Insects on Medicinal Crops

Introduction

Like any other agricultural crop, medicinal plants are subject to attack by insect pests. In the summer of 1998, a survey of medicinal crop insects was carried out on fields and plots of echinacea, valerian, milk thistle and feverfew. Sampling was done using sweep nets, aspirators and vacuum samples. Included in this factsheet are brief descriptions of the insects most frequently collected.

Stink bugs

Stink bugs are broadly oval in shape and range from 0.16 - 0.6 inch (4 - 15 mm) in length. Colour is quite variable depending upon species. The majority of stink bugs feed on plants by piercing plant tissue with their mouthparts and sucking out plant juices. Very few are host specific, including the species collected during the 1998 sampling. Some stink bugs feed on other insects.
 

Two species were found in these crops. The "two-spotted bug" (Cosmopepla bimaculata) was collected on echinacea in mid-July. It is 0.2 inches (5 mm) in length and is black with contrasting red markings. The second species is the one spotted stink bug (Euchistus variolarius), pictured above, that was collected on milk thistle in July and August. This is a large 0.5 inch (12 mm) pale brownish-green stink bug. Both species were collected in small numbers and both have numerous insect parasites that attack them and help keep their populations under control.

Lygus Bugs

Several species of Lygus bugs were collected on medicinal plants. Lygus bugs are well known pests of agricultural and horticultural crops in Canada. They have an extremely broad range of host plants. Like the stink bugs, they feed by piercing plant tissues and sucking out plant juices. In addition, some plants also react to the toxic saliva that these insects inject while feeding. This reaction has not yet been observed in echinacea, valerian, feverfew or milk thistle.
 

Lygus bugs overwinter as adults and search out host plants in the spring on which to feed and lay eggs. The nymphs develop on the host plant and complete development by mid-July when they start laying the eggs that give rise to the second generation. Nymphs are small 0.04 - 0.2 inch (1 - 5 mm), wingless and pale green in colour.

The species collected were:

• Lygus lineolaris (tarnished plant bug) on fever few, echinacea, milk thistle and valerian.

• Lygus borealis was collected on fever few, milk thistle and echinacea.

Other Plant Bugs

There were a variety of other plant bugs that were collected on these crops. These included the green grass plant bug, Trigonotylus coelestialium on feverfew and valerian; and the alfalfa plant bug, Adelphocoris lineolatus on milk thistle. None of the plant bugs were collected in large numbers and may have been simply resting on the plants as they were moving to other hosts.

Leafhoppers

A variety of leafhoppers were collected on these crops. These insects are wedge-shaped with the body tapered towards the wing tips. They are very small 0.02 - 0.12 inch (0.5 - 3 mm) and vary in colour from black to mottled brown to bright green, depending upon the species. Leafhoppers are carried in on southern winds in late May or June. They lay their eggs on a variety of crops and may produce up to four generations per year. These are also sucking insects with piercing mouthparts.
 

Feeding injury sometimes appears similar to disease symptoms. Some species, notably the aster leafhopper (Macrosteles quadrilineatus) are important as vectors of disease. The aster leafhopper is capable of transmitting aster yellows to echinacea. The survey found one plot where this occurred. Information about aster leafhopper population levels is available throughout the summer.

Several species of leafhopper were collected. These included the potato leafhopper, Empoasca fabae; the aster leafhopper, Macrosteles quadrilineatus; and three species with no common names. These are Psammotettix striatus, Cuerna striata and Neokolla hieroglyphica. The potato leafhopper is an important pest affecting alfalfa in the northeastern United States, but populations seldom reach economically damaging levels in Manitoba. Leafhoppers were collected on all four medicinal crops sampled but in low numbers. Unless populations are high, control measures should not be considered.

Treehoppers

Treehoppers are close relatives of leafhoppers. They are not known to transmit diseases and their numbers rarely, if ever, reach economically damaging levels in agricultural crops. They most commonly feed on trees and shrubs. These are small insects 0.08 - 0.2 inch (2 - 5 mm) that are often hump-backed or shaped like thorns. They vary in colour from pale green to light brown to black.

Only one species of treehopper was collected at one location in Manitoba. That species lacks a common name and is known as Publilia concara. Only four specimens were collected.
 

Aphids

Aphids are small 0.08 inch (2 mm) insects that vary in colour from pale green to yellow to pink to black. Antennae are long and there are two backward projecting tubes at the tip of the abdomen (in most species) known as cornicles. They may be winged or wingless. Aphids feed by piercing roots, stems, leaves or flowers and sucking plant fluids. Feeding causes abnormal growth, wilting or flower drop. Aphids are unique among insects in that they give birth to live young throughout most of the year.
 

Aphids were collected on all echinacea, valerian, feverfew and milk thistle plants. One farm had high levels of aphids on milk thistle.

Aphids are easily controlled by a number of methods. Often heavy rains or high pressure water spray will keep populations down. As aphids are small and slow moving, they are easy targets and preferred prey for a variety of beneficial insects such as ladybird beetles and lacewing larvae. Most organic products such as safers soap, pyrethrum and rotenone provide good control. Caution should be used before using a broad spectrum insecticide such as rotenone or pyrethrum as they will affect beneficial insects as well.

Lepidoptera

Lepidoptera are butterflies and moths. Their larval form, or caterpillars, feed on a wide variety of plants, depending upon the species. During the survey no caterpillars were collected. Gelechiid moths have been collected previously from flowers of echinacea plants. Botanical insecticides such as Bacillus thuringiensis (Bt), rotenone and pyrethrum will control these pests. Keep in mind potential effects on beneficial insects before use.
 

Dark Winged Fungus Gnats

One plot of echinacea suffered substantial damage as a result of flooding, disease and insect feeding on the roots. Dark winged fungus gnat larvae (maggots) were collected from the roots of the affected plants. It is not known whether these insects were present first and they created an opportunity for infection or if they arrived to feed on echinacea weakened by the disease.
 

For further information, contact your GO representative.