The Impact Of Insect Feeding On Vegetable Crops

Insect pests can be a major impediment when growing vegetables primarily due to the feeding damage they cause thus reducing salability or marketability. In addition, insects (and even mites) may create problems directly by vectoring diseases such as fungi, bacteria, or viruses or indirectly by pre-disposing plants to infection by these same plant pathogens.

Both of these conditions are associated with the feeding behavior of insect pests. For example, insect pests with piercing-sucking mouthparts such as aphids, whiteflies, leafhoppers, and thrips may directly vector fungi, bacteria, and/or viruses whereas those with chewing mouthparts including beetles and caterpillars may indirectly increase plant susceptibility by creating entry sites for fungal and bacterial diseases.

Direct Transmission

The direct transmission of diseases is associated with the type of feeding behavior of insects and what occurs during the feeding process. Insects with piercing-sucking mouthparts such as aphids, thrips, whiteflies, and leafhoppers are well-known vectors of many different viruses (more than 300) that can infect horticultural and vegetable crops. Insect pests may contain plant viruses on their stylets or the viruses may accumulate internally, and after passage through the stylet they are introduced into plant tissues.

The length of time required to transmit viruses varies depending on the specific insect pest. For example, aphids may transmit plant viruses within a few seconds of feeding. In addition to insects with piercing-sucking mouthparts, a number of insects with chewing mouthparts including beetles are known disease vectors. Flea beetles, for example, have been reported to transmit diseases including the fungus, Alternaria brassicicola and the bacterium, Xanthomonas campestris pv. campestris to crucifers. Cucumber beetles have been shown to transmit the pathogens Colletotrichum lagenarium, Erwinia tracheiphila, Pseudomonas lachrymans, and Pseudoperonospora cubensis.

Indirect Transmission

In general, indirect transmission is associated with insect pests either carrying spores of diseases (e.g., fungi) or creating wounds that allow entry of diseases (e.g., fungi and bacteria). In fact, there are numerous examples of insect feeding damage resulting in increased plant susceptibility to fungi and/or bacteria. The indirect transmission of diseases to plants may be associated with 1.) conidia on the body, which results in passive deposition of inoculum to plant parts (leaves and stems) associated with non-inoculated plants; 2.) feeding wounds that provide sites for entry of conidia that detach from the insect body; 3) un-damaged conidia present in feces, which are viable after passing through the digestive tract; and 4.) mouthparts that may be contaminated with spores that during the feeding process can easily enter feeding wounds.

Insect pests can transmit diseases by carrying spores on their bodies. Flea beetles, for example, have been shown to transport the spores of Alternaria brassicicola on their mouthparts, antennae, and legs. Certain thrips species (e.g., Thrips obscuratus) are capable of carrying the conidia of Botrytis cinerea on their head, thorax, legs, and abdomen. Furthermore, insects may carry fungal pathogens internally (inside their body). In this case, the means of indirect transmission involves the presence of conidia in undigested pieces of plant tissues or feces, which can then enter plants through wounds.

Flea beetle feeding injury on tomato plants leads to wounds that are colonized by Alternaria solani. Studies have also shown that feeding injury by caterpillars such as imported cabbageworm, cabbage loope, and diamondback moth provide entry sites for the fungus, Sclerotinia sclerotiorum on cabbage. In addition, pickleworm larval feeding on cucumbers may provide entry sites for fungal or bacterial diseases.

Melon aphid and spider mite feeding may predispose cucumber and muskmelon to gummy stem blight, and whitefly feeding may increase plant susceptibility to bacterial infections. Adult bean leaf beetle feeding injury on soybean pods results in an increase in fungal infections of seeds.
Another means of indirect transmission is when females insert eggs into plant parts. For example, puncture wounds on the leaves of muskmelon created by adult female leafminers (in this case, Liriomyza trifolii) provide entry sites for certain fungi.

Some insect pests can both vector and create conditions that increase plant susceptibility to diseases. The striped cucumber beetle is capable of directly vectoring gummy stem blight and feeding wounds provide entry sites for gummy stem blight. In addition, the beetle is a well-known vector of bacterial wilt.

Providing Nutrients For Fungi

Another factor to consider is that wounds created by both piercing-sucking and chewing insects may enhance disease development by releasing nutrients for fungi. In addition, feeding by-products such as honeydew are rich in sugars and phloem constituents, which may serve as a substrate for the metabolism of sooty blotch fungi.

In conclusion, many insect pests including aphids, whiteflies, thrips, and leafhoppers are associated with the direct transmission of plant diseases. In addition, chewing insects such as beetles and caterpillars may be indirectly affiliated with plant diseases. As such, it is important to suppress insect populations in order to avoid problems with not only insect pests but with any diseases that they may directly or indirectly be associated with. Therefore, in these situations, managing insect pests is more critical, which may increase the use and frequency of applying insecticides. In these situations, growers must be cautious of developing resistance problems.