Natural Enemies: Monitor Populations & Consider Food Sources

Excerpt from Fruit Crop Ecology and Management, Chapter 2: Managing the Community of Pests and Beneficials by Larry Gut, Annemiek Schilder, Rufus Isaacs and Patricia McManus

Ecological concept: Many kinds of organisms inhabit fruit production systems
Putting it into practice: Monitor both pest and beneficial species. Consider alternative food sources for beneficial species.

Monitor pest and natural enemy populations

A good scouting program is key to making ecologically sound pest management decisions. The need for control and the impact of any action are determined by monitoring pest and natural enemy populations. Sampling provides information on the organisms present, their stages of development, population densities, and the ratio of pests to natural enemies. A scout must know how an organism develops because different life stages may be monitored in different ways. For example, you would sample fruit to look for grape berry moth larvae, but use pheromone traps to monitor adults. In addition, a single life stage may move to different plant parts as the season progresses. Oriental fruit moth larvae attack terminal shoots early in the season, feed in shoots and fruit in the middle of the summer, but only infest fruit late in the season.

Sampling for predators and parasitoids is as important as monitoring pest populations. Higher pest densities can be tolerated when populations of natural enemies are also high. For example, a mite spray may be warranted in apple when there are two or three mites per leaf if there are no natural enemies, but a grower might wait until there are five or six per leaf if one predator mite per leaf is also present. Monitoring parasitoid populations can be tricky as often it is only the signs of their presence that can be readily detected. For example, aphid parasitoids that feed within their hosts cause the aphids to become puffy or mummified and tan, golden, or black in color. A round hole can be observed where the wasp has cut its way out of the aphid mummy.

Predators and parasitoids may need alternative food sources

Some predators and parasitoids use alternative food sources during the growing season. These include prey or hosts other than pests, and nectar producing plants other than fruit crops. If an alternative host is not available, the predator or parasitoid may not survive or stay long enough in the crop to control pests when needed. Predatory mites often feed on rust mites when their primary hosts, spider mites, are absent or in low numbers.

Parasitoids may require an alternative host to complete their life cycle. A small parasitic wasp, Colpoclypeus florus, can have a major impact on some leafroller populations in apple orchards. However, C. florus is often not present in high numbers early in the season because none of the leafrollers in the orchard overwinter as late instar larvae, the host size required to complete its development. Another leafroller host of this parasitic wasp overwinters as a large larva on wild rose, found in wild habitats around orchards. Larvae of the parasitoid successfully overwinter in this host on rose and complete their development early in the spring. They then emerge and can fly to colonize leafrollers in nearby orchards.

Many natural enemies require more than one kind of food to develop normally and sustain their populations. Syrphid fly adults supplement their diets by gathering and eating pollen from flowering plants. Where natural enemies have access to pollen and nectar, there is often more predation and lower abundance of pests. Similarly, parasitic wasps supplement their diets by feeding on nectar, aphid honeydew, and other sources of sugar. For example, Trichogramma species are tiny wasps that parasitize the eggs of moths, such as codling moth. Planting a cover crop or native plants along side and orchard or vineyard that includes flowering plants is a good way to provide nectar sources for these beneficial insects, but care must be taken in selecting a cover crop that is not a host for other pests.

Some pathogens also need to find alternative hosts when a fruit host is unavailable. For example, the root knot nematode can also reproduce on dandelions in vineyards. This weed can also serve as a host for viruses that are vectored by the dagger nematode. Weed management is essential to reduce these types of risk. Cover crops that suppress weeds and nematodes have been used in fruit systems and are likely to play an even more important role in the future. Verticillium alboatrum, a soilborne fungus that causes a severe wilt in strawberries, also attacks the roots of many other hosts, especially solanaceous crops like tomatoes, peppers, and potatoes. Growers are advised not to plant strawberries after Verticillium-susceptible crops.