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PROJECT TITLE: CONTROL OF STINK BUGS IN TOMATOES
RESEARCHER: Dr. David Hunt
Agriculture and Agri-Food Canada
Greenhouse and Processing Crops Research Centre
Harrow, Ontario N0R 1G0
OBJECTIVES: Adults and immature stink bugs and tarnished plant bugs feed by inserting their mouthparts into the plant or fruit and withdrawing fluid. Feeding on green fruit appears as dark pinpricks or small dimples, and these pinpricks can be surrounded by a white halo. In ripening fruit the area around the feeding site turns yellow, and white, spongy tissue or cavities are found below the feeding scar when the skin is peeled back. Damage is caused by the removal of fluids from fruit cells and the injection of enzymes by the feeding insects. This fruit damage is sometimes known as "cloudy spot". Damage is most important in tomatoes intended for whole pack canning.
Our objectives were to: 1. Determine which insect species are responsible for this damage to tomato fruit; 2. Develop damage thresholds for various species of stink bugs and tarnished plant bugs on processing tomatoes; and 3. Test various insecticides for control of stink bugs and tarnished plant bugs on processing tomatoes.
RESULTS: 1. Determined which insect species are responsible for "cloudy spot" damage to tomato fruit: Several insect species were collected using sweep nets and beating trays from in and around tomato fields that were showing "cloudy spot" damage to tomato fruit. These species were transferred to cages in field plots or greenhouses at the Research Centre. Of these insects, only the tarnished plant bug and species of stink bugs were capable of producing the fruit damage. According to a publication from the University of California in Davis, in fruit damage caused by tarnished plant bugs the fruit below the feeding site is less corky than with stink bug feeding, and it dries out more quickly causing the surface of the fruit to crack. This distinction was not noted in our trials. The tarnished plant bug also fed on the flowers and stems of tomato in our study, causing flower drop. This flower damage has been shown to reduce tomato yield in some years in the US. In the US, stink bugs carry yeast and other pathogens that may cause decay when introduced into fruit on the bugs' mouthparts. A few fields have been significantly damaged by yeast introduced by stink bugs, and this damage is scored as "mold" by state graders in California. We have not noted this type of damage in Ontario tomato fields.
Adult stink bugs are shield shaped, 10 to 15 mm long, usually green or brown, with a large triangle pattern on the middle of their backs. Immature stink bugs are similar in appearance, but lack fully developed wings. Eggs of stink bugs are barrel-shaped and are laid in groups of 10 to 25, usually on the undersides of leaves. In contrast, adult tarnished plant bugs are more elongate, 5 to 6 mm long, 2 to 3 mm wide, and reddish-brown in colour.
2. Developed damage thresholds for species of stink bugs and tarnished plant bugs on processing tomatoes:
Large numbers of stink bugs and tarnished plant bugs were collected using sweep nets, and these were used to
establish colonies at the Research Centre. Insects from these colonies at various life stages were transferred onto
tomato plants at various densities in order to determine damage thresholds. The number of adult stink bugs
required to cause significant amounts of fruit damage was much lower than for tarnished plant bugs. An average
of one adult stinkbug per tomato plant was enough to cause significant amounts of fruit damage, while 15-20
tarnished plant bugs were required to cause similar damage. Attempts were also made to sample commercial
tomato fields to determine what densities of insects were required to produce economic levels of damage.
However, field populations were low this summer due to the wet conditions, and more useful data was obtained
from cage studies.
In our study it was clear that tomato is a much more preferred host for stink bugs than for tarnished plant bugs. Stink bugs caged onto tomato plants laid eggs, and the immatures successfully developed into a new generation of adults. Tarnished plant bugs that were caged onto tomato plants were not able to produce successive generations, and generally died within a few days. Fruit and blossom damage did occur during this short period, but only when large numbers of tarnished plant bugs were present.
Both stink bugs and tarnished plant bugs are difficult to monitor effectively in tomatoes, and it is unlikely that damage thresholds could be used successfully for these pests. We have found that stink bugs spend much of the day on the ground beneath tomato plants, which complicates monitoring. According to a publication from the University of California in Davis, useful monitoring data for stink bugs can be obtained by shaking plants over a sheet or tray. They estimate that 1/3 to 1/2 stink bugs per tray shake will result in about 5% damaged fruit. Cone traps baited with an aggregation pheromone are also useful for identifying stink bug hotspots in fields, and work is also progressing on identifying a pheromone for tarnished plant bugs.
Stink bugs and tarnished plant bugs usually move into the edges of tomato fields and seldom are found in the interior of the fields, suggesting that spraying the edges of fields could be effective in achieving control. Both species, but tarnished plant bug in particular, tend to move into tomato fields when preferred hosts that are adjacent to the fields are disturbed or dry out. In previous years we have noted tarnished plant bug damage to tomato fruit when adjacent alfalfa is cut or when weedy fields are harvested or sprayed.
3. Tested various insecticides for control of stink bugs and tarnished plant bugs on processing tomatoes: Small plot trials were used to evaluate the efficacy of dimethoate (Lagon 480 E), carbaryl (Sevin XLR Li) and cyhalothrin-lambda (Matador 120 EC) for control of stink bugs and tarnished plant bugs on processing tomatoes. Satisfactory levels of control were obtained with all three of these products at the labelled rates. These insecticides are currently registered for control of the tarnished plant bug on tomatoes at the following rates: dimethoate (Lagon 480 E) 0.55 - 1.1 L/ha; carbaryl (Sevin XLR Li) 5.25 - 6.4 L/ha; and cyhalothrin-lambda (Matador 120 EC) 83 mL/ha. There are currently no registered insecticides for control of stink bugs on tomatoes. Through the Tomato Research and Services Committee it has now been recommended that the registration of these products be extended to stink bugs on tomatoes.