Host Range in Insect Parasites

Whenever we see natural enemies in our gardens or in crop habitats, we probably feel that they are benefiting us by attacking the pest species about which we are concerned. But the mere presence of predators, pathogens and insect parasites (I use the term parasite rather than parasitoid) in the habitat doesn't necessarily translate into controlling or even killing the pest of interest. The natural enemy may be attacking a prey or host far different than what we hope it is attacking. Many insect predators are generalists, meaning they will attack most prey of the right size and life stage, and slow enough to capture. Pathogens and insect parasites are a little different. Most pathogens are fairly specific to the hosts they can infect (reviewed next issue). I will concentrate in this issue on the host range of insect parasites.

We can define an insect parasite's host range as all the species of prey, or hosts, the parasite species attacks and that successfully produce parasite progeny (offspring). We may intuitively use the term "generalist" for a parasite that attacks many hosts, and "specialist" for a parasite that attacks few or one host species. Although this dichotomy is simple to use, there are likely very few true "specialists," and "generalists" can not attack any and all hosts. Confounding the definition of a host range is that most parasites CAN parasitize far more host species than they normally DO parasitize, and that parasites can attack more host species than they can use for successful development. Parasites have both a "potential" host range those species they can attack and in which they can develop and a "realized" (or observed or actual) host range those species they usually use as a host. If you were to draw an analogy between host range of insect parasites and the diet of my two children, Brussels sprouts are in their potential host range, yet are not in their realized host range.

Host ranges are influenced by many factors. Both parasite and host must coincide in time. A multivoltine (multiple generations per year) parasite may attack a number of univoltine (one generation per year) hosts, each one present at different times in the year. Conversely, a parasite that is present only at certain times may not coincide with a univoltine host that is present at another time of the year again, the difference between the potential and realized host range.

Perhaps some of the greatest differences between potential and realized host ranges are due to the sequence of processes parasites must pass through to utilize a host species. This sequence begins with habitat selection, in which the female parasite (only female parasites search for hosts for egg-laying) finds the appropriate habitat, whether or not hosts are present. This process places the parasite in the kind of habitat where a host might be found. Some parasite species search in a broad range of habitats, ranging from forests to semi-desert habitats. Other species are very specific and look only in one or very few types of habitats, perhaps even only one species of plant. Thus, for many parasites, habitat selection will determine the kinds of hosts encountered.

The second process is host finding; in this process, the parasite (already in the appropriate habitat) uses a different set of cues. The female will respond only to certain kinds of cues and not respond to others. The cues may be visual, olfactory (chemical), tactile or even auditory. Some cues are not even from the host, but can be produced by an attacked plant, "signaling" the presence of a host. Using those cues allows the parasite to find a host.

Once the host is found, the third process is host acceptance. This is a yes-no, accept-reject decision, based on a variety of different sensory inputs. Depending on the parasite species, the host may need to be a proper life stage, a certain size or shape, be healthy or not already parasitized, or even in the right microhabitat. Unlike predators, parasites are specific to a certain stage of host. An egg parasite attacks eggs, not larvae. Larval parasites do not attack pupae, and vice versa. Some species of Trichogramma wasps will attack any egg that is a certain size, regardless of what the host species is, but will only attack eggs. I have in my laboratory an ichneumonid parasite, Xanthopimpla stemmator, that will parasitize lepidopteran (moth) pupae placed inside a grass stem (or a paper straw), but will not even try to parasitize the same host pupa if it is not concealed. Some braconid parasites of leaf miners will attack (and develop in) any host that is in the proper microhabitat, even hosts as diverse as flies, beetles, and moths.

The fourth and fifth processes are whether the host is suitable for parasite development and, for internal parasites, whether development of the host can be regulated and the host's immune system can be overcome. The host has to be physiologically acceptable and provide sufficient nutrition for the parasite progeny to develop. For example, a host egg that is near hatching may not have enough yolk remaining for the parasite progeny to develop. Avoiding the host's immune system is what makes the interaction between internal parasites and their hosts very intimate, and what also limits the hosts that can be attacked successfully. Thus, these last two processes are what determine which of the hosts that are attacked actually can support development by parasite progeny. There are two general predictions that may be made about host range. Parasites of eggs and pupae likely have a broader host range than do parasites of larvae. This is because eggs and pupae are less protected (or even not protected) by immune responses than are larval stages. And, external parasites (ectoparasites) likely have a broader host range than do internal parasites (endoparasites). Again, ectoparasites do not have to contend with host immune systems, whereas endoparasites do. Ecological and behavioral factors that help to determine host range play a part in the first three processes of the sequence. The last two processes reflect host taxonomy and evolutionary factors, which also are important determinants of host range. Closely related hosts are often in a parasite's host range, either potential or realized, due to taxonomic, ecological and evolutionary reasons. For many internal parasites, overcoming the immune response is critical. Closely related hosts likely have similar immune responses; distantly related hosts likely have different immune responses. Further, closely related hosts often share ecological factors habitats, plant hosts, behaviors that may result in the parasite being more likely to find and accept the related host. As a result, host ranges more often include closely related hosts and less often distantly related hosts. But relatedness does not always determine host range. For example, the Old World braconid parasite, Cotesia flavipes, attacks lepidopteran stemborers in grasses. It was colonized in the U.S. nearly twenty years ago on the New World pest, sugarcane borer (Diatraea saccharalis). C. flavipes has in its host range some other species of Diatraea, e.g., southwestern corn borer (D. grandiosella) and neotropical cornstalk borer (D. lineolata), yet it can not develop in a closely related species, Diatraea considerata.

The totality of the sequence of processes determines host range. At any step in the sequence, an incorrect cue may result in the parasite not finding or not accepting the host, thus making a non-host out of a potential host species. This series of processes also explains the discrepancy between potential and realized host ranges, because each step of the sequence reduces the number of species the parasite will find and attack, thus limiting the realized host range. Recognizing how the sequence occurs also shows the importance of relevance when testing host range. Host range in the laboratory will nearly always be greater than that measured in the field. A parasite of lepidopteran larvae in grasses may be able to attack in the laboratory a host normally found in a deciduous forest. However, if the parasite restricts its habitat selection to grasses, it will never encounter this same potential host in the field. Thus, laboratory assays - whether testing a new parasite species against a target host or testing the safety of non-target hosts - must be interpreted with care.

For practical use, knowing the host range of insect parasites is especially important for both augmenting parasites or trying to conserve parasite populations. Who attacks whom? If we are augmenting the population of a natural enemy, we want to be certain that the parasite we are wanting to add to the habitat will actually attack the target pest. Or, if we are modifying the habitat to conserve a parasite population, we need to ensure that the parasite we are trying to conserve will attack the pest of interest.

- Robert N. Wiedenmann, Illinois Natural History Survey

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