue to consider is whether you are discussing long-term control in semi-permanent agroecosystems, for example orchards, or are you thinking about inundative release in a transient crop like lettuce. I think predators will have a more immediate impact in the latter situation. In a permanent system I think the goal should be to encourage development of a healthy, multifaceted foodweb which will be more resilient to changes that might occur.

You have to consider the biogeographical situation. In temperate climates, my experience in Europe is that parasites act very well, however in neotropical areas and in Taiwan (part temperate, part tropical) the predators are best suitable. It also depends on the species or group to be controlled, for example, for some Lepidoptera larvae (Noctuidae and Sphingidae among others) parasites are best, but for Reduviidae Chagas disease vectors, predators are best. There are some many other considerations to take in mind.

in general , parasitoids r more efficient due to host specificity, however that is a condition of an effective natural enemy regardless of being predator or parasitiod, as well as synchrony and reproduction rate among others

Usually parasitoids are more efficient because most of them are host specific,  have the ability  to search, they have co-evolved for thousands of years, use smell to locate prey even in places within fruits and branches and follow his host flying long distances. However there are some exceptions for this rule. In the other hand, predators are generalists and do not spend enough time looking for prey, but also there are some exceptions. You must identify the species  before using a natural enemy.

Pathogens (fungi, bacteria or viruses) are more efficient!

The article I mentioned above is finally published... It can give some answers on the question asked here, at least what concerns biocontrol agents in Canada.

Chris J.K. MacQuarrie, D.B. Lyons, M. Lukas Seehausena and Sandy M. Smith. (in press) A history of biological control in Canadian forests, 1882–2014. The Canadian Entomologist (early view).

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=10116692&fulltextType=RA&fileId=S0008347X15000668

the microbes are more specific for there target and more safety for human and the world and for the plant product as food and easy for us to breeding them in lab. and we can control the time of them use and easy use and management. in the second level we Usually use parasitoids they are more efficient because most of them are host specific, have the ability to search, they have co-evolved for thousands of years, use smell to locate prey even in places within fruits and branches and follow his host flying long distances. However there are some exceptions for this rule. In the other hand, predators are generalists and do not spend enough time looking for prey, but also there are some exceptions. You must identify the species before using a natural enemy.

2. Predators are mainly free-living species that directly consume a large number of prey during their whole lifetime. Parasitoids lay their eggs on or in the body of an insect host, which is then used as a food for developing larvae. The host is ultimately killed. Most insect parasitoids are wasps or flies, and may have a very narrow host range. Pathogenic micro-organisms include bacteria, fungi, and viruses. They kill or debilitate their host and are relatively host-specific. Various microbial insect diseases occur naturally, but may also be used as biological pesticides. When naturally occurring, these outbreaks are density-dependent in that they generally only occur as insect populations become denser

Parasitoids are more effective than predators because of their specificity. But Entomopathogens are more letal for the host. For example, nematodes can kill their host in the range of 24-48h because their relationship with gram negative bacteria and from 1 host, you can obtein at least 100000 IJ's (the infective stage of the nematode) that can parasite other insects.