Mint XXXI-7
Strawberry Root Weevils
Sue Blodgett & Bill Grey
Weevil adult, left; larva, right.
Identification (life cycle and seasonal history)
The strawberry root weevil (SRW) is the primary species in the root weevil complex attacking mint that includes the black vine, rough strawberry and obscure root weevil. Most root weevils overwinter as larvae in the soil, but a few adult weevils also overwinter in protected areas. Overwintering larvae mature during the end of April and early May and form earthen cells in the soil where they pupate. Adults begin emerging in early summer and are usually present in mint fields through late September. All SRW are females, capable of laying eggs without fertilization. Eggs are deposited around the bases of plants about 2 weeks after adult emergence. There is one generation each year.
Natural spread of root weevil infestations in and among fields is gradual because adults do not fly. Serious infestations can, originate when roots and rhizomes are dug from infested fields and introduced into non-infested areas.
Plant Response and Damage
The damaging larval stage begins feeding on small mint roots very soon after emerging from the eggs. Larvae feed through the summer and early fall, at which time they reduce activity until the following spring when feeding is resumed. Adult damage, which may be evident during summer months, consists of small notches on the leaf margins and stems, particularly near the soil line. Adult feeding on mint is usually inconspicuous and is not economically important. Adults are active at night resting during the day in plant debris.
Management Approaches
Degree day units (DDU) may be used to predict the occurrence of the different life stages of strawberry root weevil using a base temperature threshold of 48oF beginning January 1. Pupae are present in samples from late April to early June (350 DD) with peak pupation occurring in early summer. Peak adult emergence occurs in mid-summer (700 DD).
Larvae are best estimated by taking 1000 cm2 soil samples after harvest or the following spring. Take at least 25 soil samples from different areas of the field (one site per 2 1/2 acres), screen the soil, count the number of larvae and calculate the average number per sample. If the average number of root weevil larvae exceeds 5 – 8 larvae per square foot, suggests need for control.
Adults are sampled by taking 10 sweeps in at least 5 field locations (for a 30A field). Best times to sweep for SRW is a couple hours after sunset on a warm, still and dry evening. When 5 or more adults are collected in 10 sweep sample, treatment is warranted.
Cultural Control
Natural spread of root weevil infestations in and among fields is gradual because adults do not fly. Purchasing certified pest free rootstock can eliminate the introduction of this pest into new plantings.
Chemical Control
Spring applications to control strawberry root weevil have met with limited success largely because of low soil temperatures (< 60oF).
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Product list for Root Weevils: |
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Pesticide |
Product per acre |
Preharvest Interval, Remarks |
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Acephate2 |
1 1/3 lb (75WSP) 16 oz (97UP) |
14 days. Target adult stage. Apply in the evening (after sunset for best success) when adult emergence is compl6te but before egg lay. Apply in a minimum 40 GPA. Do not apply more than 2 2/3 lbs/A/season. Do not graze treated crop. |
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Parasitic nematodes |
3.0 billion nematodes/A |
Apply in spring or fall when larvae are active. Follow label for timing, irrigation needs and soil temperature requirements. |
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1 Label allows chemigation, |
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The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality. |
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Categories: Mint, Insects, Strawberry root weevil
Date: 06/06/2006